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Natural ventilation in non-domestic buildings

CIBSE Applications Manual AM10

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No part of this publication may be reproduced, stored in aretrieval system or transmitted in any form or by any meanswithout the prior permission of the Institution.

© March 2005 The Chartered Institution of Building ServicesEngineers London

Registered charity number 278104

ISBN 1 903287 56 1

This document is based on the best knowledge available atthe time of publication. However no responsibility of anykind for any injury, death, loss, damage or delay howevercaused resulting from the use of these recommendations canbe accepted by the Chartered Institution of Building ServicesEngineers, the authors or others involved in its publication.In adopting these recommendations for use each adopter bydoing so agrees to accept full responsibility for any personalinjury, death, loss, damage or delay arising out of or inconnection with their use by or on behalf of such adopterirrespective of the cause or reason therefore and agrees todefend, indemnify and hold harmless the CharteredInstitution of Building Services Engineers, the authors andothers involved in their publication from any and all liabilityarising out of or in connection with such use as aforesaidand irrespective of any negligence on the part of thoseindemnified.

Typeset by CIBSE Publications

Printed in Great Britain by Page Bros. (Norwich) Ltd.,Norwich, Norfolk NR6 6SA

Cover illustration: Bedales School Theatre, Hampshire (courtesy of Feilden Clegg Bradley Architects; photo: DennisGilvert)

Note from the publisherThis publication is primarily intended to provide guidance to those responsible for thedesign, installation, commissioning, operation and maintenance of building services. It isnot intended to be exhaustive or definitive and it will be necessary for users of the guidancegiven to exercise their own professional judgement when deciding whether to abide by ordepart from it.

ForewordThe need for the Institution to provide professional guidance on the design and applicationof natural ventilation in buildings was first identified when I was CIBSE President in 1992.The resulting Applications Manual was first published in 1997 with the aim of ‘providingmore guidance on energy related topics in order to realise quickly the improvements inenergy efficiency which should arise from the application of the guidance presented’.

Much has happened since 1997 in relation to energy use in buildings. The EnergyEfficiency Best Practice Programme which sponsored the first edition has been replaced bythe Carbon Trust, which has become very widely recognised for its high profile campaignsraising awareness of business use, and waste, of energy. Part L of the Building Regulations,Conservation of Fuel and Power, has been transformed and will shortly be revised once moreas Part L (2005). The Energy Performance in Buildings Directive has been adopted by theEU, and will be implemented in the UK from the start of 2006. And, late in 2004, theSustainable and Secure Buildings Act reached the statute book, to enable BuildingRegulations to address these two, sometimes conflicting, themes.

In the light of all these changes, as well as the growing practical experience of advancednaturally ventilated buildings, it is timely to issue a revised edition of this guidance. Theprinciples remain largely unchanged — as do the laws of physics on which they depend.However, experience in their application has advanced, and new examples have appeared.As a result, the material has been re-ordered, and the examples, instead of standing alone atthe end, are now incorporated within the guidance at appropriate places. This edition alsodraws extensively on work funded by the Partners in Innovation scheme of the DTI onautomatic ventilation devices. The guidance contained within this edition will enablepractitioners to apply the principles of natural ventilation based on a sound understandingof their underlying basis. In so doing further improvements in energy efficiency will beachieved.

The revision has been undertaken by one of the original authors, Steve Irving, aided byDavid Etheridge and Brian Ford of Nottingham University. The revision has again beensteered by a small group of leading practitioners from a range of professional backgrounds,with the aim of producing guidance that is as far as possible accessible to architects andengineers alike, and will assist them in adopting an integrated approach to building design.The Institution would like to thank the Steering Group, listed below, for their contributionto the project, and also to acknowledge the support of the Carbon Trust for the work.

Brian MossChairman, CIBSE Publications, Research and Outputs Delivery Committee

AcknowledgementsThe Chartered Institution of Building Services Engineers gratefully acknowledges thefinancial support provided by the Carbon Trust in the preparation of this publication.However, the views expressed are those of the Institution and not necessarily those of theCarbon Trust. The Carbon Trust accepts no liability for the accuracy or completeness of, oromissions from, the contents of the publication or for any loss arising from reliance on it.Any trade marks, service marks or logos relating to the Carbon Trust used in this publica-tion are the property of the Carbon Trust and must not be used or reproduced without theCarbon Trust’s prior written permission.

Principal authorsSteve Irving (FaberMaunsell) (Sections 1 and 2)Prof. Brian Ford (School of the Built Environment, University of Nottingham) (Section 3)David Etheridge (School of the Built Environment, University of Nottingham) (Section 4)

Contents1 Introduction

1.1 General1.2 Structure of this publication

2 Developing the design strategy2.1 Satisfying design requirements2.2 Selecting a natural ventilation concept2.3 Driving forces for natural ventilation2.4 Natural ventilation strategies

3 Ventilation components and system integration3.1 From strategy to specification3.2 Ventilation opening types3.3 Internal obstructions3.4 Background leakage3.5 Window stays and automatic actuators3.6 Control system3.7 Installation and commissioning

4 Design calculations4.1 Establishing the required flowrates4.2 Selecting a ventilation design tool4.3 Design procedures using envelope flow models4.4 Input data requirements and selection4.5 Reservoir effect

References

Index

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1

1 Introduction

1.1 General

This publication is a major revision of the ApplicationsManual first published in 1997(1). At that time, there was asignificant expansion of interest in the application ofengineered natural ventilation to the design of non-domestic buildings. The original AM10 sought to capturethe state of knowledge as it existed in the mid-nineties andpresent it in a form suited to the needs of every member ofthe design team.

Some ten years on from the time when the initial manualwas conceived, the state of knowledge has increased, andexperience in the design and operation of naturallyventilated buildings has grown. This revision of AM10 istherefore a timely opportunity to update and enhance theguidance offered to designers and users of naturallyventilated buildings.

The first edition of AM10 devoted its first section tosetting natural ventilation into the context of the range ofavailable design solutions. This aspect is now dealt with inCIBSE Guide B2: Ventilation and air conditioning(2). TheGuide sets out the various approaches to ventilation andcooling of buildings, summarises the relative advantagesand disadvantages of those approaches and gives guidanceon the overall approach to design. This edition of AM10 isintended to complement Guide B2 by providing moredetailed information on how to implement a decision toadopt natural ventilation, either as the sole servicingstrategy for a building, or as an element in a mixed-modedesign(3).

This edition of AM10 should also be considered alongsideother major sources of relevant guidance, and in particularthose in support of the requirements of the BuildingRegulations. For England and Wales, the key documentsare:

— Approved Document F1: Means of ventilation(4)

— Approved Document L2: Conservation of fuel andpower in buildings other than dwellings(5).

At the time of writing (January 2005), both these parts ofthe Regulations are the subject of major review, and so theguidance in this document will need to be interpreted inthe light of the requirements prevailing at the time of use.CIBSE Guide A(6) complements the guidance in ApprovedDocument F, and provides much fundamental data onminimum ventilation rates and thermal comfort criteria.

1.2 Structure of this publication

Following this introduction, the manual is divided intothree main sections. These chapters progress from areview of the strategic issues to a detailed development ofdesign techniques. As such, the material becomes increas-ingly technical in scope. Consequently, non-technicalreaders will probably wish to concentrate on section 2,which deals with developing the design strategy. Section 3deals with a review of ventilation components and howthey should be integrated into an overall design philos-ophy. This section will be particularly relevant to allmembers of the design team, and elements of it will berelevant to the client and the facilities management team.Section 4 concentrates on design calculations, and isprimarily targeted at the building services engineer whohas responsibility for engineering the design. Briefoverviews of the chapters are provided in the followingsections so that readers can identify the material that willbe relevant to their own requirements.

1.2.1 Section 2: Developing the design strategy

This section focuses on the strategic issues. It begins bysummarising what functions natural ventilation candeliver, and the key issues that need to be considered aspart of delivering a successful design. The section containsa detailed flow chart that can be used to assess the viabilityof natural ventilation.

Natural ventilation systems are intended to providesufficient outside air to achieve appropriate standards ofair quality and to provide cooling when needed. Since thecooling capacity of natural ventilation is limited, a keydesign challenge is to limit heat gains through good solarcontrol and careful management of the internal gains. Thesection explains how naturally ventilated buildings do notaim to achieve constant environmental conditions, buttake advantage of dynamics to provide comfortable, con-trollable conditions for the occupants.

The section continues by reviewing the different types ofventilation strategy. The most appropriate strategy isshown to depend on the type of space (i.e. open plan,cellular) and whether wind or buoyancy forces are likelyto predominate. The section aims to provide a conceptualunderstanding of how the various system concepts work,and how different design features can enhance theflexibility and robustness of the design.

Because of the increase in summertime temperaturescaused by global warming, the achievement of goodthermal comfort with low energy consumption willbecome increasingly challenging for all summer coolingstrategies (both natural and mechanical). The effectiveapplication of natural ventilation will increasingly require

Natural ventilation in non-domestic buildings

2 Natural ventilation in non-domestic buildings

careful integration with other design measures (bothpassive and active), especially in the south-east ofEngland. Global warming does not mean that the impor-tance of natural ventilation diminishes; it will still have avery important role to play as part of an integrated designapproach, as a key element in a mixed mode building, andas the lead strategy in the cooler parts of the UK. Inaddition, it might be the case that, as the climate warms,occupants will adapt themselves to that changing climate,and so the threshold at which people find conditions toowarm will also increase.

1.2.2 Section 3: Ventilation components and system integration

This section is mainly about tactics. Having used section 2to develop the strategy, this section looks at the selectionand specification of the various types of ventilationcomponent (i.e. windows, ventilators and dampers) andhow they should be integrated into an overall system.

As well as considering the technical issues of design andspecification, the section also discusses the important‘softer’ issues, such as the division of responsibilitybetween members of the design team and the componentsuppliers and system installers. This is particularlyimportant since many issues relating to the successfulimplementation of natural ventilation cross traditionalboundaries of design responsibility.

Another key issue is the inter-relationship between thesystem and the occupants. A key aspect of natural venti-lation is to empower the occupant to make suitableadjustments to window opening etc. to maintain personalcomfort without prejudicing the comfort of others. Thismeans that automatic control strategies need to becarefully integrated with user behaviour. Such issues aredeveloped in section 3.

Because of the important link between the design and theway the user operates the building, section 3 stresses thebenefits of post-completion fine tuning to ensure the fullpotential of the building is being realised to the benefit ofthe occupants.

1.2.3 Section 4: Design calculations

Section 4 is the most technical part of the manual. Itbegins by reviewing the calculations that will need to becarried out and reviews the type of calculation techniquesthat are available.

The section suggests that for basic design purposes, a classof tools known as ‘explicit envelope flow models’ are themost appropriate. They allow basic dimensioning of thesystem components. It then explains how other, moresophisticated tools (such as implicit envelope flow models,combined thermal and ventilation models, computationalfluid dynamics and physical scale models) can be used tocheck the performance of the sized system under a varietyof operating modes.

Because implicit envelope flow models are the most usefultool to the designer, this aspect is developed in depth,showing how the basic textbook equations can be manipu-lated to provide solutions to most design problems. Thesetechniques are then illustrated with a number of worked

examples, and guidance on where the relevant input datamight be found.

As an adjunct to this manual, a spreadsheet tool* has beenprepared that implements many of the design calculationsincluded in section 4. This is intended as an illustration ofhow the methods could be implemented. Users will needto confirm that the tool meets their own requirements,and adjust it as necessary to meet the particular circum-stances of the design issue they are investigating.

2 Developing the design strategy

2.1 Satisfying design requirements

Natural ventilation is one of a number of strategies thatare available to the designer. CIBSE Guide B2(2) containsan overview of the various approaches and gives guidanceon their applicability to different situations.

Natural ventilation systems need to be designed to achievetwo key aspects of environmental performance:

— ventilation to maintain adequate levels of indoorair quality

— in combination with other measures, ventilationcan reduce the tendency for buildings to overheat,particularly in summer.

The natural ventilation strategy must also be integratedwith all other aspects of the building design. Key issuesfor consideration are:

— A satisfactory acoustic environment: natural venti-lation openings also provide a noise transmissionpath from outside to inside, and this may be adetermining factor in some building locations. Inaddition, naturally ventilated buildings ofteninclude large areas of exposed concrete in order toincrease the thermal capacity of the space. Suchlarge areas of hard surface will require carefulattention to achieve a satisfactory internal acousticenvironment.

— Smoke control: since smoke can follow naturalventilation paths, the integration of the fire safetystrategy must be an important part of design fornatural ventilation.

— Health and safety(7): many natural ventilationopenings will be at significant heights above floorlevel and so the proposed Work at HeightsRegulations(8) will be particularly relevant.

2.1.1 Ventilation

The principle role of ventilation is to provide an appro-priate level of indoor air quality (IAQ) by removing anddiluting airborne contaminants. Guidance on achievingadequate levels of IAQ (to avoid mould growth and healthhazards) is given in Approved Document F(4). Higherrates of ventilation may be provided than proposed in the

* The spreadsheet may be downloaded from the CIBSE website(www.cibse.org/venttool)

Developing the design strategy 3

Approved Document, and this may enhance the percep-tion of freshness, but in most cases this will come at aprice because energy costs will increase correspondingly.In order to achieve adequate IAQ, Approved Document Fadopts a three-stage strategy as follows

(a) Extract ventilation: to remove pollutants at source,with the extracted air being replaced with outsideair.

(b) Whole-building ventilation (supply and extract): todisperse and dilute other pollutants.

(c) Purge ventilation: to aid removal of high concen-trations of pollutants released from occasionalactivities such us painting, or the accidentalrelease via spillages etc. Purge ventilation istypically an order of magnitude greater thanbackground ventilation. As well as helping toremove high levels of contaminants, purgeventilation can also help to remove excess heatfrom the space, thereby assisting thermal comfortin summer.

The whole-building ventilation rate recommended byboth the 2005 edition of CIBSE Guide A(9) and the draftApproved Document F(10) is 10 litre·s–1 per person. This isbased on the correlation between ventilation rates andhealth. Since naturally ventilated buildings cannot

provide a constant ventilation rate, it is necessary todemonstrate that an equivalent level of air quality hasbeen provided. This can be done by showing that the IAQachieved by the natural ventilation is equivalent to thatprovided using a constant ventilation rate of 10 litre·s–1

per person during occupied hours. One way of doing thisis to use the CO2 level in the space as a proxy for generalIAQ levels. By calculation, the CO2 levels in the occupiedspace can be determined based on a constant ventilationrate of 10 litre·s–1 per person during occupied hours. Asimilar calculation can then be carried out using thevariable ventilation rate typical of a naturally ventilatedscheme. In both cases, the boundary conditions of externalCO2 concentration, occupancy levels etc. must be thesame. The naturally ventilated design would be acceptableif the average CO2 concentration during occupied hours isno greater than that achieved by the mechanicallyventilated design, and the maximum concentration in thenaturally ventilated scheme is never greater than anagreed maximum threshold figure. The IAQ tool in thespreadsheet (see section 1.2.3) illustrates how thesecalculations can be carried out. This is illustrated inFigure 2.1.

Figure 2.1(a) shows the CO2 profile for a constant venti-lation rate of 10 litre·s–1 per person (equivalent to 1.2 ACHin this example), coupled with a background infiltrationrate of 0.1 ACH. Figure 2.1(b) shows a naturally ventilatedscheme having three levels of ventilation: a night-timerate of 0.25 ACH, an initial daytime rate of 1.0 ACH and aboosted rate in the middle of the day of 1.5 ACH. Theaverage concentration of CO2 in the two cases is 986.2 and971.9 ppmv respectively although, as can be seen, thenatural ventilation peaks at just over 1100 ppmv, com-pared to the constant mechanical case of 1005 ppmv.

In a similar way, if the volume of the space is sufficientlylarge, then the pollutants from the activities in the spacewill only degrade the IAQ in the occupied zone slowly,especially if a pure displacement type ventilation strategyis adopted, with the pollutants being concentrated in astratified layer above occupant level. As an illustration,consider ventilating a theatre, where there the designoccupancy is 1000 people. This occupancy will only lastfor the duration of the performance, but will build up tothat peak for the hour or two preceding ‘curtain-up’.Figure 2.2 shows the evolution of CO2 concentration inthe space when ventilating at a constant rate between17:00 to 22:00 equivalent to 8 litre·s–1 per person based on

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Figure 2.1 Comparison of constant and variable ventilation rates onindoor air quality

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Figure 2.2 Effect of volume and airtightness on indoor air quality


the design occupancy. This is 20% less than the whole-building ventilation rate recommended by ApprovedDocument F(4) but the average concentration in the spacepeaks just above 1000 ppmv at the end of the performance.If a displacement flow regime is in place, then the concen-tration in the occupied zone will be significantly lower.This relatively simple example illustrates the importanceof the dynamics of ventilation.

2.1.1.1 Control of ventilation

If natural ventilation is to be adopted, then the system hasto be able to provide controllable ventilation rates across awide range, from say 0.5 to 5 ACH or even more. Indeed, itshould be possible to shut down the ventilation rate tonear zero when the building is unoccupied, especially ifoccupancy is the principal source of pollutants. The widerange of flowrate that is required means that the differentmodes of ventilation (whole-building, purge etc.) arelikely to be provided via different devices such as trickleventilators, opening windows and/or purpose providedventilators. Such considerations will have considerableimplications for the façade design and the control strategy,requiring a high degree of design integration. This isconsidered in detail in section 3.

As well as providing the required ventilation rates, theventilators should be designed so as to minimise discom-fort from draughts, especially in winter. In office-typebuildings, this usually involves placing the inlets at highlevel, typically 1.7 m or more above floor level.

2.1.2 Control of summer overheating

Perhaps the single biggest issue that influences thetechnical viability of natural ventilation is summertimetemperatures. The cooling potential of natural ventilationis limited by the prevailing climate and by occupantexpectations of thermal comfort. As a rule of thumb, it isgenerally agreed that natural ventilation systems can meettotal heat loads averaged over the day of around30–40 W·m–2 (i.e. solar plus internal gains). If the effects ofclimate change become significant, then this rule ofthumb may need to be revised downwards, althoughpeople’s adaptation to a warmer climate may partlycounterbalance the reduced cooling effect associated withwarmer temperatures.

In most cases, achieving acceptable summer conditionsrequires three main features in the design and use of thebuilding:

— good solar control to prevent excessive solar gainsentering the occupied space

— modest levels of internal gains (people, smallpower loads and lighting loads)

— acceptance that during peak summer conditions,temperatures in the space will exceed 25 °C forsome periods of time; air temperatures may behigher still, but in a well-designed building, suchhigher air temperatures will be offset by coolermean radiant temperatures and enhanced airmovement.

These issues are discussed in the following paragraphs.

2.1.2.1 Solar control

Compliance with Part L2 of the Building Regulations(5)

requires that designers demonstrate that the building willnot overheat due to excessive solar gains. The aim of thisrequirement is to prevent the tendency to retrofit mechan-ical cooling.

The compliance procedures for checking for solaroverheating are developing with subsequent editions ofApproved Document L2. In the 2002 edition, the com-pliance check was a rather coarse filter that checked theaverage solar gains over a design July day. This set a limitof 25 W·m–2 for the average solar load in a six metre deepperimeter zone, and assumed that internal gains were amodest 15 W·m–2 total, and that other mitigating factorssuch as effective thermal mass and night ventilation werepresent. The proposals for the 2005 edition, as publishedin the ODPM’s consultation paper(11) are that detailedcalculations will be required as part of the whole-buildingcalculation approach required by the European Directiveon the energy performance of buildings(12). This newapproach means that the benefits of thermal mass andnight ventilation can be properly credited.

The forthcoming CIBSE TM37: Design for improved solarcontrol(13) will provide guidance on the solar controlperformance that will be needed to limit overheating to adefined number of hours as a function of the key designparameters. Solar control can be achieved throughmeasures such as:

— Size and orientation of the glazed areas: this will beinfluenced by the general organisation of thebuilding on its site. Shading of the windows bysurrounding buildings, and through self-shadingfrom other parts of the same building can alsocontribute to reduced solar gains.

— Tints, films and coatings in/on the glass: recentdevelopments in glass technology means thatspectral-selective coatings are able to reduce solargain without unduly reducing visible light trans-mittance.

— Blinds: internal, mid-pane or external.

— Overhangs, side fins and brise-soleil: the performanceof these forms of solar control are orientationdependent and so different forms of control will berequired on different façades. This will haveimplications for the aesthetics of the building.

The performance of these different systems (singly and incombination) can be quantified by the effective total solarenergy transmittance, or effective g-value. This is definedas the solar gain through the window and its associatedshading device during the period of potential overheatingdivided by the solar gain through an unshaded, unglazedaperture over the same period.

The design procedure suggested in TM37 is a two-stageprocess. The first is to determine the required effective g-value for the given building characteristics (window area,orientation, internal gains, thermal capacity and ventila-tion rates). The second stage is to select a solar controlstrategy that will achieve the required effective g-value.

In considering the glazing ratio and solar control features,an issue that must be recognised is that the climate has


become warmer over recent years (particularly in south-east England), and climate change predictions suggest thatthis trend will probably continue. Consequently, prudentdesign will take account of these possible future trends byeither:

(a) designing the building with some margin suchthat the building can cope with warmer conditionsand still remain comfortable; the 2005 edition ofCIBSE Guide A(9) contains data on future climatethat can be used to assess how ‘future-proof ’ is agiven design

(b) incorporating into the design means by which thebuilding can be easily and cost-effectively recon-figured (e.g. by progressing to mixed-modeoperation, see section 2.1.4) to cope with a warmerclimate.

In addition to the possible impact of global warming,other effects can result in higher local temperatures thatneed to be considered when contemplating a naturallyventilated design. The most important is the heat islandeffect in large conurbations, which can be particularlysignificant in raising night-time temperatures. In turn,this will impact on the capacity to pre-cool the buildingusing night ventilation. Guidance on the heat island effectis given in the CIBSE Guide A(9).

2.1.2.2 Internal gains

The internal gains in a space will also contribute topotential overheating. The three most important sourcesof gain are people, lights and small power loads(computers, printers, copying machines etc.). The level ofinternal gains is one of the inputs to the proceduredescribed in TM37(13) but, as a general rule, if the solarload is about 25 W·m–2 and the average coincident internalgains over the day exceed about 15–20 W·m–2 , then it willbe difficult to ensure adequate comfort at all times,especially in SE England. The key aspects of this rule ofthumb are the phrases ‘coincident’ and ‘over the day’. Thekey is to try and minimise the coincidence of the gains,and to be more concerned with daily average gains thanwith peak gains. This suggests control of gains can be asimportant as specifying high efficiency equipment. Thiscertainly applies to lighting but may also be relevant tobusiness machinery, if equipment is specified that canswitched to a low power ‘standby’ mode when not inimmediate use. Further energy and comfort benefits canbe obtained if the facilities management regime encour-ages all equipment to be switched off overnight. Asdiscussed in a later paragraph, if occupancy periods areshort, then higher levels of load can be accommodated.

In office buildings, occupant density is often quoted asone person per 10 m2, but the British Council forOffices(14) advises that, in reality, very few buildings arethat densely occupied and suggests that good practice iscurrently between 12 and 17 m2 net internal floor area perperson. It should also be recognised that occupant densitywill probably vary through the building, depending on thetype of activity taking place in each space. Occupantdensity may also vary with time as the use of the buildingchanges or working practices are modified. Occupantdensity may also vary over a much greater range (higherand lower) in other building types. For example, schoolsare often designed for natural ventilation, and yet have

areas with a very high density of occupation. As well asassessing peak occupancy levels, the variation in occupan-cy through the day should also be considered, so that arealistic assessment of total gains through the day is made.

Heat emissions from occupants vary according to the levelof activity. For sedentary activities such as office work, theaverage total heat emission from occupants is 130 W/person(average for typical mix of men and women)(6). Thedivision of this total heat emission into sensible and latentcomponents varies as a function of air temperature asshown in Figure 2.3. This variation in heat emissions withtemperature is important in the context of naturalventilation design since, as internal temperatures riseduring hot weather, the magnitude of one of the sources ofinternal gains that contributes to the rise in temperaturereduces significantly. This effect should be taken intoaccount when estimating the risk of overheating,otherwise the likelihood of overheating will beoverestimated.

Lighting is another important source of internal heat gain.The heat gain can be reduced by specifying efficient lightsources and controlling the lights so that they are dimmedor switched off when natural light levels are adequate.Periods of overheating risk are most likely to occur duringperiods of strong sunshine, when daylight levels inperimeter spaces should be sufficient for most visual tasks.It is therefore important to provide effective lightingcontrols such that unnecessary lamps are switched off.

The Society of Light and Lighting’s Code for lighting(15)

gives guidance on the efficiency of a range of light sources.As a benchmark for design, installed lighting loads shouldbe less than 3 W·m–2 per 100 lux (preferably nearer 2.5 andeven lower in large open spaces). BRE Digest Selectinglighting controls(16) provides advice on the selection andspecification of lighting controls.

One factor that needs to be considered is the interactionbetween any movable shading device (e.g. a blind) and thelighting. If blinds are closed to cut out direct solar gain,there is a danger that occupants will respond by switchingon lights, thereby losing some of the benefit of thereduction in solar gains. It should also be appreciated thatblinds are often used to control glare as much as solargain. In such situations, it may be better to provideseparate devices for the two functions, such as externalshading for solar control, with occupant controlledinternal blinds to control the glare. Ideally, the design ofthe solar control should not impact too significantly onthe daylighting in the space. This can be done in anumber of ways(17), such as:

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— louvres, lightshelves or blinds that shade the mainpart of the window but direct daylight into thedepth of the room

— selective coatings on the glass that restrict thetransmission of radiation in non-visible parts ofthe spectrum.

The third main component of the internal load is thatarising from machines such as computers, printers,vending machines etc. BCO guidance(14) suggests thatwhen diversified over areas of 1000 m2 or more, theseloads seldom exceed 15 W·m–2, although there may belocal workstations peaking to 20–25 W·m–2. These valuesmay be exceeded significantly for specialist applicationslike dealer areas. Although these figures provide usefulguidance, each space should be considered on its merits,both in terms of the anticipated activities for the space andin allowing flexibility for future change. To assist in suchassessments, section 6 of CIBSE Guide A(6) gives guidanceon heat gains from equipment.

Where there are particular items of equipment that have asignificant heat output (e.g. a large copier or networkprinter), it may be sensible to provide such a machinewith a local extract point. This will extract both heat andpollutants at source, thereby improving thermal comfortand indoor air quality.

As well as assessing the magnitude of the internal loads, itis also important to consider the duration of those loads. Itmay well be entirely practicable for a naturally ventilatedbuilding to cope with transient loads that are greater thanthe levels discussed above. In such situations, the designerhas to think about the impact of the loads on both comfortand indoor air quality. In such situations, transient effectsbecome much more significant. The thermal capacity ofthe structure can absorb significant swings in internalload, provided the structure has been pre-cooled beforethe loads build up.

2.1.2.3 Comfort expectations

A key criterion when assessing overheating is to define thethermal comfort conditions that are considered acceptable.Thermal comfort is a complex mix of physiology, psychol-ogy and culture. What is deemed acceptable will dependon activity and clothing level as well as temperatures, airspeeds and humidity.

It is clear that a naturally ventilated building will deliver amore variable temperature than an air conditionedbuilding, but that does not necessarily mean thatoccupants will be less comfortable. In summer, increasedair movement from large openings can provide anenhanced perception of thermal comfort, but care has tobe taken to avoid nuisance draughts, such as those thatmight blow papers off desks etc. Figure 2.4 shows how anair speed of about 0.25 m·s–1 is sufficient to give a coolingeffect equivalent to a 1 K reduction in dry resultanttemperature(6). Such air speeds are only acceptable in thecontext of summer cooling, but this serves to illustrate animportant mechanism by which natural ventilation canmaintain thermal comfort in summer.

The cooling benefit can be further enhanced by employ-ing a night ventilation strategy. This approach takesadvantage of the lower external night-time temperature topre-cool the building structure, and thereby lower themean radiant temperature. By lowering the mean radianttemperature, comfort can be maintained even though airtemperatures in the space might rise. By increasingthermal capacity, the amount of heat the structure canabsorb per degree rise in mean radiant temperatureincreases, thereby increasing the ability of the space tomaintain reasonable comfort conditions through the day.

The benefit of thermal mass is illustrated by Figure 2.5,which illustrates the influence of thermal mass and nightventilation on internal temperature(18). Progressivelymoving from a lightweight construction with no nightcooling to a thermally massive construction with highnight-time ventilation reduces the peak internal tempera-ture by nearly 5 K.

Thermal capacity is usually measured by the responsefactor(6), given by:

Σ (A Y) + Cvfr = —————— (2.1)Σ (A U) + Cv

where Σ (A Y) is the sum of the products of all the surfaceareas bounding the space and their correspondingadmittances (W·K–1), Σ (A U) is the sum of the products ofthe external surface areas and their corresponding thermaltransmittances (W·K–1) and Cv is the ventilation heat losscoefficient (W·K–1).

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Figure 2.4 Effect of air speed on dry resultant temperature(6)Figure 2.5 Effect of thermal mass and ventilation rate on peak indoortemperature(18)

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24

240 6 12 18Time / h

LightweightLightweight withnight vent

HeavyweightHeavyweight withnight vent


Admittances and transmittances for a range of construc-tions are given in CIBSE Guide A(6,9). Lightweight spacesare those with a response factor of around 2, medium-weight spaces have a response factor of about 4 andheavyweight spaces are those with a response factor of 6 orabove.

Good natural ventilation design will also provideoccupants with so-called adaptive opportunities to adjusttheir situation in response to changing conditions. Thismight involve:

— using blinds or other moveable shading to cut outdirect solar radiation

— providing the opportunity to increase air move-ment by opening windows or the use of desk fans.

Coupled with less formal dress codes, these opportunitiesallow occupants to adjust their environment to suit theirown preferences, allowing greater freedom of choice thanis usually practical in an air conditioned environment.However, this does require careful design of the windows,their position and opening mechanisms, such thatoccupants can readily adapt their environment. This isdiscussed in detail in section 3.

Another important factor is that recent research hasshown that in prolonged spells of warm weather, people’sexpectations for comfort change.

2.1.3 Acoustic performance

The presence of significant external noise sources is one ofthe main factors that inhibit the application of naturalventilation. There are two main solutions to this problem.

— Place the ventilation inlets on the sides of the buildingaway from the principal noise sources: if the noisesource is road traffic, this has the added benefit oflocating the ventilation inlets away from thesource of pollutants. The Canning Crescent Centrecase study described in the output of the NatVentproject(19) is a good example of this approach. Theventilation concept used is shown in Figure 2.6.However, it should be noted that there aresubsidiary issues that need to be considered.Figure 2.6 shows the fresh air intake located in thevicinity of the courtyard carpark. Although this

will not be a significant noise source, it may be asource of pollution, particularly if there are idlingvehicles near the intake such as cars queuing forthe exit or delivery vehicles at a loading bay.

Integrate acoustic baffles into the ventilation opening:good acoustic performance in conjunction withnatural ventilation is often a key designrequirement for schools. BB93(20) provides guid-ance on ways to reconcile the needs of acousticperformance and natural ventilation. Figure 2.7 isa detail taken from that publication, illustratinghow an acoustically protected ventilation openingmight be integrated with the design of the windowsill and the casing of the perimeter heating.

Another important acoustic issue is that of absorbinginternally generated noise in spaces with large areas ofhard surface, which are often associated with thermallymassive buildings. The use of absorbent partitions ordecorative hangings is one approach; another is to usecarefully profiled ceilings that result in acoustic focuspoints below the absorbent floor level, such as used in theInland Revenue building in Nottingham(21).

Fresh airintake

Roof light

Exhaustair

Exhaustair

Courtyardcar park

Busy highstreet

Figure 2.6 Flow schematic:Canning Crescent Centre

2 layers ofself-extinguishingfire retardentnylon mesh

Overhanging sill toprevent directpassage of sound

Insect mesh

Ventilation flap

Linear grill

Fixeddivider

Radiator

Acoustic infill gluedto aluminium casing

80 mm cavityinsulation

Ground level

Figure 2.7 Ventilation opening with acoustic protection


2.1.4 Natural ventilation and mixed-mode

The above paragraphs give an overview of the factors thatneed to be considered when assessing whether naturalventilation is appropriate to the building under consider-ation. It should be recognised that not all parts of abuilding have to be treated in exactly the same way.Different strategies may be applied to different parts of abuilding, or at different times. This is the so-called ‘mixedmode’ approach, which is discussed in greater detail inCIBSE AM13(22). The various approaches to mixed-modeare as follows:

(a) Contingency mixed-mode: where flexibility of spaceis required, then it is important to ‘design-in’ thepotential to upgrade the services so that additionalcooling can be installed to meet tenant require-ments or the changing climate. This provision willinclude space allowances for additional distribu-tion systems incorporated into floor and/or ceilingvoids. The cost of the additional flexibility willneed to be set against the savings in initial andoperating costs accruing from the avoidance ofunnecessary air conditioning.

(b) Zoned mixed-mode: recognises that different partsof any building will have different uses. Airconditioning is provided only to those parts of thebuilding where there is a real need. In areas oflower heat gain, heating and natural ventilationonly would be provided. Such an approach relieson the requirements of the individual spaces beingreasonably constant over the life of the building.Such an approach can also create tensions, if onegroup of occupants feels that another group hasbeen provided with what they believe is a betterworking environment.

(c) Changeover mixed-mode: recognises that the coolingrequirements of any space will vary from season toseason. An example of changeover mixed-modewould be to use mechanical ventilation in extremeweather conditions (hot and cold), but rely onnatural ventilation in milder weather. This reducesthe problem of cold draughts in winter, and allowsthe use of mechanical night ventilation for pre-cooling in hot summer periods.

(d) Concurrent mixed-mode: provides mechanical andnatural ventilation simultaneously. The mechan-ical system is designed to provide the fresh airrequirement, with additional ventilation byopening windows to provide summer cooling. Themechanical system can also provide night venti-lation without the security problems that may beassociated with opening windows. In very hotweather, energy wastage may occur if mechanicalcooling is provided as part of a concurrent mixedmode system, since excess natural ventilation mayimpose an unnecessary fresh air load.

Figure 2.8 provides a flow chart that takes the userthrough a broad-brush decision tree to identify the mostappropriate forms of ventilation.

2.1.5 Proceeding with the design

If on the basis of the above considerations it is concludedthat natural ventilation is viable, either as the sole strategy

or as a major component of a mixed mode approach, thenext stage is to develop the design concept. Three basicsteps need to be considered.

(a) To define the desired air flow pattern from theventilation inlets through the occupied space to theexhaust: this will be a function of the form andorganisation of the building, which in turn willdepend on its intended use and the details of thesite. For example, if a heavily congested roadbounds one side of the site, then it would beinadvisable to draw air into the building from thatside because of the problems of pollution fromvehicle exhausts.

(b) To identify the principal driving forces which willenable the desired flow pattern to be achieved: certainstrategies tend to be driven by wind pressures,others by temperature differences. In some cases,these natural forces will need to be supplementedwith fans. Good design will ensure that thedominating driving forces are in sympathy withthe intended flow distribution. This topic is thesubject of the rest of section 2. Section 3 then givesdetails on the ventilation components that areavailable and how they should be integrated into acomplete system.

(c) To size and locate the openings such that the required airflow rates can be delivered under all operating regimes:this is in itself a three-stage process:

(1) The flowrates have to be determined basedon the air quality and thermal comfortrequirements defined in the brief.

(2) The openings have to be sized and locatedto deliver those flows under design con-ditions.

(3) A control system must be specified tomaintain the required flowrates undervarying weather and occupancy conditions.Section 4 gives details of the calculationmethods that can be used at this stage ofdesign.

2.2 Selecting a natural ventilation concept

The first consideration is to plan the flow path throughthe building. This involves much more than drawing aseries of arrows on a sectional view of the building. Theform of the building has to be designed to facilitate thechosen strategy; the strategy then has to be engineered toensure the air can flow along the chosen path at therequired flowrates under the naturally available drivingpressures. It should be appreciated that the building willneed to operate in a number of different modes. In theheating season, driving forces will tend to be strong andthe building will need to operate in ‘minimum fresh air’mode. During mid-season, the building will move into acooling mode in which outside air can be used to providethe cooling but driving forces will reduce. During peaksummer conditions, outside air temperatures will exceedinternal temperatures; this will reverse the direction of thestack-induced pressures but will also change the ventila-tion effect from cooling to heating. Therefore the building


may need to revert to operating in minimum fresh airmode.

Figure 2.9 illustrates the logic behind the development ofthe strategy for the Anglia Polytechnic building.

Figure 2.9 illustrates some key points, as follows:

— Cellular spaces will need to be ventilated in adifferent way to open spaces.

— Internal planning arrangements must allow the airto flow through the space towards the exhaust.

— In many buildings, the natural buoyancy ofwarmer air will involve an upward air movementso that substantial areas of exhaust opening mustbe provided at high level.

— Pollution sources (such as exhausts and flues mustbe positioned so as to minimise the likelihood ofpollutants being re-ingested into the ventilationsystem)(23).

Fresh air must flow from outside, through the occupiedareas and then to the exhaust point. The pattern of airflow must be considered for all operational regimes,winter and summer, as well as special operating modessuch as cooling by night ventilation. This must includeconsideration of the impact of variations in weather con-ditions. For example, although one wind direction mightpredominate, the strategy will have to be sufficientlyrobust to work in all likely weather conditions. As well asthe general pattern of air movement through the building,the needs of the occupants and the way in which these

Figure 2.8 Selecting a ventilation strategy

No

No

Start

Aremax. heat

gains more than30–40 W.m–2?

Can courtyards or

atria reduce width to less than

15 m?

Does the building

have a narrow plan?

No

No

No

No

No

No

No

Can re-design

reduce gains to 30–40 W.m–2?

No

Yes No

No

Yes

Yes

Yes

Yes

Yes

YesYes

Yes

Yes

Yes

Yes

Is seasonal

mixed-mode acceptable?

Is this a peak

season?

Is zonal mixed-mode acceptable?

Is this a

perimeter zone?

Is tight temperature

control required?(≈ ±1 K)

Is humidification

required in winter?

Is close control

of RH required? (better than

±10%)

Are noise and

pollution levelsacceptable?

Can occupants

adapt conditions with weather

changes?

No

Mechanical ventilation and humidification

Mechanical ventilation

Natural ventilation

Comfort cooling

Full air conditioning

Is occupancytransient?

Can capacityeffects absorbswing in temp.

and IAQ?

Yes

No

Yes

Yes

No

See also AM13: Mixed mode

ventilation

Yes

Yes


interact with the ventilation system must be considered.These issues are discussed in detail in section 3, but thepractical issues which impact on the viability of anystrategy must be kept in mind in the early stages of design.Trying to design around problems is rarely as successful asavoiding them in the first place.

The ventilation system is made up of a number of keycomponents:

— Ventilation inlet(s): where the fresh air is drawn intothe ventilation system

— Supply point(s): where that fresh air is deliveredfrom the ventilation system into the occupiedspace. Usually, the inlet to the ventilation systemis the same component as the supply point, (e.g. atrickle ventilator or an open window), but some-times the ventilation inlet and room outlet may beseparated by a distribution system such as a floorplenum. In such cases, access for cleaning will bean important issue.

— Flow path: through the occupied space to theexhaust point.

— Exhaust path: such as a stack.

— Ventilation outlet: where the air is exhausted outsidethe building.

Circulation areas such as stairwells or corridors can beused as distribution routes to deliver air to rooms that donot have direct access to the perimeter of the building.However, care must be taken to avoid these routes actingas ‘short circuits’ by providing a path of lower resistancefrom the supply to the exhaust, thereby preventing outsideair entering the occupied zones.

Consideration must be given to the route the air will takethrough the building. For example, it may be beneficial todraw the air from one side of the building in order to:

— avoid noise and traffic fumes from a busy road

— draw cooler air from a shaded side of the buildingin order to maximise the cooling potential.

As the air flows through the space, it will pick up bothheat and pollutants and therefore becomes less ‘fresh’.Therefore, if the air is taken from one occupied zone toanother, the downstream zone will have worse indoor airquality and probably poorer thermal comfort. It is there-fore preferable either to extract from each zone direct tooutside or via spaces with transient occupation such ascirculation spaces or toilet areas.

In addition to the overall flow pattern, consideration mustalso be given to the detail. The ventilation air has to beintroduced into the space in such a way as to avoidstagnant areas which may lead to local discomfort due topoor indoor air quality or poor thermal comfort. Thismeans that the openings must be well distributed.

As discussed in section 2.1.1, the range of ventilation ratesthat will be required may vary over an order of magnitude.In order to provide effective control over such a widerange, different ventilation openings may be required forthe different ventilation modes.

2.3 Driving forces for natural ventilation

The magnitude and pattern of natural air movementthrough a building depends on the strength and directionof the natural driving forces and the resistance of the flowpath. Good design involves the appropriate application offundamental principles to the general strategies describedin the following sections.

It is important to appreciate that the discussions thatfollow describe steady-state ventilation scenarios. The twoimportant driving forces of wind and buoyancy (caused byheat gains in the space) are always varying. As conditionschange, the ventilation rate will also change, and the flowdirection might even reverse under extreme conditions.These steady-state ‘snapshots’ are therefore an idealisationof reality. The flow pattern needs to be thought throughfor each of the main operating modes (see discussion atthe beginning of section 2.2). The most critical operatingmode in terms of sizing vents is the condition where theventilation is providing summer cooling with a smallinside–outside temperature difference. This mode willrequire the largest vent sizes, with the control systemthrottling back on this condition at the other operatingconditions. The size of the vents will be dictated by therequired flowrate and the available driving pressurescaused by wind and buoyancy.

In order to understand the relative importance of windand buoyancy forces, Figure 2.10 shows the pressuredifference driving the flow as a function of the drivingforce (temperature difference or wind speed). In Figure2.10(a), two height differences are shown, 3.5 and 10.5 m,typical of the distance between the top floor of a buildingand a stack outlet and the whole of a 3-storey building andthe same stack outlet. The lines on the graph show thestack pressures corresponding to the inside–outsidetemperature difference given along the x-axis. In a similarway, in Figure 2.10(b) two lines are shown representingwind driven pressure difference for a typical range ofpressure coefficient difference between inlet and outlet(0.1 to 0.5). The shaded boxes show the typical conditionswhich exist at the summer design condition, namely an

Open plan/open doorwaysrequired for crossventilation

Design ofwindowsis critical

Kitchen exhaust well above openings

Roof vents positionedwith regard to varyingwind direction

Avoid downstandsin high level air flow

6 m max.depth fornormalsingle sidedventilation

Outside noisedicatates use ofinlet vents

Minimum permanent ventilationneeded in winter

Figure 2.9 Ventilation schematic: Anglia Polytechnic


inside–outside temperature difference of about 3 K and awind speed of between 1 and 2.2 m·s–1 (depending onterrain conditions).

Figures 2.10(a) and 2.10(b) illustrate that at the summerdesign condition, the stack and wind driving forces arelikely to be comparable, unless the design can deliver verylarge differences in wind pressure coefficient betweeninlet and outlet. It is difficult to achieve a high differencein pressure coefficient for all wind angles, and becausewind direction is inherently variable, a design that relieson a large difference in pressure coefficient is unlikely tobe very robust. For this reason, design practice is often tosize the openings based on stack effects alone, and toengineer the design so that any wind effect will enhancethe driving forces (e.g. through careful design of the stackoutlet).

The ventilation through the building is driven by thesedifferences in pressure. The ventilation rate is determinedby the pressure difference acting across a ventilation pathand the resistance of that path. Section 4 gives the detailedmathematical treatment of these effects, but the presentsection concentrates on explaining the physical processesthemselves. The section then summarises how theseprocesses can be harnessed to deliver a variety of naturalventilation concepts.

2.3.1 Density differences (‘stack effect’)

Warm air is less dense than colder air. Therefore, if twocolumns of air at different densities are connected, thecooler, denser air will fall and displace the warmer, lighterair upwards. Consider a very simplified building in theform of a box (see Figure 2.11). At most times of the year,the air inside the building is warmer than the external airas a result of the building heating system and/or the effectsof internal gains. Hydrostatics require that the pressure ata point decreases with height, the rate of decrease beingproportional to the density of the fluid. This is shown inthe Figure 2.11 by the two pressure gradients. Because theoutside air is colder, it is denser, and therefore the pressuredecreases with height more rapidly outside the building(the blue line) than inside (the red line).

Because the internal and external pressures change withheight, there will exist pressure differences across the

separating walls. If holes are now placed within theseparating wall, an air exchange will be set up inproportion to the pressure difference acting across it. Inorder to visualise the effects, the pressure gradients actinginside and outside can be placed against a scale such thatthe point where the pressures are equal occurs at the midpoint of the wall, as shown in Figure 2.12. When this isdone, the horizontal distance between the lines repre-senting the two gradients is a measure of the pressuredifference between the inside and outside of the building.In order to simplify the subsequent diagrams, these onlyshow that part of the building enclosed by the green line.

Three physical relationships control the flows that are setup. These are:

— Conservation of mass: the airflow into the buildingmust be balanced by the exhaust flows.

— Hydrostatic pressures: the pressure gradient isproportional to the air density. If the air is at auniform temperature, the density is constant andthe gradient is a straight line (this is the caseshown in the diagrams). If temperature varies withheight, then the slope of the pressure gradient willalso vary with height and become a curved line.

Pres

sure

diff

eren

ce /

Pa

5·0

4·0

3·0

2·0

1·0

0·050 1 2 3 4

Inside–outside temperature difference / K

3·5Height difference

10·5

(a) Stack only

Pres

sure

diff

eren

ce /

Pa

5·0

4·0

3·0

2·0

1·0

0·050 1 2 3 4

Site wind speed / m·s–1

0·1Pressure coefficient difference

0·5

(b) Wind only

Figure 2.10 Relative driving pressures

Figure 2.11 Schematic of pressure gradients

ColdWarm

Height Height

Pressure

Pressuregradients

Pressure


— The flow equation: flow through cracks and open-ings depends on the area, the discharge coefficientand the pressure difference; the bigger the holeand/or the bigger the differential pressure, thegreater the flow.

These relationships are developed in mathematical termsin section 4.

Consider the two cases shown diagrammatically in Figure2.13. In Figure 2.13(a), the size of the two holes are thesame; in (b), the size of the lower hole is twice that of theupper. In order to satisfy the law of the conservation ofmass, the airflow that develops will be such as to equalisethe flow through the two openings. Because in (a) the twoholes are the same size, the pressure drop across eachopening must be the same in magnitude, but acting inopposite directions. Hence the relative position of the twogradients adjust themselves until the pressure drops acrossthe two openings are the same. The height at which thetwo gradients intersect is the neutral pressure level (NPL),which, in (a) is midway between the two openings.

In (b), because the lower hole is bigger, its resistance toflow is lower than in the upper opening. Therefore, inorder to fulfil the law of mass conservation, the pressuredrop across the lower opening must decrease, and thatacross the upper will increase until the flows equalise oncemore. The neutral pressure level is still defined by thepoint at which the two pressure gradients intersect, butnow it can be seen that this level occurs at a point muchlower down the wall.

In all cases, below the NPL air will flow from the cold sideto the warm and, above the NPL, from the warm to thecold. For most of the year, the temperature outside thebuilding will be cooler than inside, and so the air will berising up through the building. However, in very warmweather, it may be that the outside temperature exceedsthe internal, in which case the flow will reverse. If this isundesirable, the designer can make unoccupied parts ofthe building deliberately warmer than outside so as tomaintain a constant direction of flow. This can beachieved through solar chimneys, or by trapping solargain at the top of an atrium (see 2.4.3.2).

It is also important to note how changing the size of anopening influences the position of the NPL. This isparticularly relevant in the context of real natural ventila-tion design, where the building has several storeys. Figure2.14 shows a three storey building, with outside air beingdrawn in at each level and being exhausted via a high leveloutlet above the top occupied floor. In this example, it isassumed that the same ventilation rate is required at eachoccupied level.

In this context, the sum of all the inflows has to balancethe single high-level outflow. To achieve the required flowdistribution, the NPL has to be forced above the height ofthe highest occupied floor, otherwise warm air from lowerfloors will exhaust through the top occupied floor, result-ing in poor thermal comfort on the top floor.

As will be seen from Figure 2.14, the driving pressure atthe ground floor level is much greater than that at thesecond, which is correspondingly greater than that at thethird. If equal flowrates are to be obtained, this must beachieved by varying the opening size at each level. If therequired opening size on the third floor level was thoughtto be too large, the available pressure drop could be madegreater by forcing the NPL still higher. The problem is thatthis will reduce the available pressure drop at the exhaustvent, requiring a bigger vent size. The alternative wouldbe to raise the height of the stack, but this would have cost(and possibly planning) implications.

The fact that the ventilation openings are smaller at lowerlevels has some security benefits, particularly where anight ventilation strategy is employed.

Another important point is that, since the magnitude andbalance of the air flows is very dependent on the size andlocation of the openings, it is essential that there are nounintentional flow paths through the building envelopesince these could undermine the design intent. Thishighlights the importance of achieving a good standard ofairtightness in the envelope(24). Problems have beenexperienced in a building with a poorly sealed atrium roofand a poorly designed entrance lobby. In this situation,there are much larger than expected openings at both highand low levels, resulting in very strong draughts at theentrance and through the atrium.

Figure 2.12 Representation of airflows

Cold

Warm Height

Pressure

Pressuredifference

Warm Warm

ColdCold

(a) (b)

NPL

NPL

Figure 2.13 Impact of size of openings on neutral pressure level; (a)openings of equal size, (b) openings unequal


2.3.1.1 Local stack effects

Stack effects do not occur just over the whole height of thebuilding. Stack pressures will be exerted over anyvertically spaced openings that are inter-connected. Forexample, in a large window opening, air will tend to flowin at the bottom and out at the top. This creates an airexchange mechanism in the room even if it is isolatedfrom the rest of the building by a well-sealed door. In abuilding designed to promote stack induced flows, theselocal stack effects will be superimposed on the overallpattern of air movement.

2.3.2 Wind

Wind driven ventilation is caused by similar physicalrelationships as stack driven ventilation, except that thepressures are the result of varying surface pressures actingacross the building envelope rather than differences inhydrostatic pressure. As the wind approaches the façade ofthe building, the air is slowed and the pressure rises. As itflows around the building, the air accelerates over the roofand sides, resulting in a lowering of the local surfacepressure. At the downstream face, the flow separates fromthe roof and sides resulting in low pressure recirculationzone. This is shown diagrammatically in Figure 2.15.

The distribution of pressure over the surface of thebuilding depends on the following:

— Wind speed and its direction relative to the building:the local wind speed is affected by the type ofterrain surrounding the building (e.g. opencountry or city centre). The more congested theterrain, the slower will be the local wind speedrelative to the quoted meteorological wind speed.

— Shape of the building: this provides the opportunityfor the architectural form and detailing to enhancethe potential for wind driven ventilation.

Air will flow through the building from areas of highsurface pressure to areas of low pressure. In very generalterms, building surfaces facing into the wind willexperience positive pressures; leeward surfaces and thoseat right angles to the wind direction will experiencesuction. As shown in Figure 2.15, wind velocity increaseswith height. The velocity pressures of the wind increase asthe square of wind speed and so the wind pressures onhigh-rise buildings can be very large during periods ofhigh wind speed. The surface pressure acting on thebuilding is related to the velocity pressure of the wind bythe wind pressure coefficient, which will vary from façadeto façade, and will also vary across a façade.

Because the wind develops a pressure difference across thewidth of the building, the normal approach to wind drivenventilation is to design flow paths that are largelyhorizontal, with air entering and leaving the building atthe same vertical level. However, by careful design of thetop of a stack, wind forces can be used to induce verticalflows through the building (see section 2.4.2).

NPL

Figure 2.14 Application of stackeffect to a realistic building design

Wind velocity Wind pressure field

Figure 2.15 Wind pressures acting on the building


Careful orientation of the building relative to thetopography of the site and the prevailing wind directioncan maximise the potential of wind driven ventilation,although the variability of wind direction must always beconsidered.

Site conditions can be enhanced by landscaping, such asthe planting of trees to act as shelter belts. If there aremultiple buildings to be developed on the site, then therelative positions of the buildings can enhance the flowpattern of the wind. Consideration must be given to theeffects of high wind speeds in winter. In order tounderstand these complex effects, it may be necessary toundertake wind tunnel tests (see section 4.2.7).

When the wind acts on a large opening, especially if theangle of attack is non-orthogonal, then different windpressures will act on different areas of the opening. This isone mechanism whereby ventilation can be driven inspaces that only have a single opening. The othermechanisms are wind turbulence (section 2.3.2.1) andbuoyancy (section 2.3.1.1).

2.3.2.1 Wind turbulence

Wind is never steady in either magnitude or direction.Consequently, the wind exerts a continuously varyingpressure field over the building. In most situations, thetime-averaged pressure is sufficient to determine theaverage ventilation rate, but there are situations where theunsteadiness or turbulence of the wind can be an impor-tant factor in determining the ventilation rate. The mainsituation is where the space is being ventilated through asingle opening.

In such situations, a steady pressure difference cannotdevelop across the space because there is not a separateinlet and outlet. Although a steady pressure may begenerated at the face of the opening, all that this does is toraise the pressure in the enclosed space to that steadypressure. As has been intimated, the pressure will never besteady, and so the opening will experience a rapidlyfluctuating pressure, alternately higher and lower than themean pressure in the enclosed space. This means that thepressure difference across the opening will be contin-uously reversing direction, sometimes forcing air into thespace sometimes extracting air from it, hence creating an

air exchange mechanism. Although the mechanism can beexplained in physical terms, and anecdotal evidencesuggests it can provide adequate ventilation for rooms ofshallow depth, information on the magnitude of sucheffects is not available at present.

2.3.3 Combined wind and stack effects

The above discussions of wind and stack effects havepresented an idealised explanation of the mechanisms thatdrive natural ventilation. In reality, these effects never actin isolation. Analysis of the data given in CIBSE GuideA(6) Tables 2.18 and 2.19 shows that the wind speedcoincident with the temperature that is not exceeded for99.6% of the time is in excess of 3.5 m·s–1 for all UKlocations. Figure 2.16, based on data in CIBSE Guide J(25),illustrates the frequency of wind speeds for two temper-ature ranges: 20–34 °C and 26–34 °C. It demonstrates thatmost of the time there will be significant wind speeds tohelp drive natural ventilation. It should be noted thatthese are meteorological wind speeds, and that the effectsof the terrain type will be to reduce the apparent windspeed at the building location, especially in urban areasand city centres.

The important point to note is that the natural ventilationrate that will occur will be the result of the combinedeffects of wind and stack. Therefore, although the designmay be concentrating on utilising one mechanism, theeffects of the other must not be ignored. For example, inFigure 2.14 if the wind were blowing from the left, theopenings would all be on the leeward side, and this wouldresult in a negative wind pressure on the outside of eachopening. The pressures acting on each opening can becombined (taking account of their direction).

This is shown diagrammatically in Figure 2.17, in whichthe outline of the building has been removed for simplic-ity. Figure 2.17(a) shows the two stack pressure gradients,with the length of the horizontal arrows representing themagnitude of the resulting pressure difference across eachopening. Figure 2.17(b) shows the variation in windsurface pressure with height. Note that this pressureprofile is relative to an arbitrary datum pressure. In orderto calculate the pressure drop across the opening, thisprofile must be related to the internal pressure in thebuilding. For the required flow pattern to occur inpractice (inflow at the lower three openings and exhaustvia the top), the internal pressure would adjust itself sothat the variation in pressure difference with height is asshown in Figure 2.17(c), where the pressure differencechanges sign somewhere between the second floor ventand the outlet. This adjustment of the internal pressure isachieved by judicious sizing of the ventilation openings.The internal pressure that achieves the required conditionis shown on the wind pressure profile. It can be seen thatalthough the external surface pressure is negative at allopenings, the internal pressure is even lower for the threeinlets, resulting in the wind and stack effects reinforcingeach other at these levels, as indicated by the length anddirection of the green arrow between the internal pressureline and the wind pressure profile. The opposite is true atthe outlet. Figure 2.16 Frequency of summertime windspeeds in the UK

Freq

uenc

y of

occ

urre

nce

/ %

100 42 6 8Windspeed / m·s–1

45

40

35

30

25

20

15

10

5

0

20–34 26–34


Clearly, if the distribution of wind pressure over thesurface of the building were different to that assumed inthe figure, a different flow pattern might result. It shouldbe stressed that the overall flowrates cannot be determinedby calculating the flow due to each mechanism separately,and then adding the results together. This is because theflow through a typical ventilation opening is non-linear,and so it is the pressures that must be added and then thecombined pressures used in the flow equation. Themathematical treatment of this subject is dealt with indetail in section 4.

2.4 Natural ventilation strategies

In the following sections, the basic forms of ventilationstrategy are reviewed and related to the form and layout ofthe building for which they are best suited. The rules ofthumb for estimating the effectiveness of natural venti-lation given in the following paragraphs are based oninformation provided in BRE Digest 399(26). The relevantdesign calculations contained in section 4 are also cross-referenced.

2.4.1 Single sided ventilation

Single sided ventilation relies on opening(s) on one sideonly of the ventilated enclosure. It is closely approximatedin many cellular buildings with opening windows on oneside and closed internal doors on the other side.

2.4.1.1 Single opening

With a single ventilation opening in the room, see Figure2.18, then in summer the main driving force for naturalventilation is wind turbulence. Relative to the otherstrategies, lower ventilation rates are generated and theventilating air penetrates a smaller distance into the space.As a rule of thumb, the limiting depth for effectiveventilation is about twice the floor-to-ceiling height. It isalso possible to get buoyancy driven exchanges through asingle opening provided the opening is reasonably large inthe vertical dimension. See cases 2 and 3 in section 4.

2.4.1.2 Double opening

Where multiple ventilation openings are provided atdifferent heights within the façade, see Figure 2.19, thenthe ventilation rate can be enhanced due to the stackeffect. See case 1 in section 4.

The stack induced flows increase with the verticalseparation of the openings and with the inside to outsidetemperature difference. As well as enhancing theventilation rate, the double opening increases the depth ofpenetration of the fresh air into the space. As a rule ofthumb, the limiting depth for effective ventilation is about2.5 times the floor-to-ceiling height

To maximise the height over which the stack pressures act,it may be necessary to separate the ventilation openings

Stack pressures Wind pressures

Variations in surface wind pressure

Internal pressure

Combined pressuredifference (inside –outside)

Warm

NPL

+ =

Cold

Combined effect

(a) (b) (c)Figure 2.17 Combining wind andstack pressures

Figure 2.18 Single sided ventilation, single opening

W

W ≤ 2 H

Hh approx

1·5 m

W

H

W ≤ 2·5 H

Figure 2.19 Single sided ventilation, double opening


from the window itself. Care needs to be taken over theposition of any low level inlet, because it may create low-level draughts in cold weather. One solution is to place thelow level inlet behind the radiator. However, in mid-season it may be that the internal and solar gains aresufficient to maintain satisfactory internal temperaturesand so the radiator may have switched off, in which casethere may still be a draught risk. Another potentialproblem is that if wind effects are significant, an openingbehind a radiator may act as an outlet as well as an inlet. Ifit is acting as an outlet, then some of the heat from theradiator will be lost to outside without warming the zone.

2.4.2 Cross ventilation

Cross ventilation occurs where there are ventilationopenings on both sides of the space, see Figure 2.20. Airflows in one side of the building and out the other sidethrough, for example, a window or door. Cross ventilationis usually wind driven, but can be driven by densitydifferences in an attached vertical chimney. As the airmoves across the zone, there will be an increase intemperature and a reduction in air quality as the air picksup heat and pollutants from the occupied space.Consequently there is a limit on the depth of space thatcan be effectively cross-ventilated. The rule of thumb forthe maximum distance between the two facades is fivetimes the floor-to-ceiling height. This implies a narrowplan depth for the building. This is usually achieved witha linear built form, although a similar effect can beachieved by ‘wrapping’ the building around an opencourtyard. The narrow plan depth associated with thisapproach has the added benefit of enhancing the potentialfor natural lighting.

The main design challenge with such an approach is toorganise the form of the building such that there is a

significant difference in wind pressure coefficient betweenthe inlet and outlet openings. This will be more difficultto achieve for the courtyard approach, because thecourtyard and the leeward side of the building will be atsimilar pressures.

The second issue that needs careful consideration is theresistance to airflow. Insufficient flow may be generated,particularly in summer conditions, if openings on one sideof the building are closed, or if internal partitions(particularly full height ones) restrict the flow of air acrossthe space. In such situations, the ventilation mechanismwill revert to single sided.

The normal approach to cross ventilation is via openingwindows (see case 4 in section 4), but other approacheshave been used with success. One example of thisapproach is the ‘wind-scoop’, see Figure 2.21, whichcaptures the wind at high level where the dynamicpressure of the wind is higher, thereby creating additionalpressure to drive the air through the building to theexhaust on the leeward side. When designing a windscoop, the effect of stack pressures must be considered,since these may act in opposition to the intended flow.

W

H

W ≤ 5H

Figure 2.20 Cross ventilation

Figure 2.21 Wind scoop

Damper anddiffuser

Wind direction

Separated4 m duct

Figure 2.22 Roof-mounted ventilator

Figure 2.23 Underfloor ventilator


The success of the strategy is enhanced by a dominantprevailing wind direction, (e.g. a coastal site). Where winddirection does vary frequently, then it would be necessaryto have multiple inlets, with automatic control closing theleeward and opening the windward vents. Because of theincreasing wind speed with height, the wind pressure willbe greatest at the top of the structure, thereby generating apositive pressure gradient through the whole building.

A modern variant of the wind scoop is the roof-mountedventilator(27), see Figure 2.22. This uses the pressuredifference across a segmented ventilation device to driveair down through the segment facing the wind and intothe space. The suction created by the negative pressure onthe leeward segment then draws the air back out of thespace. Flow rate can be controlled using a damper, with airdistribution being achieved via a diffuser module*.

In order to improve air distribution into deeper spaces, itis possible to use ducted or underfloor ventilationpathways, see Figure 2.23. This approach provides freshair directly to a second bank of rooms. It also provides amethod of ventilating a building from only one side if apollution source (e.g. a busy road) prohibits openingwindows on the other facade. Because of the low drivingpressures with natural ventilation, it is important todesign the supply duct for very low pressure drops.

2.4.3 Stack ventilation

Stack ventilation is driven by density differences. Theapproach draws air across the ventilated space and thenexhausts the air through a vertical flow path. This meansthat occupied zones are cross-ventilated, in that air entersone side of the space and exits via the opposite side. Thesame rule of thumb for effective ventilation thereforeapplies as for wind driven cross ventilation, i.e. the limit isabout five times the floor-to-ceiling height. The air mayflow across the whole width of the building and exhaustvia a chimney, or it may flow from the edges to the middleto be exhausted via a central chimney or atrium. Adetailed review of stack driven ventilation is given byRiain and Kolokotroni(28), and the required designcalculations are described in cases 5 to 8 in section 4.

In order to achieve the required flow distribution withoutexcessively large outlet ventilator sizes, it is usually thecase that the stack outlet needs to be at a height at leasthalf of one storey above the ceiling level of the top floor.The calculation methods given in examples 5 to 8 insection 4 explore this issue. Designing the stack outlet sothat it is in a wind induced negative pressure region willenhance the effectiveness of the stack. This requirescareful design of the position and form of the stack outlet.

2.4.3.1 Chimney ventilation

Chimneys provide a means of generating stack drivenventilation. The essential requirement is for the air in thechimney to be warmer than the ambient air. If thechimney has a large surface area exposed to the prevailingweather, this has to be well insulated in order to maintainthe required temperature differential.

It is possible to enhance the temperature (and hencedensity) differences by using solar heat gains to increasethe air temperature in the chimney. In this mode, glazedelements are included in the chimney structure. Theglazing allows solar radiation to be captured within thechimney by absorbing surfaces that then release heat tothe air, thereby promoting buoyancy. The ‘pull’ of thestack is enhanced by having as high a temperature aspossible over as much of the length of the stack aspossible. Therefore having low level glazed elements inthe stack will facilitate a temperature lift along the fulllength of the chimney, always provided the low levelglazing is not shaded by trees or other buildings.

Care has to be taken to ensure that there is a net heat gaininto the chimney during cooler weather (i.e. the solar gainmust be greater than the conduction loss). If this balanceis not achieved, the buoyancy effects will be reduced andthe chimney will be less effective. In cold weather, theconduction heat loss will result in low surface temperaturefor the glass. This may be sufficient to generate downdraughts that may inhibit the general upward flowthrough the chimney.

An advantage of the solar chimney is that it is likely to beplaced on the sunny side of the building in order tocapture the solar radiation. Consequently, the ventilationair will be drawn from the opposite (shaded) side, result-ing in cooler air being drawn into the building.

Another important part of the design of any type ofchimney ventilation is the detail of the outlet, whichshould be located in a negative wind pressure zone. Thisnegative pressure zone can be created by careful design ofthe roof profile and/or the chimney outlet. If the outlet isnot properly designed, positive wind pressures may act onthe chimney, making it act as a wind scoop (see Figure2.21) thereby disrupting the flow strategy.

As a means of providing adequate ventilation on very hotand still days, consideration might be given to installingextract fans in the chimney to pull air through thebuilding. This should be arranged in such a way that thefans do not provide a significant resistance to flow whenthe chimney is operating in natural draught mode.

* The extent to which good air distribution can be achieved will belimited by the available driving pressure, which will rarely be as great asthe 10–30 Pa that is dissipated across the diffusers used in mechanicalventilation systems.

Figure 2.24 Solar chimneys at BRE’s Environmental Building


All of these features are built into the ventilation strategyof the BRE Environmental Building, see Figure 2.24. Amore detailed description of the design strategy is givenelsewhere(19).

Chimneys provide no functional purpose other thanventilation. Consequently they are sized just to satisfy thepressure drop requirements. They can be in the form of asingle linear chimney or several smaller chimneys distrib-uted around the building to suit the required ventilationflowpath. For example, if the building faces on to a busyroad, it would be possible to place the inlets on the facadeaway from the noise and pollution source with thechimneys on the road side.

2.4.3.2 Atrium ventilation

An atrium is a variant on the chimney ventilationprinciple. The essential difference is that the atrium servesmany additional functions. For example, it provides spacefor circulation and social interaction. Because it providesattractive, usable space, the location of the atrium is anintegral part of the organisational planning of thebuilding. The design of atria is discussed in detail bySaxon(29). The multiple-use nature of the atrium restrictsthe flexibility to locate the atrium to maximum advantagefor ventilation purposes, since many other criteria nowhave to be satisfied.

A significant advantage of atrium ventilation is that, asshown in Figure 2.25, the air can be drawn from bothsides of the building towards a central extract point,thereby effectively doubling the plan width that can beventilated effectively by natural means. (The same effectcould be achieved by a central spine of chimneys.) Itshould be noted that the flow pattern as shown could notbe maintained by stack effect alone, since the air could notpass through the top occupied level and exit through theatrium roof at the same vertical height level unless windeffects were also present.

The atrium also provides the opportunity of gettingdaylight into the centre of a deep plan building. If theatrium can provide good daylighting to its adjoiningoccupied space*, then much of the occupied space can bedaylit for much of the time. The atrium can also act as abuffer space to reduce winter conduction losses from thesurrounding accommodation.

Because atria are usually designed to capture natural light,they are in effect large solar assisted chimneys. The

particular strategy for capturing solar gain within theatrium will depend upon the precise function of theatrium itself. To promote natural ventilation, the air in theatrium should be as warm as possible for as great aproportion of the atrium height as possible. If the atriumis open to the surrounding space, or if it provides high-level walkways between floors, then excess temperatures atoccupied levels may be unacceptable. In such situations,the design should seek to promote a stratified layer ofwarm air above the occupied zone by capturing the solargain at high level on absorbing surfaces, which can thenconvect heat to the adjacent air†, see Figure 2.26. Theseabsorbing surfaces could be:

— elements of the structure

— solar baffles or blinds which act as shading devicesto prevent direct solar gain passing through theatrium to the occupied spaces; the baffles can bearranged to allow a view of the sky allowing gooddiffuse lighting without excessive glare.

It should be noted that the effectiveness of a solar chimneyis determined more by the length over which the elevatedtemperature is maintained than by the magnitude of thetemperature elevation. This means that, unless theelevated temperature can be maintained over a reasonablevertical height, the driving pressure for ventilation will below. Therefore, if the design is seeking to achieve a high-level stratified layer, then the stack outlet will need to behigher than would otherwise be the case. This effect canbe explored using the equations in section 4 that deal withstratified stacks.

As with the chimney strategy, the roof vents must becarefully positioned within the form of the roof so thatpositive wind pressures do not act on the outlets. It isnormally possible to organise the outlets such that they

Figure 2.25 Atrium stack ventilation (Barclaycard Headquarters)

Glazed roof heats airto promote stackeffect in atrium15 m deep offices allow

natural ventilation andmaximum daylighting

* Note that penetration of daylight will be reduced at lower floors

Absorbingstructure

Solarbaffles

Figure 2.26 Absorbing solar gain at high level

† Note that enhanced stack pressures only act across that part of the stackthat is at the elevated temperature; this is known as the ‘stratified stack’effect, see section 4


are always in a negative pressure zone. This is achieved byeither:

— designing the roof profile so that the opening is ina negative pressure zone for all wind angles, or

— using multiple vents which are automaticallycontrolled to close on the windward side and openon the leeward side.

Natural ventilation can be supplemented on hot still daysby the use of extract fans in the atrium roof. Subject toFire Officer approval, these might also form part of thesmoke control system, although the vent would then haveto comply with all the necessary performance standardsfor smoke ventilation.

2.4.4 Double-skin façade ventilation

The double-skin façade is a special form of solar chimneywhere the whole façade acts as an air duct. The main aimis to provide a very transparent envelope, but to trap thesolar gain in a cavity between an inner and outer skins,and then ventilate the heat away to outside before it canhave a significant impact on environmental conditions inthe adjoining occupied space.

In most cases, it is the cavity between the two façade layersthat is driven by natural ventilation as a means of remov-ing the solar gains absorbed on the blinds. However, thereare some designs where ventilation air can be taken fromor exhausted to the cavity(30). Figure 2.27, illustrates the‘shaft-box’ façade. A particular benefit of this approach isthat it provides a practical means of naturally ventilating

tall buildings by subdividing the building into a numberof separate vertical stacks

Such designs are a specialist subject and so the reader isreferred to a specialist publication(30) for further infor-mation. As well as the thermal and lighting performanceof the façade, particular issues for consideration includecondensation and fire and smoke spread.

2.4.5 Mechanically assisted strategies

The previous sections have described strategies that areessentially natural, but which can use mechanical meansto supplement the ventilation during hot extreme weather.Other approaches rely on mechanical ventilation for thesupply or the exhaust side of the system during normaloperation.

For example, one approach is to use mechanical supplyventilation with natural exhaust. Such an approach maybe required in very deep plan spaces where it is impossibleto arrange all the occupied space to be within the requireddistance of the natural ventilation inlet. Mechanicalsupply, usually via an underfloor displacement ventilationsystem, is used to ensure good distribution of the fresh air.The air then rises within the space and is exhausted athigh level into a chimney or an atrium. This approach alsoallows the building to be sealed against external noise orpollution sources.

With this approach, all the exhaust air is extracted at acommon point. Because of the design strategy, that air willbe warm, thereby providing the potential for heat recoverywhen required.

Mechanical extract/natural supply systems are mostappropriate if there are particular areas where there aresignificant sources of internal pollution (e.g. ‘wet’ rooms,print facilities, smoking rooms etc.). The pollutants areremoved at source and the precise route for the ventilationair is more controllable.

2.4.6 Night ventilation

Night ventilation is not an additional mode of ventilation;rather it is a different operational strategy. Night ventila-tion takes advantage of the natural diurnal variations intemperature to promote ventilation cooling. Night venti-lation offers many advantages:

— Because of the lower night-time temperature, theinside–outside temperature differences will begreater, enhancing both the stack driven flowratesand the cooling capacity of the outside air.

— By cooling the fabric of the building by night ven-tilation, there is a reduction in the mean radianttemperature of the space, which enhances theoccupants’ perception of thermal comfort duringthe following day.

— By ventilating during unoccupied periods, thepotential problems of draught and noise in theoccupied space are avoided. However, if mechan-ical night ventilation is used, then the noiseimpact on neighbouring buildings must be consid-ered, especially as background noise levels will belower.

Figure 2.27 Shaft-box double-skin façade (reproduced from Double-skinfaçades(30) by E Oesterle, R D Lieb, M Lutz and W Hausler)

Exhaust airopening

Air-intakeopening

Openingto shaft

1st floor

Room depth

Fresh air

Bay width

2nd floor

3rd floor

4th floor

5thfloor

6th floor

7th floor

Services

Exhaust airopening

Ventilationstack

Pivotingcasement

Air-intakeopening

Exhaust air


Night ventilation involves some additional issues thatneed to be considered as part of the briefing and designprocesses. These include the following:

— Night ventilation via opening windows is asecurity risk; this can be overcome by usingopening limit devices, separate ventilation open-ings (such as motorised dampers or louvers), or amechanical system.

— Appropriate controls are needed to avoid over-cooling the space, with subsequent discomfort thefollowing morning.

— Good thermal contact (and thus high heat transferrates) must be ensured between the ventilating airand the thermally massive elements of thebuilding (usually the underside of the floor slab).Guidance on how to provide thermal mass in thebuilding fabric and how to provide the necessarythermal contact with the ventilation is given inBRE Digest 454(31).

These issues are discussed in some detail in section 4.7 ofGuide B2(2), which includes a number of design strategiesfor achieving thermal contact between the mass and thespace.

3 Ventilation components and system integration

3.1 From strategy to specification

Achieving successful natural ventilation (whether man-ually or automatically controlled) depends critically on thedetailed design, specification, installation, commissioning,operation and usability of the ventilation components andon the careful integration of the ventilators, actuators andcontrols. The key areas requiring attention to detail are:

— the principles upon which the system is designedand developed

— the products which are specified

— the processes of design, specification, installation,commissioning, and operation.

3.1.1 Principles

The purposes of the ventilation and control strategy needto be effectively thought through: how will the system becontrolled and managed and how will individual occu-pants be affected by and interact with the system and itsvarious components? These strategic issues are dealt within section 2.

3.1.2 Products

The design challenge is to seamlessly combine com-ponents and subsystems that are often developed fordifferent contexts. The various components and sub-systems that need to be considered include the following:

(a) Windows, ventilators and rooflights whichinitially may have been designed solely for manualcontrol.

(b) Actuators intended for occasional use, as in smokeventilation, or developed for other purposes suchas mechanical ventilation systems, greenhousesand industrial ventilators.

(c) Dampers normally used in mechanical ventilationsystems. Although tried and tested in mechanicalventilation applications, when used for naturalventilation dampers can fall short of expectationsin terms of appearance, building-in details andworkmanship, insulation and airtightness whenclosed.

(d) BMS systems, and the way they are integrated withthe manual control that is often desirable innaturally ventilated buildings. For example,providing manual control of windows by theoccupants at low level and automatic control ofhigh level vents where they are either out of sightor difficult to reach. BMS software must anticipatemanual interventions and allow for these withinits control logic.

Different types of actuators will be appropriate in differentsituations. The range of types available and the factorsinfluencing choice are described in section 3.5. It is vital,however, that the link between actuator and ventilator isnot overlooked. The following questions should beconsidered:

(a) Is it properly connected to the opening elementand to the frame or building fabric?

(b) Are its connection points and fixings strongenough and are any linkages properly aligned?

(c) Will the element it opens close and seal effec-tively?

(d) How does it connect to the control system and tosupplies of pneumatic or electric power? In onebuilding, the need for such cables had beenforgotten and it proved too difficult and expensiveto retrofit them.

(e) Will it stand-up to the duty cycles required? Careneeds to be taken so that the automatic controlsystem does not constantly exercise the actuator,leading to a very short service life.

(f) Can it be maintained safely and conveniently?

There are fewer such questions when the opening,actuator and controls are procured as a fully integratedsystem, so that installation can be largely maintenance-free. Even then it can be important to check whetherproducts have been adequately developed: manufacturerssometimes ‘bolt on’ actuators to standard natural venti-lation products without sufficient thought.

3.1.3 The decision making process

Decisions need to be made in different parts of the supplychain, for instance:

(a) Who specifies what? An architect will often specifythe window and a services consultant the actuatorand control. Will they be compatible? How willthey be put together? What about structuralengineering issues?

Example 3.1 Offices for a housing association

Ventilation components and system integration 21

(b) What are the implications for construction pro-gramming and commissioning? Should theactuator be fitted at the factory or on site? Sitefitting has often given rise to problems withmechanical integrity, alignment and fine adjust-ment (e.g. of end-limit switches). Some suppliershave a better record than others. Insist that sitefitting is by specialists, and planned in advance.

(c) What about trade responsibilities? Ventilationopenings, which are usually seen as part of thebuilding fabric, now become part of the buildingservices. This may bring trades together at stagesand in sequences in which they would notnormally be collaborating. Do specification doc-uments and management of subcontract packagestake proper account of this? There are manyadvantages if the window supplier is given respon-sibility for integration of vent, actuator andcontrol. See Example 3.1.

(d) Have preparations been made for effective oper-ation, management and fine-tuning? Occupiersoften assume that natural ventilation will lookafter itself and so are not prepared to operate thecontrols. In addition, neither they, nor thedesigners or installers, make provision for the fine-tuning that is normally needed during the firstyear or so of operation. While the desirability ofsuch activities is not unique to natural ventilationsystems, they tend to be more critical to itssuccess.

Having developed a clear overall ventilation strategy asearly as possible, it must be kept under review. Thefollowing questions should regularly be revisited.

(a) How will local and automatic controls be integrat-ed? The need for local override by occupants mustbe carefully considered. Generally, vents which areout of sight of the occupants are automaticallycontrolled and not normally subject to local over-ride. Conversely, high level vents which are visibleshould have local override control. Feedback onvent status is crucial.

(b) How many different types of ventilation devicewill be required to cope with all seasonal andoccupational requirements?

(c) How will ventilator geometry affect available freearea and airflow rates?

(d) Will they seal well when closed?

(e) Will they clash with other devices, e.g. fixed ormovable blinds?

(f) What protection will be required against highwinds and rain penetration?

(g) What restrictions are there in terms of noise andsecurity?

(h) What actuators and linkages will be used and howwill they be integrated and fixed?

(i) Is protection against insects and small animalsrequired?

(j) What controls and feedback devices will beprovided for occupants and management?

(k) What provision has been made to promoteoccupant awareness of the system?

(l) Has a clear division of responsibilities for allaspects of the ventilation and control scheme beenestablished?

3.1.4 Responsibilities for effective integration

Clear division, definition and ownership of responsibilityis vital for a successful outcome. If the boundaries ofresponsibility are not clearly defined, some problems maynot be ‘owned’ by anyone. Important elements may evenbe left out of the specification entirely. For example, afactor often overlooked is that ventilators (usuallyspecified by the architect) and actuators (often specifiedby an engineer) are joined by linkages. Experiencesuggests that in many installations, linkages and fixingsare major points of weakness and sometimes completefailure.

For window-type ventilators, the architect will normallyspecify the ventilator and the building services engineerthe actuators and control system. Contractually, where theactuators form part of the window package, the windowmanufacturer becomes responsible for procuring andfixing the actuators, and for testing the performance of theintegrated assembly. This will allow many problems to besolved before the assembly arrives on site. Even if somesite assembly (e.g. installation of projecting actuators) isrequired, it will have been more carefully planned.

To prepare a complete specification for an integratedwindow-type assembly, the architect will need to obtainspecification information on actuators, controls, linkagesand fixings from the engineer. In order to be able to judgethe appropriateness and completeness of this specification,the architect will need to be aware of all the criteria thatneed to be covered.

Sourcing from a single manufacturer can help to eliminatethe risk of integration problems but this is not essential.However, successful integration of ventilator, control,

The windows on the south side include fixed, manually-operable andmotorised elements with concealed provision for actuators, controlsand wiring. The actuator’s chain drive attaches to the window at thesame point as the manual latch, making secure fixing easier andhelping the windows to close tightly. With tried-and-tested, factory-

assembled componentsintegration was assured andthe installation has beenvirtually trouble-free.

The initial outcome was lessfortunate on the north side,which used purpose-mademotorised flaps and concealeddampers in site-builtenclosures beneath louvres inthe window cills. Hereintegration was less easilyachieved with air leakage boththrough and around thedampers. Unclear indicationof control status causedproblems, particularly withheat loss and discomfort inwinter.


actuator and linkage components from different manufac-turers requires:

(a) a knowledge of the technical issues, and

(b) clear lines of responsibility.

Window and actuator suppliers all recommend earlydialogue during which issues can be raised, e.g. providingsuitable fixings for the actuator, suitable linkages, the needfor any reinforcement of the frames. It is also recom-mended that the window/ventilator supplier be asked tosupply shop drawings showing the actuator type, linkageand fixings.

For damper-type ventilators, the building servicesengineer is more likely to specify the assembly, completewith its actuators and controls, while the architect will beconcerned with its appearance, finish, weather-tightnessand building-in details. Ideally, a detail will be developedwhich allows the damper assembly to be simply insertedas an engineering item into a prepared opening in thebuilding specified by the architect.

The responsibility matrix, Table 3.1 (pages 24 and 25),identifies the various issues in the design that need to beconsidered, and the interactions between the differentmembers of the client/design team that will be necessaryto ensure appropriate design solutions are developed inrelation to the specification of vents, actuators andcontrols. This can form a useful ‘aide memoire’ for theteam during the detailed development of the design.

3.2 Ventilation opening types

Having defined the ventilation strategy (see section 2) thetype of vent opening type needs to be chosen. Naturalventilation openings are of three broad types:

(a) Windows, rooflights and doors

(b) dampers or louvres

(c) background ‘trickle’ ventilators

3.2.1 Windows, rooflights and doors

Their major advantages are that they are familiar tooccupants and that hinged versions can be made to shuttight relatively easily. In comparison with louvres anddampers:

(a) they have a shorter crack length

(b) effective seals are easier to provide

(c) closing forces can be higher and better distributedabout the perimeter.

Window stays vary widely in terms of their ‘throw’,geometry and robustness. Particular care must be taken toensure that the effective free area is achieved (see section 2and Ventilation capacity below). Most patterns of windowand door were originally designed for manual operation.Windows used for automatic control may require adapta-tion to accept motorised actuators, and need strengtheningto accommodate forces applied at different places and indirections that may not be parallel to that in which thewindow opens.

Different window designs may be assessed under thefollowing criteria:

(a) Ventilation capacity: this is clearly related to theway the window opens and the surrounding head,cill and jamb details. Determining the effectivearea for a particular window type must be donecarefully to ensure a sound basis for subsequentcalculations of volume flow rate. With top hungvents, for example, the triangular opening eachside of the open window is significant; in theevent, an estimate is usually the basis for cal-culations. Figure 3.1 illustrates the differencebetween structural opening, throw, and effectiveopening. Window sills, reveals, internal andexternal blinds all have a major impact on the finaleffective area which is achieved. Ensure that thestrategy is carried through into detail design byproviding continuity in the design team. Table 3.2(page 26) considers the ventilation capacity ofdifferent window types.

Figure 3.1 Structural opening,effective opening and traveldistance for a top hung window

Top hungwindow

Effectiveopening

Deep external sill

Travel

Stru

ctur

al o

peni

ng

Example 3.2 Automated fanlights at Building 16, BRE Garston

(courtesy of Feilden Clegg Bradley Architects)

Contact between the incoming air and thebuilding structure is enhanced by using a‘wavy’ ceiling slab that allows night air to passthrough it. The automated fanlights at theperimeter use chain drive actuators.

Hand-held infra-red controllers permit usersto over-ride not just the automated windowsbut also the external louvres and the electriclighting from any point within the space. Inspite of this good control, occupants havecommented on insufficient feedback owing tothe slow response times of windows andlouvres, and not being able to see theautomated windows which are behind the lowpoints of the ‘wavy’ ceiling profile.

(b) ‘Controlability’: where it is considered desirable andacceptable for occupants to control opening ofwindows, stays should be both adjustable androbust. With manual and automatic operation, thestay increases the angle of opening of the windowand the effective area will increase, but therelationship is not linear, and this will also varyaccording to the opening type(32).

(c) Comfort: generally, the use of opening windowsresults in high levels of user satisfaction whenlocal occupant control is part of the strategy.However, the use of the same opening for winterand summer ventilation may be unsatisfactory incertain situations. Where, for economic or otherreasons, this approach is taken then the impact ofthe window geometry on draughts in winter ormovement of papers in summer must be consid-ered. This can be a reason for maintaining lowlevel openings under manual operation and highlevel openings under automatic control. Theprovision of automatically controlled high levelwindow openings for night ventilation can incor-porate manual override to provide occupants witha greater degree of daytime control.

(d) Security: it can be argued that open windows areonly likely to be a security problem at ground orfirst floor levels of a building. Restricting thelength of throw of stays or actuator arms may besufficient in many situations. A facility to lockmanually opening vents in a secure positionshould be provided.

(e) Sealing: sealing of windows is usually achieved byEPDM expanded rubber gaskets. In aluminiumframed windows the tolerance achieved is usuallyvery fine and good sealing is achieved. However,with some steel framed and timber windows,racking or twisting of the frame can occur, particu-larly with large windows, resulting in poor sealing.Automatic vent gear must be installed so that theopening frame is pulled tight against the subframe.

(f) Integration with vent actuators: the choice of windowtype and its integration with different actuatoroptions and (where applicable) internal blinds,requires careful consideration if the performanceof one or other of these components is not to becompromised.

The main window types and related automated actuatoroptions are described in Table 3.2 (page 26).

3.2.2 Dampers or louvres

Motorised dampers are widely used in mechanical ventila-tion systems, where they are usually effective and reliable.However, when used for natural ventilation they have amajor disadvantage in that they do not shut as tight asmost windows: this is due to:

— longer crack lengths

— difficulties with rotating seals

— problems with mechanical strength and closingforces.

Additionally, they are often characterised by poor insula-tion standards, with subsequent condensation problems.


In many naturally ventilated buildings constructed in thelast 15 years, excessive air infiltration has caused problemswith heat loss and discomfort in winter. Some availableproducts, including some glass louvres, have good sealsbut need to be carefully selected and checked for construc-tion and performance.

For most dampers, the shaft for the actuator motorprojects out of the frame, from the end of one of thedamper shafts. This is ideal for ductwork and air handlingunits as the motor can be installed, checked and main-tained externally. However, for natural ventilation, theprojecting shaft stops the dampers being installed in theplane of a wall, so either the dampers need to be fittedwithin a duct spigot or lever operation may be necessary.

The different damper types which may be used for naturalventilation are described in Table 3.3 (page 27).

3.2.3 Background trickle ventilators

Trickle ventilators are designed to provide a minimumfresh air rate, but are designed to be controllable (and canbe shut down) particularly in winter. Building RegulationsApproved Document F*(4) recommends an opening areafor background ventilation which is related to floor area,with a minimum provision in all habitable rooms of4000 mm2. Trickle ventilators can be in the window frame,part of the glazed unit or independent of the window.Adequate winter ventilation can also be achieved by anautomatically controlled window element. See Example3.3 (page 27).

To minimise cold draughts, the ventilators should be athigh level (typically 1.75 m above floor level) and designedto promote rapid mixing with room air. As trickleventilators are only intended to promote ‘backgroundventilation’ (about 5 litre·s–1 per person), their mainpurpose is to supply fresh air in winter months. Theapparent need for control, even over trickle ventilators,has led to the development of pressure regulated trickleventilators that respond to variations in pressure differ-ences around the building, especially during periods ofhigh wind speed. Their advantage is that they throttledown in windy weather, but still require further develop-ment and there are only a few products currently on themarket.

3.2.4 Combined openings

When designing ventilation openings, it is common to usecombinations of opening types in an overall design. Forexample, motorised dampers or louvers may be used tocontrol fresh air supply to an underfloor plenum, and thento a space from which air is exhausted via high levelwindows.

Different opening types may be combined in a singlewindow unit (e.g. an opening window with a trickleventilator in the frame), or they may be independent.Combinations of window types in a single window unit areworth considering. For example, a hopper over a centrepivot window has many advantages. The hopper canprovide night ventilation and also help provide air tooccupants deep into the room. The centre pivot allowshigh summer ventilation rates and is especially beneficial

* Under review at time of writing (January 2005)


Table 3.1 Principal responsibilities and interactions

Category Client (owner) Architect Building services engineer Client (facilities manager)

Briefing Define: Comment on: Comment on: Comment on:— site/location — area/volume requirements — performance criteria — performance criteria— activities — plan depth/massing— occupancy — orientation— performance criteria — spatial relationships— control preferences — performance criteria

Ventilation strategy Take ownership of strategy Define relationships of supply Define flowrates to achieve: Question implications for:and exhaust for different — indoor air quality — occupants satisfaction

Question implications for: purposes: — cooling — operation— lifecycle cost — zoning — local extraction of — maintenance— occupant satisfaction — airflow paths pollutants — CDM issues— control options — geometry — control— CDM issues — application of principles

— CDM issues Pre-commissioning— operational instructions

CDM issues

Draft simple instructions for users

Ventilation openings Implications for: Preliminary sizing of vent Preliminary sizing of Access for:— appearance openings ventilation openings for — commissioning— operation supply and exhaust — cleaning— maintenance Opening type — maintenance— access Performance prediction— replacement Weathering in relation to briefing

criteriaSecurity

Pollution and noise controlClashes with other elements

Access for maintenanceAirtightness/thermal performance

Interface with building envelope

Actuator Implications for: Choice of type in relation to: Implications for:— appearance — weight and size of — operation— achieving required free openings — maintenance

area opening — type of opening — replacement— achieving required — location (low or high level)

ventilation purpose — opening parameters

Linkage Number of openings operated Commissioning requirements Implications for:— operation

Geometry and connections Maintenance and cleaning — maintenance

Maintenance and cleaning

Controls Implications of control and Control logic Control logic: Implications for:logic for occupant operation — status feedback — operationand satisfaction Operation during construction — automatic/normal/fire — maintenance

modesStatus feedback — occupant override

— wind/rain/snow thresholds

Installation and Recognise importance Recognise importance Commissioning requirements Access for commissioningcommissioning

Ensure time in programme Ensure time in programme Access Commissioning requirements

Witnessing

Operation and Allow for training Fine tuning Fine tuningmaintenance

Simple user instructions CDM issues


Specialist subcontractor or Contractor Commentsproduct designer

The structure of this table assumes a conventional contract. Where the contractor is engaged in design then the responsibility chain will change.

It is important that all involved take ownership of the agreed strategy. This includes the quantity surveyor. Elements which are fundamental to the working of the building cannot subsequently be regarded as optional extras in cost cutting exercises.

Implications for: Subcontract package Generally the vent supplier should be made responsible for integration of vent — actuator type boundaries, coordination with actuator linkage and controls.— linkages and fixings and programming— effective area achievable— frame reinforcement Advise on maintenance

procedures

Implications for fixings: Recommend that actuators Engineer must also allow for power and control requirements and circuiting, as — frame reinforcement form part of the window well as maintenance, alteration and replacement.— noise package where appropriate— speed of operation

Advise on: Check linkages and fixings Where vent, actuator and control are not integrated, the architect will become — forces acting on fixing have been defined by design responsible.

points team

Pre-commissioning demonstration of system

Undertake commissioning Commissioning requirements Allow for modifications during first year of operation.according to commissioningplan Ensure time in programme

CDM issues

Supply O&M manuals

Undertake maintenance


to occupants near the perimeter. The different sizes ofopening also allow finer control over the ventilation rateby progressively opening the hopper and opening themain window, then opening both together.

As the design of the unit is developed, the other functionalrequirements of the window (lighting, shading, security,transmission losses) need to be considered. Considerableeffort is being put into developing multi-functionwindows that address all these requirements.

Many ventilation strategies rely on shafts to take airvertically through a building. Similarly, ducts (includingfloor voids) are used to provide horizontal distribution.The criteria for sizing these airways are very differentfrom those for conventional mechanical ventilationsystem because of the need to keep pressure drops withinthe range available from natural driving forces. Thismeans that space must be allowed to incorporate largerthan usual ducts or shafts. See Example 3.4 (page 28).

Table 3.2 Main window types and actuator options

Window type Actuator options (if automatic Commentscontrol is required)

Horizontal pivot Linear These windows have a high ventilation capacity and the geometry promotes Chain good distribution of supply air. Internal blinds are not practical but

interpane blinds may be a useful alternative. For an opening of 22° then the effective area is 34% of the area of the structural opening. Shorter stay lengthcompared with casements, reduces wind pressures on actuators.

Vertical pivot Linear This type provides a similar effective area as horizontal pivot. Internal Chain blinds are also not practical Vulnerable to driving rain, and will provide a

‘wing wall’ effect (either positive or negative) in response to the prevailing wind.

Top/bottom hung Linear Wider range of actuator options. Length of throw is significantly increased Chain relative to pivot windows. (i.e. more costly and more vulnerable under high Rack and pinion winds). Although bottom hung inward opening is a useful geometry when Lead screw located adjacent to the ceiling for night vent cooling, top hung is a better Lever arm geometry at low level to direct flow towards occupants for daytime

ventilation.

Side hung (casement) Chain Not easy to link to automatic opening gear (lever arm must rotate as it Rack and pinion extends). Also ventilation characteristics are strongly influenced by wind

speed and direction (as with vertical pivot windows).

Tilt and turn Cannot be linked to actuator These ventilation characteristics have been studied in several buildings, where it was reported that the ‘tilt’ setting provides too much ventilation in winter and insufficient cooling for occupants in summer.

Sash (sliding) Linear These windows have ventilation characteristics similar to the vertical and Linear sleeved cable or rod horizontal pivot window. The effective area is maximum 50% of the

structural opening.

Louvres Rotary Usually glazed or aluminium. Advantages are that they can be made secure Linear and still function satisfactorily, and therefore have potential application for

night ventilation. However, when closed louvres generally have a very poor seal. This is the case with most louvre or damper installations.


Also of crucial importance is the need to keep the inlet ductsclean to minimise problems of poor air quality. This willrequire the provision of inlet screens and access for cleaning.

Since shaft outlets are at high level, they are in a region ofhigher wind speed. This means that the wind pressuresacting on the shaft are also likely to be high. Wind effectswill probably dominate the pressure distribution throughthe system, except at very low wind speeds. It is vital,therefore, that outlets are designed to create wind pres-sures that reinforce the intended flow direction. Usually,this means creating a negative pressure coefficient at thetop of the shaft, the exception being the wind scoop, seeExample 3.5 (page 28).

3.2.5 Shafts and ducts

Orme et al.(33) provides information on the requiredpressure coefficients. For isolated buildings with no localflow interference, the minimum height of the stack aboveroof level to avoid back-draughting is given by:

h = 0.5 + 0.16 (θ – 23) a (3.1)

where h is the height above roof level (m), θ is the roofpitch (degrees) and a is the horizontal distance betweenthe outlet and the highest point of the roof (m).

Occasional back-draughting may not be regarded as aproblem, depending on the situation.

For roof pitches of less than 23 degrees, the outlet must beat least 0.5 m above the roof level. This simplifiedrelationship represents a minimum stack height, greaterheights may provide higher suction pressures. This can bebeneficial, since a tall shaft can generate a suction greaterthan that generated on an opening on the leeward verticalface of a building.

More information on the pressure coefficients over roofs isgiven in BS 6399(34). Model testing is available for complexroof profiles or where surrounding buildings or otherobstructions disturb the wind.

As well as the position of the roof outlet, the geometry of acowl also affects the pressure coefficient. The cowl shouldprevent rain entering the stack; it can also accelerate flowclose to the outlet to reduce static pressures.

3.2.6 Ventilator design decisions

A preliminary list of items that need to be considered bydesigners is given in Figure 3.2 (page 29), which may bephotocopied and used as a checklist.

3.3 Internal obstructions

Windows and ventilation openings create resistances toairflow where the air enters or leaves the building. Inseries with these flow resistances will be internal resist-

Table 3.3 Types of damper suitable for natural ventilation

Damper type Construction Comments

Standard blade Standard dimensions: heights/widths Mainly used for mechanical ventilation systems and can be installed in from 100 mm to 2400 mm in steps either rectangular or circular ductwork. This is also provided with a motor of 100 mm. Bespoke sizes available. platform. This type of damper is used to control ductwork volume flow rates All dampers have a position indicator in connection with ductwork balancing. and manual adjustment device.

Can provide control in natural ventilation systems. In the closed position the leakage air flow of a closed damper is low (though higher than a window). In the open position the blades are turned in the direction of flow and do not cause a significant pressure loss.

Sealed blade Normally supplied with a motor Often used to close a ductwork branch, as in the closed position the leakage platform. Available in diameters from air flow is low. In the open position the blades are turned in the direction of 100 mm to 1000 mm or rectangular flow and do not cause significant pressure loss. Can also used for the control in heights/widths from 100 mm to of volume flow rates.2400 mm in steps of 100 mm.

Has better sealing than standard damper type. Consists of a casing andblades usually made of painted hot-galvanised or acid-proof steel. There are gaskets built into the edges of the blades and between the end of the blades and the outer frame.

Example 3.3 Natural ventilation of corporate offices on a ‘green field’ site

4

5

6

7

This office building used fixed trickleventilators and natural ventilation to maintainindoor air quality, plus a mechanical system(supplemented by chilled beams) for summercooling. In practice the amount of trickleventilation has proved excessive, andsometimes draughty, and many of theseventilators have now been blocked-up.However, in other respects natural ventilationvia the atrium is working well. High levelatrium vents are operated by paired pneumaticactuators.

Example 3.4 Vertical shafts and terminations used to enhance natural ventilation; Queen’s Building, de MontfortUniversity (courtesy Short Ford Architects)


ances created by internal partitions and doorways etc.Thus, the way the internal spaces are compartmentalisedinto individual work areas needs to be considered as partof the overall ventilation strategy. (Resistances arediscussed in detail in section 4.)

If cross-ventilation is employed, care needs to be taken toensure that the ventilation path is not obstructed by full-height partitions and closable doors. If cellular office spaceis required, transfer grilles will be needed, as a minimum,to allow the air to move across the building. The resist-ance of these transfer grilles will need to be included indesign calculation when sizing the façade openings.

As well as providing a resistance to the airflow, internalpartitions, furniture, filing cabinets etc. can influence theairflow pattern. Where this occurs, the occupant does notexperience the full benefit of the fresh air, since it bypassesthe occupied space, moving across the space at ceilinglevel. This is directly analogous to the problems that canoccur in mechanical ventilation system if obstructions areplaced between the supply diffuser and the occupant’swork place.

Example 3.5 Wind cowl at the Jubilee Campus,University of Nottingham (courtesy Hopkins Architects)

Cavitywall

Cavitywall

Cavitywall

Office

Externalair

85 W/m2

85 W/m2

4·0 m3/s

3·6 m3/s

6·0 m3/s2·2 m3/s

ClassroomClassroom

Laboratory75 W/m2

Concourse75 W/m2

(peak

Drawing studio

Auditorium

28 W/m2

Detail of termination (photo: B Ford) Isometric view


3.4 Background leakage

It is very important to appreciate that purpose providedopenings such as windows and ventilators are not the onlyroutes through which outside air flows into the building.All buildings have leakage paths at joints between build-ing components and through some building materials.The extent of these leakage paths can have a significanteffect on overall ventilation performance since it is thesize and distribution of the ventilation paths that controlsthe magnitude and direction of the airflow.

Background leakage is usually quantified by installing apressurising or depressurising fan in an external openingand measuring the airflow rate required to maintain atarget pressure differential across the buildingenvelope(24). Building Regulations Approved DocumentL2(5) now sets maximum allowable air permeabilitystandards and, for buildings with floors areas greater than1000 m2, the achievement of the standard must bedemonstrated by means of a pressure test.

For controllable natural ventilation, the CIBSE ‘goodpractice’ recommendation(3) is that the airtightness should

not exceed 7 m3·h–1 per m2 of permeable envelope area foran imposed pressure differential of 50 Pa across theenvelope (‘best practice’ = 3.5 m3·h–1 per m2). To achievethese targets, careful attention needs to be given to thedetailing of component jointing and sealing techniques.BRE has produced a report to give guidance on methodsof reducing air infiltration in large, complex buildingssuch as offices(35). The worst allowable for offices underBuilding Regulations Part L* is 10 m3·h–1 per m2.

3.5 Window stays and automatic actuators

3.5.1 Actuator and linkage options

The types of automatic actuator commonly used fornatural ventilation, together with their linkage options,typical applications, and comments on their suitability, areas follows:

(a) Linear push-pull piston: a motor propels a push-rodforward. These are most commonly pneumatic butelectro-hydraulic versions are also available.Advantages include mechanical simplicity, robust-ness, fire resistance of pneumatic units andgeneration of large forces. Disadvantages includelarge projecting cylinders and mechanical damageto windows, linkages and fixings which are notsufficiently robust. Travel is typically 200–500 mmbut longer distances are possible with largecylinders. They are most widely used for rooflightsand high-level windows.

(b) Projecting chain drive push-pull: an electric motordrives a chain over a sprocket wheel, providinglinear motion to push out a window. They aregenerally modest in size and mechanical strength;and with limited travel of typically 150–200 mm. Auseful feature is that the motion tends to be atright angles to the axis of the actuator body, whichcan therefore be tucked away in the plane of awindow frame or recessed and concealed within it.Their compact size and unobtrusive appearancemakes them best suited to smaller windows suchas inward and outward opening fanlights.

(c) Rack-and-pinion: a rotary electric motor drives ageared shaft which engages with one or moreracks, providing linear motion with less bulkyprojections than linear actuators. Typical traveldistances are 500 mm but travel distances of1000 mm or more are possible. They are particu-larly useful for windows which require pairedactuators (one on each side). With a commonpinion, the two racks move together, which avoidstwisting the window (as can happen, for example,if one of a pair of linear actuators fails; sometimesbreaking the window). They are most commonlyused for rooflights with relatively light frames.

(c) Linear sleeved cable or rod: driven by a rack-and-pinion, worm gear or chain drive electric motorand allow linear motion to be transferred, forexample, to sliding sashes.

(e) Rotary: these are most commonly applied withdampers and louvres, often rotating one of the

Ventilator design checklist� Types of ventilators to be used

� Sizes of the individual openings (effective area) for winter and summer use

� Loadings, both in normal service and in extreme situations, e.g. high winds

� Influence of ventilator geometry on free area and airflow rate

� Actuators and linkages to be used, and their integration and fixing

� Control strategy required, including appropriate integration of manual and automated controls, and the scope for user over-rides

� Need for feedback on operational status, both to individuals and to the control system

� Provision for control equipment and the associated transformers, compressors, switches, indicators, wiring and tubing; together with the associated safety and protection requirements

� Need to avoid adverse effects, such as ingress of noise, rain, fumes, insects and intruders

� Need to avoid clashes: physically (with internal curtains and blinds, external shutters and sunscreens,insect screens) and operationally (with building services systems)

� Health and safety issues in installation, operation andmaintenance, particularly safe access for maintenanceand cleaning

� Need to avoid hazards from unexpected operation, e.g. trapping fingers or knocking people over

� Need to avoid ‘grey’ areas of responsibility for design,installation, testing and commissioning.

Figure 3.2 Checklist: ventilator design decisions

* Under review at time of writing (January 2005)


shafts directly, with mechanical linkages to theother louvres. Sometimes they also operate shaftsconnected to cranks and lever arms to providelinear motion. There are two main types: one witha bi-directional motor used for opening andclosing, and one which motors in one directiononly and uses a spring to return, which can beuseful for fail-safe operations.

(f) Lead screw: these actuators form the majority oflinear actuators and are sub-divisible into highand low power applications. For general applica-tions from 200 N to 2000 N, a low friction nut isdriven along the lead screw to provide motiveforce.

(g) Lever arm: these often found in Victorian schoolsand hospitals. Continued use is testament to therobustness of the gear. It is often regarded asvisually intrusive and unnecessarily cumbersometoday (but are still being manufactured).

(h) Gas struts with cables: a linear variant on the springreturn motor is where an automatic catch releasesthe window and a gas-filled strut opens it, or holdsit open. Closure is then affected by pulling on acord, either manually or by means of small electricwinch.

3.5.2 Actuator and linkage selection

There are many factors that need to be considered whenchoosing an actuator. These include:

— Location of vent and manner of opening: there arethree common variants: a hinged flap, a set ofcentre-pivoted louvres, and a panel sliding onrunners. Linear actuators are most commonly usedfor rooflights and chain drives for low levelwindows, where it is more important for actuatorsto be unobtrusive and for them not to obstruct theinterior.

— Weight and size of vent: the motor should be sized tosupport the appropriate level of forces, includingwind, snow and ice loads. Large windows mayrequire two (or more) actuators.

— Angle of attack: owing to geometrical restrictions,the actuator force cannot always be applied exactlyin the direction of motion and can sometimes be

nearly at right angles to it. This can lead to largeshear forces and moments on fixings.

— Travel and free area to be achieved: chain actuatorsare usually limited to 150–250 mm, though someconcealed units can reach up to 500 mm. Thechains have limited buckling strength, so they aremost commonly used to open top or bottom-hungwindows by a small angle. Linear piston actuatorstypically have travels up to 600 mm, while rackand pinion drives can go up to 1000 mm and more.

— Available space: actuators tend to swing as awindow opens so articulated linkages are required.Actuator and linkage components must clearly notclash with each other, or with the surroundingstructure, during their full range of travel.

— Travel speed: low travel speed is desirable to reduceloadings on the actuator and to reduce noise.However, where occupant control is used, visibleresponse is also important so people can see thattheir control actions have had a result. If theactuator moves too slowly or is not visible fromthe point of operation, then some visual feedbacksuch as an indicator light or display should beprovided. For rooflights, rapid closure is desirableto minimise entry of rain. Two-speed operationmay be desirable; slow for normal operation andfast for manual over-ride and emergencies.

— Smoke ventilation: sometimes the ventilator mayneed to open or close within a specified time. Forexample, smoke control often requires any ventila-tion device to be fully open within 60 seconds. Foruse as a smoke vent, the ventilation opening,actuator, linkage and controls must be tested as acomplete package in accordance with BS 7346:Part 1(36).

— Linkages and fixings: linkages and fixings must becarefully considered for appropriateness, mechan-ical strength, durability and ease of installationand maintenance. The supplier of the ventilatorshould be made responsible for the fixing andmechanical commissioning of the actuator andlinkage. This should include adjustment of anyend stops and limit switches which, if not carefullydone, can easily lead to mechanical damage to theactuator, linkages, fixings or ventilator.

Example 3.6 Bedales School Theatre, Hampshire (courtesy of Feilden Clegg Bradley Architects)Five bays of vent openings on the northand south sides of the auditorium,located below raised perimeterwalkways, supply air below perimeterseating via a plenum. These vents havefixed timber louvres to the outside witha double bank of fully modulatingmotorised vents behind. Performance isgenerally satisfactory. Early problems ofdraughts on north side have been solved,but at high level, poor adjustment oflever arms results in large gaps when inthe ‘closed’ position. Infiltration lossesdue to large crack length of motoriseddampers at low level, and poor sealing tohigh level window vents cause heatingenergy consumption to be higher thanpredicted.

(photo: Dennis Gilvert)


— Electrical safety/level of protection: electrical actu-ators tend to be either 240 volt or 24 volt. Theformer is cheaper (but needs more relays) and thelatter is safer (but needs transformers and largerdiameter cables). Although normally indoors,some actuators may be exposed to high humidity,drips, or driving rain. Occasionally they may needto be flame or explosion proof (when pneumaticoperation would be preferable). For electricalequipment, the level of protection is defined bythe IP rating (from IP 00 to IP 68). The first digitrefers to ingress of dirt and dust (from 0(unprotected) to 6 (dust-tight)) and the second toingress of water (from 0 (unprotected) to 8 (sub-mersible)). Linear actuators in exposed positionsmight be IP 65 (dust-tight and protected againstwater jets from any direction) while chain driveactuators sheltered by an opening window mightbe IP 33 (resistant to objects above 2.5 mm, waterdrips, and water spray at an angle of up to 60° fromthe vertical). IP 54 is a reasonable cost compromise(particularly important for motor housing, but thismay be difficult to achieve for chain drives). Forproducts with a lower rating a secondary housingfor the motor may be required.

3.5.3 Forces imposed on an actuator

The forces imposed are determined by:

— the weight of the ventilator

— external and uncontrollable forces especially wind,snow and ice

— the positioning of the actuator and linkage

— maintenance loads (sometimes).

How these forces are transferred to the actuator dependson the angle of the ventilator, the location of the fixingpoints, the geometry of the linkage (which will oftenchange with the extent of opening), the geometry of theactuator, the speed of opening, and any frictional andadhesive forces.

3.5.3.1 Ventilator weight

The effect of self-weight depends on geometry. A rooflightwill need a considerable force to open it (and to keep itopen) but only requires the actuator to resist the weightduring closing. A top-hung window will require much lessforce to open and to be resisted during closure, and acentre-pivot window even less. A bottom-hinged windowwill need very little effort to open, but may need asignificant force to close, depending on the opening angle.If a fanlight is to open no more than 10–15°, then theforces required can be often met by a small chain driveactuator producing 500 N, with a maximum static load oftwice the dynamic load, or 1000 N.

Issues to be considered are:

— the weight to be supported when shut and whenfully open

— moments on pivot points when shut and whenfully open.

3.5.3.2 External forces

Wind, snow and ice loads may often be far higher than theweight of the ventilator itself, particularly for rooflights.High suction loads may also require latches to ensure thatthe unit remains closed under extreme conditions.Rooflights used for smoke ventilation are required tooperate under any conditions, including a once in 50-yearoccurrence (approximately 160 mm of fresh snow). If thesystem is for natural ventilation only, then there may beno requirement to open under snow load but this may stillaffect the design of actuators, linkages and safety measuresto avoid actuator failure when attempting to operate underexcessive loadings.

Issues to be considered include:

— snow, ice and wind loads in all positions

— possible requirement for catches to keep theventilator secure in the closed position.

3.5.3.3 Actuator position

The positioning of actuators and linkages will often beaffected by the size and shape of the ventilator, theavailability of fixing points, the presence of obstructions,and aesthetic considerations. Angular and leverage effectscan then multiply the forces on the actuator. The smallerthe distance from the pivot point to the actuator, thegreater the force required, but the shorter the stroke.


— multiplication of forces owing to actuatorpositioning and angle

— mechanical strength of the ventilator

— effect on specification of actuator, fixings, linkages.

3.5.3.4 Other forces on the actuator

Where possible, actuators should operate slowly. Highspeed operation will cause greater stresses on the unit andlarge shock loadings at each end of its travel. Where theactuator acts as a strut, holding the ventilator open,buckling loads will also need to be considered.

Engineering advice should be sought from the supplier ofthe assembly when analysing the forces on the mech-anism. The complexity of predicting the performance ofthe different elements suggest that testing of the finalassembly prior to final production is essential.


— dynamic and buckling stresses.

3.5.4 Number of actuators

Studies of a number of different installations(37) haverevealed that a good general rule is to have one actuatorper ventilator. If one actuator powers several ventilators,difficulties in obtaining good initial alignment, togetherwith accumulating backlash in the linkages, nearly alwaysmeans that not all the ventilators close tightly, leading toproblems with airtightness and security. This can beaggravated by racking of the ventilator frames, distortions


from poor fixing, flexing of the linkages, lever arms andbrackets or from shock loads from wind or at the ends oftravel.

If the end stops are not correctly set, or drift as backlashaccumulates, the more powerful motors necessary tooperate multiple ventilators can easily run on beyond theend of the linkage’s available free travel and cause damageto themselves, the linkages or the fixings. A current limiton the motors can be applied to prevent this fromoccurring.

If one window is powered by two actuators (typically oneon each side), it could easily be damaged if one actuatorfails or if they operate at different speeds. If two fixingpoints are needed (typically on windows more than1200 mm wide; much less if lightly-framed) then it willoften be preferable to use a single actuator with a linkage,for example a rack-and-pinion or a cable drive, or an anti-skew control system.

3.5.5 Reliability of actuators

A typical mean time before failure is 30 000 operationsunder normal conditions. This represents about fivecomplete operations per day for 250 days per year for a 25-year product life. While at first sight this seems a lot, onseveral recent buildings it has been found that the controlsoperated the actuators very frequently, leading to failuresafter a year or two, or even more rapidly if the motorsoverheated. It is therefore important that the controlsystem is designed not to move the actuators too often. Itis important that a hysteresis loop is integrated into thecontrol.

3.5.6 Linkages and fixings

The fixing of the actuator arm to the window frame is notalways straightforward. Window manufacturers willnormally require a plate to be bonded to the frame, eitherby self-tapping screws or adhesive. The number of leverarms required per opening (to lift the window evenlywithout twisting or racking and to be capable ofwithstanding wind loads) is usually based on the recom-mendation of the vent gear suppliers. The Europeanlegislation that requires all products to be ‘fit for theirintended purpose’ does not protect the designer, however,and very often problems arise because the windows areeither too heavy or too large for the gear installed.(Controls should, of course, normally close vents in highwind speeds).

The protrusion of vent actuator gear into the space makessome gear unsuitable for low-level windows. This hasmade chain drive and cable drive popular for low-levelapplications (but only for lightweight windows). Wheremultiple vents are to be opened in line at high level thenrack-and-pinion gear is suitable and sufficiently robust.Very long lever arms sometimes must be curved in orderto avoid being obstructed in travel.

Linkages and fixings are as important as any other part ofthe system but are often taken for granted, making them acommon source of failure. Fundamentally, it must beclear:

(a) how the linkage transmits the motion of theactuator to the ventilator without distortion, and

(b) how the actuator and linkages are fixed to theventilator and the support structure so that theentire assembly is secure and can function satisfac-torily through tens of thousands of cycles.

Although the ‘one actuator per vent’ rule is ideal, it willnot always be possible to avoid using multiple linkedvents. When these are required, particular care should betaken in specifying the linkage and how it is fixed. Whereseveral windows are driven by a long set of linkages onone shaft or push rod, it can be difficult to get goodalignment when they are all driven from one end. It isbetter to place actuators in the centre if possible.

Having established the loads on the fixings, the method offixing must be given careful consideration, together withthe need for reinforcement of the ventilator and its frame.The fixings, brackets and attachment points should alwayshave a significant safety margin to cope with added shockloads from wind buffeting and actuator operation, andwith any rotational and shear forces. As a general rule,fixings with self-tapping screws and rivets will not beadequate: thread-locked screws or bolts are normallyrequired, screwed into threaded fixing plates.

3.6 Control system

3.6.1 Background

The amount of ventilation a building needs, from control-ling indoor air quality only in winter to maximum coolingin summer, can easily vary by an order of magnitude.During unoccupied periods even less ventilation may bedesirable. The amount of air flowing through a givenopening in a given position can also vary widely as thedriving forces (wind, natural buoyancy) change withindoor and outdoor conditions. In addition, the internalresistance to cross-ventilation will vary as doors etc. areopened and closed.

With these wide variations in the amounts of air neededand the associated driving forces, it can be difficult toachieve the full range of requirements using the sameventilation device. For example:

— The large openings required for summer ventila-tion will need to be shut tight during winter whenmuch less air is required but natural buoyancypressures are higher.

— Winter ventilation will require much smalleropenings.

— In summer, people may welcome air movement forits additional cooling effect (provided they canpersonally decide whether or not to sit in adraught) but in winter people will complain ofdraughts unless the incoming air is either well-diffused or preheated.

Designers need to develop control strategies very carefullyand present them clearly and diagrammatically in a step-by-step manner. Designers are warned against expectingthe suppliers of controls and building management


systems to provide control strategies based on second-guessing the design intentions.

3.6.2 Flowrates through ventilators

The flow rate through a ventilator is a function of theopen area, but often this is not directly proportional to theactuator movement. In particular, the thickness of theventilator and the way it fits into the building fabric cangreatly affect the relationship. With a centre pivot orhinged window, the lower opening can be greatlyrestricted by the frame, the window cill and the reveal,particularly for the first 100 mm or so of travel. This canprovide useful fine control but it may restrict the requiredopening area for summer ventilation, particularly if safetyconsiderations also restrict the permissible amount ofwindow opening. For centre pivot or bottom-hung hopperwindows, the upper opening may also be restricted by thereveal and by any internal blinds. External shading mayalso restrict flowrates and openings (and may also preheatthe outside air, particularly if the shading is dark in colourand of a metallic finish).

3.6.3 Development strategy

A wide variety of control strategies is possible(38) andmany will be used in combination. It is necessary toconsider how the system will work at all times, not just innormal operation modes but in emergencies such as powerfailures and fire alarms. The principal purposes are:

— Localised heat and pollutant removal: this may beassisted by time-, temperature- or air quality-controlled advanced natural ventilation, particu-larly via ventilation stacks over heat sources.

— Indoor air quality and heating: this is the principalwinter control mode. Key control problems areproviding sufficient, but not excessive, back-ground ventilation whilst avoiding draughts.

— Daytime cooling: key control problems during theheating season include telling whether overheatingis a result of insufficient ventilation or too muchheating and, in hot weather, determining whether

additional ventilation will necessarily cool thebuilding. (However some people may welcome theair movement even if the air is no cooler.)

— Night pre-cooling: strategies are summarised else-where(39). However, previous studies(40,41) suggestthat much simpler algorithms are easier toimplement and can work better. They includeaiming to keep embedded sensors in exposedfabric at a constant temperature and progressivelyreducing night-time setpoints if daytime tempera-tures climb. However, care must be taken not toover-cool the building at night in a quest toachieve lower afternoon temperatures or theoccupants may object to low morning temper-atures.

Natural ventilation may also be part of a mixed-modestrategy where mechanical ventilation or cooling assiststhe natural systems. Integrated controls are particularlyimportant in such applications(22,38).

3.6.4 Operating natural ventilation openings

Ventilators can be controlled in a number of ways. Theseinclude the following (in order of increasing sophistica-tion):

— On/off: the opening has two positions, fully openand fully closed. This coarse method is best suitedto buffer spaces and atria in which close control ofcomfort conditions is not essential. It is also moresuitable for outlet positions at high level andremote from individual workstations than to inletpositions near workstations, where it may causedraughts.

— Bank control: the space has a number of windowsor ventilators, in different banks each with on/offcontrol. This allows the ventilation rate to bevaried by opening-up banks of ventilators insuccession. The banks can be multiple identicalventilators or ventilators of different capacitieswhich can provide either background or rapidventilation.

Example 3.7 Natural ventilation control at Powergen headquarters, CoventryThe automated windows at the top of the atrium at Powergen’s mixed-mode headquarters areopened under on/off control as necessary to let heat out of the top and to provide a negativepressure to help draw air in through any open perimeter windows. Occupants can open themiddle and lower manually controlled lower windows in the facade to suit their individualrequirements, but are asked to shut them when they go home.

If overnight precooling isrequired (i.e. evening officetemperature is over 23 °C)the automatic controls openthe atrium windows and theupper motorised fanlights inthe facade to draw air inover the exposed ceilings.The inlets are banked. If theair temperature in any zonefalls below 18 °C its bank ofwindows are closed andreopened if it rises backabove 20 °C. All automatedwindows are closed at anairspeed of 10 m·s–1 in theday and 20 m·s–1 at night.


— Stepped control, with each opening having a number offixed positions: this simplifies the controller/actuatorinterface, but does not offer such close control as amodulated system.

— Fully-modulating operation: the actuator position isinfinitely variable in response to a control signal.This method can potentially give greater controlbut, in practice, conditions may vary more rapidlythan the control and vents are capable of respond-ing. The best strategy is to keep the controlalgorithms simple. Vent position feedback is stillimportant.

Control may be either ‘open loop’, where a signal is sent toactuators which are presumed to motor to the requiredposition or ‘closed loop’, in which the actuator position ismonitored and fed back to the control system. Open loopcontrol is widely used because it is cheaper, but casestudies(37) have revealed widespread problems withuncertain status and undetected faults. With stepped ormodulating operation, positioning errors can also mountup cumulatively in the course of a day.

As a general rule, the further the automated device is fromindividual occupants, the coarser can be its control. Forexample, it may be acceptable for opening lights, which letheat out at the top of an atrium, to be under on/off orbanked control while control of the inlets will need morefinesse. See also Example 3.2 (page 22).

It is also important to recognise that different zones in abuilding may have different ventilation and/or coolingrequirements, and that these requirements may vary withtime as the use of the building changes. This means thatthe development of the ventilation system and the controllogic will need to reflect such spatial and temporalvariations. As with all aspects of design, there will need tobe a compromise between providing ultimate flexibilityand the costs of so doing.

3.6.5 Control sensors

The specification, type and location of control sensors canhave a major effect on the success of a control strategy. Inparticular:

— Avoid unrepresentative ‘hot spots’ whenmeasuring inside and outside temperatures. Acommon problem is where the sun shines on asensor or warm exhaust air passes over it.

— Cold spots may also be a problem; for examplesensors in the path of incoming outside air orsubject to air infiltration through cable entries.

Direct measurement of pollutants (in particular carbondioxide concentrations) can be effective in controlling airquality but the sensors can be expensive and/or unreliable.Suitable locations for sensors can also be more difficult to findin naturally-ventilated buildings than in common mechanicalventilation extract ducts. An alternative is to use timeschedules or occupancy sensors to activate the ventilation.

3.6.6 Combined user and automated control

Naturally ventilated buildings have frequently sufferedfrom insufficient or inappropriate user control. Automatic

controls have sometimes also opened windows, only to letin noise, fumes, dust, insects and small animals orswitched into draughty night-cooling modes whenbuildings were still occupied.

Sensible integration of user and automated controls istherefore critical to the success of natural ventilation. Thesubject is discussed in general terms elsewhere(39,42).Occupants will always know more about what is going onin their part of the building than will an automated systemand therefore must be allowed to make sensible choices.

User behaviour is governed by six key rules:

— If occupants perceive conditions as ‘good enough’,they will accept them, even if they are not ideal.

— If local conditions are perceived as not goodenough, occupants will wish to take action toimprove them, e.g. by pressing a button, openingor closing a window, or telephoning a help desk.They will rarely act in anticipation of becominguncomfortable. The ensuing action also needs tobe local; one person may want their window openwhile their neighbour may not.

— When users decide that they need to intervene, theactions they take will tend be those which theyfind simplest, most convenient and most acces-sible. The designer’s task is therefore to make theactions that would best suit the design intentionsto be simple, speedy and intuitively obvious to theuser. For example, if the design strategy requires ahigh-level window to be operated first, then itshould be easier to do this than anything else.

— Rapid response is essential. Occupants who areunable to take action to alleviate their discomfort,or unable to tell whether their actions have hadany effect, will become disgruntled.

— If an automated control action (such as windowopening) is perceived to be capricious or to makematters worse, occupants will get angry if they findthemselves unable to countermand it.

— Users will alleviate discomfort but will rarely re-set controls when the need has passed.

For effective involvement, occupants must be informedabout how the building is designed to work, so that thegoals of good comfort, air quality and energy efficiency areall achieved. Ideally, the strategy should be clear,straightforward and, as far as possible, intuitive so thatpeople need to be told what to do only once. The controlrequirement will often change with the context for theuser. For example, occupants sitting beside a window aremost affected by the draughts from the lower casementswhile those in the centre of the room are more likely towant to adjust the fanlights.

Suitable combinations of automated and manual controlwill not only provide backup in the event of BMS malfunc-tions, but will also empower the occupants, improvingtheir adaptive opportunities(43) and making variations inconditions more acceptable than when there are nooverride facilities. The automated system must not usurpcontrol too rapidly after user intervention. As a generalrule, manual override settings should be retained until theend of the day or at least until there is some major changein internal or external conditions. The ideal system will


operate in the background, allowing users to undertakeoverrides as they wish, and will afterwards quietly andunobtrusively restore systems to their most appropriatesafe, comfortable and efficient default states. Simple, non-technical descriptions of the system are extremelyimportant. Such descriptions are a feature of the log bookrequirements of Building Regulations ApprovedDocument L2(5). Guidance on building log books is givenin CIBSE TM31(44).

3.6.7 Control schedules

Many control strategies currently use on/off (open/close)control of inlets and particularly outlets. Better controlcould be obtained where each of the variables are‘scheduled’ over a range to determine the maximum travelof the ventilating device. The control should be based onagreed limits of internal temperature (and perhaps relativehumidity). External wind speed should influence thecontrol strategy only to maintain safety.

Other criteria, such as outside air temperature, rainintensity, wind speed and wind direction, may be recordedfor performance assessment purposes, but are generallynot required for the control strategy. Further tuningshould be carried out on-site to determine the mostappropriate limits for these schedules, based on thedominant factors for the particular site.

3.6.8 Ventilator status indication

As part of an effective strategy, it is important to know, atthe point of local control, the status of the ventilator andthat control action is being undertaken. Window-typeventilators are often used where they are visible, but eventhen it is not always easy to tell whether it is tight shut orpartially open. In such situations, status lamps or otherforms of indication are helpful. Dampers or louvres arefrequently hidden from view. In these cases status feed-back is particularly important but is rarely includedbecause of cost. Status lamps can also be used to adviseusers of the system’s preferred state. For example, if it ishotter outside (or very cold outside), a red light by thewindow can advise the occupants that the system wouldprefer the window not to be opened.

3.7 Installation and commissioning

3.7.1 Installation

The objective is to provide a ventilation system whichmeets the specified requirements and satisfies the users.This requires properly managed resources to be allocatedto the process of constructing a commissionable system.The design project team should be responsible for estab-lishing the commissioning strategy. The system installermust study the strategy and contract documents carefully,so that the strategy and the designer’s requirements arefully understood and unresolved items identified so thatthe commissioning process can proceed.

The development of a commissioning plan, its executionand signing off as being completed satisfactorily is arequirement of Part L2 of the Building Regulations(5).Commissioning specialists, whether employed by the

installer or independently, need to become involved asearly as possible, so that their experience can be applied toplanning and programming commissioning and pre-commissioning tasks. The installer and commissioningspecialist should:

— establish effective lines of communication betweenthem and other parties involved

— review the contract documents to determine therequirements for commissioning, taking nothingfor granted and seeking clarification where neces-sary

— produce a realistic programme with the commis-sioning activities phased alongside the installationprogramme

— regularly review the programme during instal-lation to establish the effect of any modificationsand delays on the planned static completion andpower-on dates, and on any other dates critical tothe commissioning activities

— obtain from equipment suppliers and manufac-turers their latest information for all itemssupplied; standard details which are not modifiedto suit the particular project should always betreated with caution; manufacturers’ literatureshould be checked for installation requirementsadditional to those specified

— progressively record ‘as-installed’ information onat least two sets of drawings: one ‘clean set’, toform the basis for the record drawings and oper-ating and maintenance documentation, and one‘site set’ for use by the commissioning specialist

— establish systematic site control procedures toassist the progressive monitoring of the standardof the installation practices maintained on site

— establish an equipment and materials procurementprocedure which includes an effective means ofchecking each delivered item against the specifiedrequirements

— retain all documents and literature provided witheach delivered item of equipment for use by thecommissioning engineer (and for inclusion in theoperating and maintenance manuals).

If the installation is done carefully, commissioning ismuch quicker and easier, and achieved with greateraccuracy. Installers must therefore:

— ensure that their operatives and supervisors areadequately trained

— give appropriate instructions on good housekeep-ing, workmanship, detailed system arrangements,accessibility and inspection

— ensure that any remedial work has been completedand that all the works comply with the specifiedrequirements before commissioning commences.

During installation, there should be a planned system ofcontinuous inspection and monitoring for correctness,quality and good engineering practice. It can benefit frompro forma sheets which methodically register complianceand monitor the progress of any remedial action. Thesewill help to:


— establish a consistently high standard of workman-ship, and maintain it throughout the contract

— stop defects accumulating which need to be putright before proper commissioning can start

— stop defective work being temporarily hidden,only to resurface during commissioning.

3.7.2 Readiness for commissioning: static completion

To be certified as ready for commissioning, an installationmust have:

— been installed complete and in accordance withthe specification

— been subject to final inspection, with all out-standing remedial works completed

— been successfully tested for air leakage in accor-dance with the specification and/or relevantHVCA and CIBSE publications(24)

— been satisfactorily cleaned in accordance with thespecification

— had all spaces in the vicinity of system equipmentand components requiring safe access for commis-sioning cleared of all obstructions

— been made safe and ready to set to work.

An installation in this state of readiness for commission-ing is said to be statically complete.

In general, the commissioning of BMS control systemsshould be in accordance with BSRIA(45) and CIBSE(46)

codes. The commissioning (setting to work) of a BMS-controlled, naturally ventilated building is comparativelystraightforward. However, fine tuning (which will need togo on for at least one year after handover) can be moredifficult. The initial setpoints for the control system willgenerally be advised by the designers.

Commissioning and tuning a building’s natural ventila-tion system requires the inlet and outlet vent positions tobe manipulated until the system is ‘in balance’. This isbest indicated when there are no untoward draughts onthe occupants under the maximum external wind velocityconditions, no areas of overcooling or overheating, reason-able carbon dioxide concentrations and no perceived airnoise. The difficulty in obtaining this condition is thatvariations in the weather create ever-changing conditionswhich must be incorporated within the tuning process.

3.7.3 Practical completion

In order to demonstrate successful completion of commis-sioning, it will be necessary to simulate summer or wintersensor values or to manipulate the setpoints in order tomimic the summer, winter or some intermediatecondition. If the requirements of the BMS control systemspecification can be proved to the consultant in this way,‘practical completion’ will be possible.

3.7.4 Fine-tuning after practical completion

It is recommended that the designers’ appointments andthe contract include provision for fine-tuning in the year

following handover. Typically this may require three visitsto site in different seasons. How long each visit requireswill depend on the size of the building and the complexityof the system; a small building might need only one dayeach time, while a larger one (say up to 10 000 m2) mightneed a week.

On the visits, the contractor should check system oper-ation, focus on any complaints and fine-tuning problemsreported by the occupants, and implement changes whilststill on site. The contractor must develop a plan for fine-tuning beforehand. This should include a log bookmaintained by both the occupant and the contractor inwhich any reported problems are recorded, with thesystem status and the inside and outside conditions at thetime also being noted. The history built up (which shouldinclude details of any changes made to control strategiesand setpoints) can be very helpful when considering anycontrol alterations which need to be made.

For fine-tuning to be effective, the controls engineer andthe building operator must work together and haveconfidence in each other, so avoiding situations in whicheach manipulates the system in a different way. A commondirection should be decided, with each party notifying theother of changes made to the system, via the log book.

In innovative buildings (particularly those which combineactive and passive systems), contractors, facilities, andmaintenance staff may find it difficult to develop mentalmodels of the behaviour of an unfamiliar system and toidentify clearly what interventions need to be made. Theadded perspective of the designer can often assist discus-sion and lead to more effective resolution of problems. Itis therefore recommended that the designers keep in closetouch with the fine-tuning activities and plan to attendthe site for at least one day in the middle of each fine-tuning visit.

An example of a fine-tuning programme is shown in Table3.3.

4 Design calculations

4.1 Establishing the required flowrates

The first step in designing a natural ventilation system isto determine the airflow quantities necessary to satisfy therequired indoor air quality (IAQ) and thermal comfortcriteria. These flowrates will vary for each season of year.The ventilation system must therefore be designed todeliver the required quantity of air to the right places, atthe right time.

4.1.1 Ventilation for controlling indoor air quality

Ventilation is needed primarily to satisfy indoor airquality requirements. This requirement is therefore thebasis for establishing minimum fresh air rates. The basisfor determining an acceptable average ventilation rate isconsidered in section 2. Any increase in fresh air rateabove the minimum necessary for acceptable IAQ will have

Design calculations 37

an energy penalty when the space requires heating and anenergy benefit when the space requires cooling (except forthose periods when external temperatures exceedinternal).

In some circumstances it is possible to reduce the designrate by making use of the ‘reservoir’ effect of the airwithin the space. This is dealt with in section 4.5.

4.1.2 Ventilation for controlling summer overheating

The estimation of the flows required for summer coolingwill be based on a summertime temperature prediction.This will establish the air flows that are needed (day andnight) in order to limit the peak temperature in the spaceto acceptable limits. There are a number of thermalanalysis methods available which can be used. Eachmethod has a part to play in the design process. In thefollowing list, each successive method is increasinglyflexible in the types of building it can analyse, butincreasingly demanding on the user in terms of the inputdata required.

The methods are:

(a) Design charts based on parametric analysis: theBRE’s Environmental Design Manual(47) is an earlyexample. The user can work only within the rangeof variables covered by the published designcharts.

(b) Manual methods included in CIBSE Guide A(6).Many computerised versions of this type ofadmittance-based technique are available commer-cially. The designer can explore a wide range ofvariables within the constraints of the method.The method is based on a periodic analysis, thepredominant period being 24 hours. This can belimiting for buildings with night cooling andthermal mass, where ‘coolth’ can be stored fromone day to another. Another problem is that theventilation rate is often constrained to be constantfor 24 hours, whereas a night cooling strategyimplies a high ventilation at night and a reducedrate during the day.

(c) Simulation software: these programs, which arebased on the numerical solution of equations ofheat transfer in buildings, can be used withsequences of real weather data (i.e. not restricted torepeating sinusoidal weather patterns). Theseprograms also tend to provide extra features whichmay be required when analysing passive buildings(e.g. those with sophisticated shading devices).They are also able to predict the frequency ofoverheating, rather than just instantaneous peaktemperatures, thus giving a better indication ofoverall comfort. Despite these advantages, itshould be stressed that the data input require-ments for such models can be demanding andexperience is required to obtain reliable results.

Users of all types of model must first satisfy themselvesthat the chosen technique is appropriate to the particulardesign task. CIBSE Applications Manual AM11(48) givesguidance in this respect.

4.1.3 Data requirements

To use any of the above techniques, the designer will needto assemble a set of input data covering:

— building description: glazing ratios, orientationetc.

— schedules of internal heat gains (see section 2.1.2)

— weather data (see below)

— ventilation strategy: this will include an under-standing of:

(a) the intended air flow path, i.e. is fresh airentering the zone at the ambient externaltemperature, or is the air being transferredfrom another internal zone?

(b) the timing of the ventilation flow (e.g.daytime only, night ventilation, purgingwith short periods of high ventilation etc.)With natural ventilation, constant venti-lation rates are unlikely to exist, evenwithin a single operating period, due tovariations in wind speed and inside/outsidetemperature differences.

Using these input data, the calculation will predict thepeak internal temperature. The designer must then adjustthe input parameters through an iterative process untilsatisfactory comfort conditions are achieved, see section2.1.2. Computer-based methods can be useful inidentifying the main heat gain likely to cause overheating

Table 3.3 Example fine-tuning programme

Visit Actions to be carried out

First visit Before fine-tuning can begin, all control points on the system should be checked to ensure that they are working properly. All ventilation devices should be examined to ensure that they close tightly, do not let in rain, and their opening/closing mechanisms are in sound condition and not obstructed. A spot check of thecalibration of relevant sensors should be carried out. Trend logging of relevant points should be set up.

Note: it is often easiest to assess the performance of the system and to identify any important problems if the control strategies are simple to start with, even if this means temporarily disabling some of the more advancedfeatures. Similarly, it can help to start looking at only basic monitored data (though it may well be worth collecting extra data for later analysis)

Second visit The data from the monitoring set up on the first visit should be analysed. This will give an initial indication of the performance of the system and its controls, and the need for any fine-tuning. Set-points and control algorithms may need adjusting, but take care in making adjustments if there is not enough data to make a firm assessment of the situation.

Third visit Tuning of other variables should be concentrated upon. For example, is ventilation system working effectively with the current wind speed and direction set-point or isit being limited unnecessarily?

Completion A report detailing the ventilation system’s performance and associated controls should be produced, incorpo-rating information covering the operation of the system,set-points, example graphs and details of changes made. The O&M manual will also need updating to take account of the changes in strategies, schedules and set-points which have been undertaken.


(it may be high solar gain or an internal gain in a par-ticular area). Careful analysis of the results can direct thedesigner to the key design issues that need to beaddressed.

The above analysis will give a picture of the performanceof the building under ‘design’ weather conditions.However, some clients may wish to know how thebuilding will perform throughout the year. This requiresmultiple analyses using representative weather data fordifferent proportions of the year(25). For greater accuracy,an hourly dynamic simulation for all 365 days of arepresentative year can be carried out.

Having obtained a set of parameters that are capable ofachieving the comfort conditions required by the brief,the next stage is to size the ventilation openings togenerate the required air flow. This stage of the process isdiscussed in section 4.3.

4.1.4 Weather data

The design of natural ventilation systems uses weatherdata in two different ways:

— as a means of establishing the required ventilationrates (day and night) that will be required tomaintain acceptable thermal comfort

— as a means of establishing the driving forces thatwill drive the ventilation.

These two uses of weather data are considered below.

4.1.4.1 Weather data for thermal comfort analysis

It is important that the analysis of summer comfort isbased on an external climate that is related to an agreeddesign risk. The different types of model described insection 4.1.2 require different types of data.

The admittance procedure described in section 5 ofCIBSE Guide A(6) is based on analysis of a cyclic repeatingday. Table 2.19 of Guide A gives maximum daily designtemperatures for different degrees of risk, i.e. 2%, 1% and0.4% exceeded. This would need to be combined withinformation on the diurnal range to establish the profile ofdaily temperatures. (Guidance is given in section A5.8 ofCIBSE Guide J(25)). Coincident wind speed data can alsobe established from information in Guide J. Oneimportant issue is that maximum overheating may notoccur when outside temperatures are highest. If theoverheating is primarily driven by solar gain, shadingdevices that work well in July can be less effective in, say,September when the sun angle is lower.

Simulation programs use weather data sets containinghourly records of the primary weather parameters (dry andwet bulb temperatures, direct and diffuse radiation, windspeed and direction etc.). These data sets can be based onreal weather sequences or synthesised data which areprocessed to represent some form of ‘typical’ year. TheCIBSE recommended weather sets are:

— Test Reference Year (TRY): which is a synthesisedtypical weather year suitable for analysing energyuse and overall environmental performance(25).

— Design Summer Year (DSY): which is an actual yearcontaining a near-extreme summer, and is the onerecommended by CIBSE for use in overheatingassessment(49).

Figure 4.1 shows the difference between the TRY and DSYfor the three locations. This is given in terms of thefrequency with which a given temperature is exceededduring the year for which CIBSE has recommended datasets.

The increased severity of hot weather with latitude isapparent from Figure 4.1. These data are based on ananalysis of the 20-year period from 1976 to 1995. It is nowaccepted that global warming is a reality and estimates ofthe impact of climate change have been made in terms of arange of possible scenarios(50). Figure 4.2 shows someexamples of the anticipated increase in summer tempera-tures. (The examples show the range of scenarios but notall individual scenarios.) This likely trend is exacerbatedby another aspect of the climate change predictions,namely the increased incidence of very hot days. Forexample, the frequency with which the temperatureexceeds 30 °C is expected to increase by a factor of eight by2080 for the Medium-High scenario. Coupled with thesummer temperature increase is an increase in solarradiation, particularly in the southern parts of the UK. Itis predicted that this will be coupled to a reduction insummer cloud cover. This means that the increase in solarradiation will be seen as increased direct beam rather thandiffuse radiation. Because of its directionality, beamradiation is easier to control using external shadingdevices and/or selective location of window position. Thisadds to the need for improved guidance on solar controlmeasures (see section 2.1.2).

Freq

uenc

y ex

ceed

ed /

%

2·5

2·0

1·5

1·0

0·5

0·03515 20 25 30

Dry bulb temperature / °C

Edinburgh DSYEdinburgh TRYManchester DSYManchester TRYHeathrow DSYHeathrow TRY

Figure 4.1 Frequency of extreme temperatures for TRY and DSY

Figure 4.2 Examples of possible increases in mean summer temperaturedue to global warming

Mea

n te

mpe

ratu

re c

hang

e / K

6

5

4

3

2

1

0SE low SE high NE low NE high

2020 2050 2080


4.1.4.2 Weather data for sizing ventilation openings

The sizing methods explained in this section are based onsteady state calculation methods. It should be appreciatedthat the purpose of the calculations is to size the openingsto deliver the ventilation flows that the thermal analysishas shown to be needed. That ventilation rate is notrequired 100% of the time. For example, with stack drivenmechanisms, as the inside and outside temperaturesequalise, the driving pressures will approach zero and, nomatter how large the opening, the flows will be very small.However, as shown in Figure 4.3, even if a flow of 5 airchanges per hour could be maintained, the cooling effectwould be very small once the temperature difference dropsbelow about 3 K.

Consequently, typical practice is to size openingsassuming an inside-to-outside air temperature differenceof about 3 K in order to set a practical upper limit for thesize of the ventilation openings. As the air temperaturesequalise, the achievement of comfort is more reliant onradiant cooling from the thermal mass (and by usingshading devices to protect from direct solar gain) than onventilation cooling.

It will be important to test that the design and the controlsystem are able to maintain adequate comfort duringtypical weather sequences. Simulation models using DSYweather data can then be used to simulate the operatingperformance of the building and its robustness to userbehaviour.

Selecting the wind conditions for sizing openings is agreater challenge than specifying temperatures because itis necessary to specify the wind speed that is used in thedefinition of wind pressure coefficient. This problem isdealt with in section 4.4.1.

4.2 Selecting a ventilation design tool

Having determined the flowrates that the natural ventila-tion system will need to deliver, the next stage is to sizethe components of the ventilation system so that the flowswill be delivered. For example, the summer design

condition may determine the maximum sizes of theopenings such as windows, whereas the winter designcondition may determine the minimum sizes of openingssuch as trickle ventilators. The control system (manual orautomatic) will then need to adjust the opening sizebetween these limits depending on the prevailing ventil-ation need. Once the initial sizing has been done, thedesigner may wish to examine off-design conditions andto determine more details of the flows and temperaturesinside the building.

There is a range of design tools available, which can besummarised under the following headings:

— envelope flow models

— computational fluid dynamics (CFD)

— combined thermal and ventilation models

— physical scale models.

The design process is progressive and iterative, differenttools being more suited to different stages in thedevelopment of the design. Envelope flow models are thesimplest tool and are recommended for the initial sizing ofopenings at the chosen design conditions. The other toolsprovide more detailed information and are usually moresuited to later stages in the design.

The various types of design tool are briefly describedbelow, with guidance on their most suitable range ofapplications. An extensive description of envelope flowmodels and physical models, with a comparativediscussion of CFD can be found in Building ventilationtheory and measurement(51). More detailed descriptions ofCFD can be found elsewhere(52,53). Because of theirrelevance to the initial stages of design, envelope flowmodels for single-cell buildings are described in detail insection 4.2.1 and worked examples are presented insection 4.3.

4.2.1 Envelope flow models: single-cell

Envelope flow models solve the equations that govern theflow of air through openings in the envelope of a building.They rely on assumptions about the internal density (tem-perature) distribution. The basic equations for a single-cell can be illustrated by considering an opening in theenvelope.

4.2.1.1 Basic equations for an opening

It is common practice to express the relationship betweenthe flow rate through an opening and a pressure differenceacross it by means of the discharge coefficient and aspecified geometric area:

(4.1)

where i identifies the opening, qi is the flow rate throughthe opening (m3·s–1), Cd i is the discharge coefficient (—),Ai is the area of the opening (m3), ∆pi is the pressuredifference (Pa) and ρ is the air density (kg·m–3).

The discharge coefficient is defined and measured understill-air conditions with uniform density with the flow

q C Ap

i i ii= d

2 |∆ |ρ

Sens

ible

coo

ling

/ W·m

–2

60

50

40

30

20

10

0

650 1 2 3 4Temperature difference / K

2·5 ACHVentilation rate

5 ACH10 ACH

Figure 4.3 Effect of temperature difference and air change rate on sensiblecooling capacity of ventilation air (height of space taken as 2.75 m)


being generated by a fan. When the flow is generated by adensity difference (i.e. in the absence of any wind effects),it has been shown(51) that the pressure drop in equation 4.1is given by:

∆pi = PE0 – PI0 – ∆ρ0 g zi (4.2)

where PE0 and PI0 are the external and internal hydrostaticpressures respectively at ground level (Pa), ∆ρ0 is thedensity difference at ground level (kg·m–3), g is thegravitational force per unit mass (m·s–2) and zi is theheight of opening i above ground level (m).

The density difference is defined by:

∆ρ0 = ρE – ρI (4.3)

where ρE and ρI are the densities of the external andinternal air respectively (kg·m–3).

Equation 4.2 is quite general. It applies to an openingaligned in any direction, irrespective of whether the flowis inward or outward, and whether the inside temperatureis higher or lower than the outside temperature. However,it is important to note that the height of the opening, zi, isthe height where the flow leaves the opening, i.e. theheight of the outlet. This follows from the outlet boundarycondition, namely that the pressure at the outlet isdetermined by the pressure of the surroundings, and theassumption that the temperature of the air remainsunchanged as it flows through the opening. This meansthat for a long opening in the vertical direction, such as achimney, zi will change with the flow direction, see Figure4.4. For most openings, such as windows and air vents, ziwill not change with flow direction.

If the density of the internal air varies with heightequation 4.2 becomes(51):

(4.4)

Equation 4.4 allows non-uniform density distributions (ascan occur with atria and chimneys, for example) to bedealt with.

4.2.1.2 Effect of wind

When the wind blows the external pressure around theopening is changed by the pressure due the wind, pw i ,

∆p P P g z g zi i

zi

= − − + ∫E I E I

0

d0 0 ρ ρ

which adds to the hydrostatic pressure and the pressuredrop equation becomes(51):

∆pi = PE0 – PI0 – ∆ρ0 g zi + pw i (4.5)

where pwi is the pressure of the wind at the point on theenvelope at which opening i lies (Pa).

It is common practice to obtain pw, i from wind tunnel testswhere wind pressures are quoted in the form of thepressure coefficient:

pw i – prefCp i = ———— (4.6)

0.5 ρ U2

where pref is a reference pressure (Pa) and U is the windspeed (m·s–1).

Thus the pressure difference across an opening whoseinlet or outlet is situated in the external flow can bewritten as:

∆pi = PE0 – PI0 – pref – ∆ρ0 g zi + 0.5 ρ U2 Cp i

(4.7)

The treatment of wind effect involves several assumptions.For example, the size of the opening is assumed to besmall enough for the value of pwi obtained from a pointmeasurement on a wind tunnel model to be appropriate.This is reasonable for small openings such as air vents, butmay be less so for open windows. An associated assump-tion is that the external wind flow around the openingdoes not affect the discharge coefficient of the opening.This is more reasonable when the outlet of the opening isexposed to the wind (i.e. outward flow from the space)than when the inlet is exposed. None of these assumptionsis needed for the treatment of ventilation due to densitydifference alone and envelope models are more accurateunder this condition.

4.1.2.3 Conservation of mass for the envelope

The principle of mass conservation applied to the fixedvolume defined by the envelope gives a relationshipbetween the flow rates of the openings:

∑ ρi qi = 0 (4.8)

i.e. the nett mass flow into the building is equal to zero. Inthe context of design procedures it is an acceptableapproximation to ignore the differences between densitiesso the equation can be simplified to ∑ qi = 0.

4.1.2.4 Solution of the equations

An envelope flow model thus consists of the followingthree basic equations. For the envelope:

∑ qi = 0 (4.9)

and for each opening:

(4.10)q C A Sp

i i i ii= d

2

0

|∆ |ρFigure 4.4 Single-cell envelope illustrating definition of zi

PI0PE0

qi

Zi

IρEρ

Zi

Zi


and:

∆pi = ∆p0 – ∆ρ0 g zi + 0.5 ρ0 U2 Cp i (4.11)

Note: in equation 4.10, it is important to include the signof the pressure difference, which has been introduced intoequation 4.10 by the symbol ‘S’; the convention used hereis that S = +1 for flow entering the space and S = –1 forflow leaving the space.

Equation 4.11 is the same as equation 4.7 except that (PE0 – PI0 – pref) has been denoted by ∆p0 for brevity.Although the bracket encloses three terms, it is only theinternal pressure PI0 that is a variable; PE0 and pref areconstant for a given analysis.

For the general case of N openings there are 2 N+1equations. The unknowns are qi , ∆pi and ∆po. Theequations can be solved by determining the value of ∆po atwhich the continuity equation is satisfied. In physicalterms, when the ventilation pattern of a building ischanged from one steady state to another (e.g. by openinga window), the internal pressure adjusts until the flowsthrough the openings are such that the continuity equa-tion is again satisfied. Mathematically this adjustment isdone by an iterative procedure. When the equations aresolved in this way it is referred to as an implicit method.

The equations can be solved directly (i.e. without the needfor iteration) by specifying the value of ∆po and the valuesof qi (magnitude and direction) to find the values of Aiwhich will give that particular flow pattern. When solvedin this way, the model is referred to as an explicit method.This method is particularly useful in the initial designstages for sizing openings, such that the openings give therequired flow rates under a specified design condition.Worked examples are given in section 4.3. Using theseareas, the implicit procedure can then be used for off-design calculation of flow rates.

The explicit procedure is easy to use for openings with aconstant discharge coefficient, i.e. one that does not varywith flow rate (Reynolds number). Most purpose-providedopenings fall into this category and manual calculationsare not difficult, as illustrated by the worked examples insection 4.3. The explicit procedure is not appropriate foradventitious openings (see section 4.4.4) because suchopenings cannot be sized in the same way as air vents andwindows. Implicit methods are more appropriate fordealing with adventitious openings because the effect ofassumptions about the size and distribution of the adven-titious leakage can be quickly assessed. However, a meansof accounting for adventitious leakage with the explicitmethod is given in section 4.4.4. Although envelope flowmodels can account for adventitious openings, this is notfeasible with physical models, see section 4.2.7, because ofthe very small dimensions of the openings at model scale.Nor is it practically feasible with computational fluiddynamics (CFD), because of the need to specify the detailedgeometry of a large number of small openings. Most CFD

packages will allow special inlet and outlet flows to bespecified as boundary conditions and adventitious open-ings could be dealt with in this way.

Envelope flow models are best suited for dealing withopenings with unidirectional flow. For large openingssuch as windows it is possible for bi-directional flow to

occur and this is more difficult to treat, although there areways of doing so(51).

4.2.2 Multi-cell envelope flow models

Multi-cell envelope flow models are the general case ofsingle-cell envelope flow models. They include the effectof the internal partitions, not only in the sense thatpartitions can increase the resistance to flow through thebuilding, but also in the sense that they allow temperaturedifferences between rooms to be defined. The equationsthat are solved and the assumptions that are made are inmost respects the same as for single-cell models. Zonalmodels are a step further than multi-cell models in thatthe rooms are further divided into a number of zones.They do not offer any real benefits to the design of naturalventilation, unless they form part of a combined thermaland flow model.

4.2.3 Unsteady envelope flow models

There are more advanced envelope models that canaccount for the variation of external wind pressures withtime due to wind turbulence. They predict the instan-taneous flow rates when the external pressure fluctuationsare specified. Under most circumstances the errors inventilation rates arising from a neglect of unsteady effectswill be small and can be neglected(54). However, unsteadyenvelope models can be useful in the design of ventilationchimneys in order to avoid flow reversal.

4.2.4 Non-dimensional graphs

When results of theoretical calculations or experimentalmeasurements are expressed in non-dimensional form,they can have much wider application. Wind tunnel dataare often presented in this form, because a small numberof tests at one condition can then be applied to many otherconditions. One example of this is the use of the pressurecoefficient, which can be used for any wind speed.

Although not strictly a distinct design method, graphsgenerated from envelope flow models allow quickcalculations to be made(54) and may be useful at any stageof a design to give a quick estimate of proposed changes.Worked examples are given in sections 4.3.5 and 4.4.4.

4.2.5 Computational fluid dynamics (CFD)

Whereas envelope flow models solve the equations ofmotion for the bulk flow through openings, CFD solves thefundamental equations of motion for individual elementsof fluid at all points in the specified space. Of all thedesign tools, it is unique in the sense that it predicts theair motion at all points in the flow. By including thermalequations, CFD also predicts the effects of buoyancy andthe temperature field. CFD can address questions such asstratification and local air movement. Therefore themethod is particularly applicable to large spaces such asatria, although such flows may be inherently unsteady andthe use of a steady CFD solution may not be realistic.Because the computations are extensive, CFD is moresuited to generate ‘snapshots’ of how the design wouldwork at a given moment, rather than for the initial sizingof openings.


The applications of CFD to the design of naturally venti-lated buildings include:

(a) calculation of velocity and temperature fields inrooms and single-cell buildings

(b) calculation of velocity and temperature fields inmulti-cell buildings

(c) calculation of surface wind pressure distributionson buildings

(d) whole-field calculations, i.e. the combination of (a)and (c)

(e) calculation of flow through components, e.g. dis-charge coefficients of chimneys.

The first application is now quite common. Figure 4.5 isan example that illustrates the use of CFD to airtemperature in a building with top-down ventilation(55).CFD is generally used for more complicated cases but thisexample has been chosen because it can be compareddirectly with the results of the saltbath technique shownin Figure 4.7 (page 44).

Commercial CFD software is widely available. Most soft-ware packages consist of three distinct parts:

— Pre-processor: where the user sets the boundaryconditions for the solution, including the specifi-cation of the computation grid and the number ofcells. The number of cells is likely to be of order10 000 for simple cases such as a small room and oforder 1 000 000 for large buildings.

— Solver: which carries out the numerical solution.This is an iterative procedure, which continuesuntil convergence is obtained, i.e. a stable solutionis reached with error terms below pre-definedlimits. Solution times can vary from a few hours toseveral days, depending on the complexity of theboundary conditions, the number of cells and thecapacity of the computer. The onus is on the userto assess whether convergence has been satis-factorily achieved. This is not necessarilystraightforward, partly because the criteria used bythe software may not relate to the part of the flowthat is of interest and partly because instabilitiescan occur.

— Post-processor: where the results are presented in asuitable form. Since the solution generates vastamounts of data it is important to be able to

present it in a meaningful way. One common formof output is the contour plot where parameterssuch as velocity and temperature that lie in certainranges are identified by different coloured areas.This type of flow visualisation can be helpful forpresentations to clients.

It is important to appreciate that the solution obtainedwill depend to a greater or lesser extent on the followingparameters:

— Boundary conditions: specifying the boundaryconditions for a calculation is not a trivial matter.Even for a relatively simple case of a single roomwith wind alone, the velocity conditions at eachopening need to be specified, which is not alwayseasy or obvious. If heating or cooling is involved,the thermal boundary conditions at the surfaces ofthe room must also be specified and this is evenmore difficult. Inevitably there will be uncertain-ties in specifying boundary conditions and thiswill be reflected in uncertainties in the CFD results.The sensitivity of the results to the conditions maybe determined by carrying out specimen calcula-tions. Although software packages include adviceon specifying boundary conditions, the user needsexperience of the conditions that occur in practice.It is possible to specify boundary conditions forwhich there is no converged solution or for whichthe solution is unstable; such problems are morelikely when buoyancy effects are significant(56).

It is normal practice to carry out steady calcula-tions, i.e. the boundary conditions do not varywith time, and it is implicitly assumed that thecalculation will converge to the correct steady statesolution. This assumption is not necessarily validbecause with complicated flows involving buoy-ancy there may be more than one steady solution.In principle it is better to carry out an unsteadycalculation in which the full flow field is specifiedat time t = 0 (i.e. the initial values of velocities etc.at all points) and the remaining boundary con-ditions are specified as functions of time. Suchcalculations are extremely demanding and arerarely carried out. Nevertheless, when looking atthe fine detail of a steady solution it should beremembered that such a solution may well be asimplification of reality.

— Calculation grid: most software packages reduce theamount of effort required to generate an appro-priate calculation grid. However, with complicatedand/or large building geometries, skill is requiredto obtain a grid that minimises computation timeand gives the required precision in the areas ofinterest. Clearly the results should be independentof the chosen grid and the number of cells and as ageneral rule this should always be checked.Examples of the problems of convergence, griddependency and stability that can arise with CFD

are discussed elsewhere(57). It should be noted thatenvelope flow models can also suffer from conver-gence problems, due to inappropriate numericalconvergence criteria, the existence of more thanone solution to the equations or to the presence ofvery large openings with very small pressuredifferences.

Figure 4.5 Example of CFD results for air temperature; cool air entersthrough the chimneys and leaves through the atrium


— Assumptions inherent in the equations: although CFD

solves fundamental equations, it still relies onassumptions and approximations. The main onesrelate to the properties of turbulence and to heattransfer, particularly close to surfaces. CommercialCFD codes usually offer a range of turbulencemodels and the user can investigate the effect ofchanging the model. Turbulence models are stillthe subject of considerable research and develop-ment. For example, it is now generally recognisedthat the large eddy simulation (LES) model isbetter than others for predicting pressure distribu-tions on simple buildings, because it gives betterprediction of flow separation. However the calcula-tion of wind pressures on complex arrangements ofbuildings that are routinely measured in windtunnels is a major challenge and the use of CFD as avirtual wind tunnel is still open to question.

If CFD is used purely for qualitative flow visualisation (e.g.demonstrating flow patterns to clients), the uncertaintiesassociated with boundary conditions, grid size andturbulence model are probably not important, providedconvergence is obtained and the results are not griddependent. However, if quantitative results are important(e.g. meeting certain temperature requirements), theinexperienced user may well be advised to consult experi-enced users. This is particularly true when buoyancy andheat transfer effects are included, since the uncertaintiesare more likely to be significant.

4.2.6 Combined thermal and airflow models

One of the difficulties in designing natural ventilationsystems is the estimation of internal temperature distribu-tion. Especially in summer conditions, the temperature ofeach space will depend on the ventilation rate, which willitself depend on the temperature distribution, particularlywhen using buoyancy-driven strategies.

In principle this difficulty can be overcome by combininga ventilation model (envelope flow or CFD) with a thermalmodel. Ideally the two models would be completelyintegrated such that the governing equations are solvedsimultaneously. A simpler strategy is to solve the twomodels separately, with some form of link between theirsolutions. Examples of this strategy and the sort ofproblems that can arise can be found elsewhere(56,58,59).

There is a class of relatively simple model that combinesan envelope flow model with the internal flow associatedwith buoyant plumes, leading to simultaneous predictionof ventilation rate and temperature stratification withinthe space. Although capable of giving good quantitativeagreement with simple physical models, the assumptionsmade about the internal air motion and heat transfer meanthat this type of model is not at present suitable forquantitative design.

4.2.7 Physical models

Although the majority of ventilation design is carried outusing mathematical models, there is a place for physicalmodels.

4.2.7.1 Wind tunnel testing

Environmental wind tunnels, such as that shown inFigure 4.6, are the main source of information on windpressure coefficients, see section 4.4. The results ofmeasurements are available as design data for genericbuilding forms; alternatively, bespoke measurements canbe made for individual building designs. Bespoke meas-urements may be required even for simple building shapesif there are specific local obstructions (e.g. a nearby high-rise building) or an unusual site layout.

Wind tunnels can also be used for measuring directly theventilation rate of a building, subject to certain limitationson the size of the building and the openings. The buildingneeds to be large enough such that the volume of theenvelope at a model scale of, say, 1/100 allows a tracer gasmeasurement inside the model. Similarly the full-scaleopenings need to be large enough for them to be accu-rately modelled. For example, an open window can beadequately modelled at 1/100 scale, but a domestic air ventmay not be (due to dependence on Reynolds numbercausing errors in applying the results to full-scale).However, if the requirements can be met, wind tunneltesting offers relatively high accuracy in the determinationof ventilation rates due to wind because the effects of windturbulence are inherent in the measurements. It is alsopossible to determine ventilation rates under the com-bined effect of wind and buoyancy. Simple scaling lawscan be applied to calculate the full-scale ventilationrate(54).

Wind tunnel testing can also be used to optimise theperformance of specific ventilation openings, such aschimneys or wind towers. This may take the form ofsimply measuring wind pressures at the outlet of theopenings to ensure that they are negative. It is nowpossible to measure instantaneous flow rates in modelchimneys and thereby determine directly how susceptiblethe chimney is to flow reversal(60).

4.2.7.2 Modelling with liquids

The simulation of air motion in rooms with liquids hasbeen employed in research for many years. The directapproach of using heated water is relatively simple inpractical terms but, as with all scale modelling techniques,the conversion of model results to full-scale conditions

Figure 4.6 Environmental wind tunnel with building model


can be difficult. The saltbath method(61) is a relativelyrecent development and is primarily used for testingbuoyancy-driven ventilation strategies. A small scalemodel of the envelope is immersed in a transparent bathcontaining saline solutions of different concentrations thatsimulate the temperature (density) differences.

The technique is useful for giving the non-technicalmembers of the client and design teams a good visualisa-tion of airflow behaviour. The flows can be visualisedeither by direct dye injection or by shining light throughthe transparent model, the variation in the refractiveindex of different salt concentrations causing shadows tobe cast on a translucent screen. Quantitative estimates ofthe specific temperature distribution can be made bymeasuring the solution concentration using a conductivityprobe. However it should be noted that heat transferboundary conditions at surfaces are not completelysimulated, and therefore local effects, such as solarpatching on the floor of an atrium, cannot be dealt with.Figure 4.7 shows top-down ventilation in a two-storeybuilding connected to an atrium(61). The blue regionsindicate regions of cool air and the orange and red regionsindicate where the warm air gathers in the space.

4.2.8 Application of tools

Table 4.1 gives a broad indication of where each designtool might be used in the various stages of a project.

4.3 Design procedures using envelope flow models

It is assumed that the basic configuration of the buildingand the intended flow strategy have been agreed (seesection 2.2). In particular, it is assumed that the requiredflow rates through openings have been quantified (in mag-nitude and direction) under specific wind and temperatureconditions. The next stage is to size the components of theventilation system so that the flows will be delivered.Procedures for the initial sizing of openings based onexplicit envelope flow models are illustrated below. This isdone for a summer condition, but exactly the sameprocedures can be applied to winter conditions. Beforecarrying out the calculations there are several preliminarysteps:

— Step 1: identify isolated rooms and spaces: these arerooms that can be considered to be isolated fromother parts of the building (in terms of theirventilation) and can therefore be treated by verysimple explicit envelope flow models. To fallwithin this category the openings in the internalpart of the envelope need to be much smaller thanthe openings in the external part of the envelope.This can often be the case for the summer designcondition, but it is less likely to be valid in winter.Nevertheless the designer may wish to size theopenings on this basis for both design conditions.The two basic cases are single-sided ventilationand crossflow ventilation, as described in section4.3.1.

— Step 2: determine which parts of the building can betreated as a single space (i.e. a single cell): in a singlecell all the rooms are connected by openings thatare much larger than the openings in the externalenvelope. With such buildings it is necessary toconsider all openings simultaneously when sizingthem. This can be done with explicit single-cellenvelope flow models, as described in section 4.3.2.If the building falls into the category whereinternal openings are neither very large nor verysmall in relation to the external openings, it may

Figure 4.7 Example of a saltbath test; corresponding CFD results areshown in Figure 4.5

Table 4.1 Uses of design tools

Project stage Tool Comments

Initial concept Explicit envelope flow models Use for initial sizing of openings at design conditions

Optimisation Implicit envelope flow models, single Use for looking at off-design conditions to test the robustness of and/or multi-cell the design

Detailed analysis of ‘special’ Computational fluid dynamics (CFD) Use for determining details of velocities, temperatures and comfortspaces and components parameters inside the space

Saltbath Use where stratification is a particular issue

Wind tunnel Use where wind effects are important, e.g. for siting of stack outlets

Performance assessment Linked thermal and flow models Use for overall seasonal/annual performance assessments

Data generation Wind tunnel Use for determining pressure coefficients

Computational fluid dynamics (CFD) Use for determining pressure coefficientsUse for determining discharge coefficients


be desirable to make use of multi-cell implicitenvelope flow models. However, in practice itwould be simpler to size the openings using anexplicit model and then use an implicit model tocheck the effect of the internal openings.Alternatively, the openings could be sized for thetwo cases (single-cell and isolated spaces) and thelarger values chosen (assuming that the buildingwill operate in both modes).

— Step 3: specify the input data required to obtain asolution: this will include the positions of theopenings, their discharge coefficients and therelevant wind and temperature conditions (seesection 4.4).

— Step 4: size the openings: using the relevant pro-cedure (see sections 4.3.1, 4.3.2, 4.3.3 and 4.3.4).

— Step 5: check practicality of the sizes of openings:checks should include:

(a) What proportion of each facade does theopening represent? In the winter designcase, the Building Regulations recommendcertain minimum areas of trickle venti-lator. Depending on the design of thebuilding, the proposed area may be greateror less than the recommended value. If theproposed area of trickle ventilation is less,the designer will need to demonstrate thatthe required air flows can be delivered,possibly by taking account of adventitiousopenings.

(b) What is the velocity of the air through theopening and the implication for draughts?

(c) How sensitive is the design to operationalmisuse? (For example, if internal doors areshut in a cross flow design, are transfergrilles necessary?)

If checks such as these identify shortcomings in thedesign, revised parameters must be substituted in theequations and the viability of the amended design re-assessed.

4.3.1 Isolated rooms and spaces

A simple example of a building with isolated spaces androoms is shown in Figure 4.8. On the ground floor, therooms are isolated and separated by a corridor. The uppertwo floors are open plan, but isolated from each other.

Four common cases are shown below with the appropriateequations for sizing the areas. Cases 1, 2 and 3 areexamples of single-sided ventilation. The ground floorrooms in the building shown in Figure 4.8 rely purely onthis form of ventilation. Case 4 is an example of crossflowventilation. The two upper floors rely on this form ofventilation when wind is dominant but on what isbasically single-sided ventilation when buoyancy isdominant (i.e. case 1 or 2 applied to each wall). Cases 1and 4 are simple examples of envelope flow models. Cases2 and 3 rely more on empirical data.

Case 1: Single-sided, two vents, buoyancy driven

This case represents single-sided ventilation through twoidentical vents driven by buoyancy alone, see Figure 4.9.

The area A of each opening required to give a ventilationrate q for a specified value of h is:

(4.12)

where A is the area of each opening (m2), q is theventilation rate (m3·s–1), Cd is the discharge coefficient, TIis the internal temperature (°C), ∆T is the differencebetween the internal and external air temperatures (K), gis the gravitational force per unit mass (m·s–2) and h is theheight between the openings (m).

A typical value for Cd is 0.6.

Aq

C

T

T g h=

+

d

I( )∆

273

Figure 4.8 Illustration of isolated rooms and spaces

q A

A

q

h

∆T

Figure 4.9 Case 1: single-sided ventilation, two identical openings,driven by buoyancy alone

Case 2: Single-sided, single vent, buoyancy driven

This case represents single-sided ventilation through anopen window driven by buoyancy alone see Figure 4.10.

q

A

q

h

∆T

Figure 4.10 Case 2: single-sided ventilation, single opening, driven bybuoyancy alone


The area required to give a ventilation rate through asingle opening of area A is again given by equation 4.12but in this case h is the height of the single opening ratherthan the distance between openings.

Case 2 is basically the same as case 1, but the value of Cd istypically 0.25. This is lower than the value in case 1 partlybecause only half the area A is available for air entry.

Case 3: Single-sided, single vent, wind driven

Case 3 represents single-sided ventilation through an openwindow, driven by wind alone see Figure 4.11.

(4.14)

where A is the total ventilation area (each wall) (m2) and∆Cp is the difference between the wind pressure coeffi-cients Cp1 and Cp2.

Note: in cases 1 and 4 it is assumed that the openings areidentical; different equations will apply if the openingshave different areas or discharge coefficients (see theexamples in CIBSE Guide A(6) and BS 5925(62)).

4.3.2 Single-cell buildings, uniform temperature

Examples of single-cell buildings are given in Figure 4.13.In case 5, the spaces are all connected to an atrium, theaim being to induce fresh air into all of the occupiedspaces with all stale air passing out through the upperoutlet. The aim is the same for case 6, but the spaces areconnected to a large stack. As far as the explicit model isconcerned they can be treated in the same way.

Case 7 is similar to case 6, but here the space is connectedto a well-defined chimney. The difference between cases 6and 7 is that the stack in case 6 is wide open to theoccupied spaces. Thus the temperatures within the stackand the space will tend to be the same at any given height.Also the resistance to flow of the stack can be neglectedrelative to the resistance of the outlet. In case 7 thechimney is closed to the surroundings, except at the inlet,and has narrow dimensions. This means that the tempera-ture within the stack can be considerably different to thatin the room (e.g. solar-powered chimney). The chimneywill support density differences in the horizontaldirection, but the stack will not. It also means that thewhole chimney must be considered when evaluating itsdischarge coefficient.

Case 8 is an example of a space ventilated by twocombined chimneys. For any given wind direction, oneacts as an air inlet and one as an air outlet.

Worked examples using the explicit method for cases 6, 7and 8 are given below. (The procedure for case 5 is exactlythe same as for case 6.) It is initially assumed that thetemperature is uniform throughout the building. Therelevant properties of the building are given for eachexample. The calculations can be carried out by hand butthey are ideally suited to a spreadsheet*.

Note: the results of the calculations are presented here to aprecision of four or five significant figures. This is done toallow the user to check their calculations. For an actualdesign, there is little justification for calculating the openareas to more than two significant figures. For the workedexamples given here, the density differences ∆ρ0 and ∆ρchave been evaluated using equations 4.33 and 4.34. Toreproduce the present results it is necessary to use thesame equation although, as noted in 4.4.2, the equationsare approximate.

A q C UC

=

−

dp∆

2

1

Figure 4.11 Case 3: single-sided ventilation, single opening, driven bywind alone

q

A

q

The area required to give a ventilation rate for a specifiedvalue of wind speed is:

A = q / C U (4.13)

where U is the wind speed (m·s–1).

The value of the coefficient C depends on (a) the geometryof the opening, (b) the position at which the referencewind speed is measured and (c) the flow field around thebuilding. Reported values range from about 0.01 to 0.05when U is measured at the same height as the building(51).The value is low because the air entry arises as a result ofturbulent diffusion.

Case 4: Crossflow ventilation,wind driven

In this case, ventilation is by wind alone see Figure 4.12.

The ventilation area required to give a ventilation rate qfor a specified value of ∆Cp is given by:

A / 2

Cp2Cp1

q / 2

U

A / 2

q / 2

Figure 4.12 Case 4: crossflow ventilation driven by wind alone* A suitable spreadsheet may be downloaded from the CIBSE website(www.cibse.org/venttool)


To use the explicit method it is necessary to specify a valuefor ∆p0. The value must be compatible with the flowpattern specified for the design but there is still someflexibility in the value. Guidance on specifying a value for∆p0 is given in Examples 4.1 to 4.4.

Example 4.1: case 6: buoyancy alone (uniform internaltemperature)

The aim is to achieve the flow pattern shown in Figure4.14, i.e. fresh air enters all rooms and all of the stale airexits through the upper opening. This flow pattern meansthat the pressure difference must change sign at a heightwhich lies somewhere between z3 and z4. The height atwhich ∆p0 = 0 is known as the ‘neutral height’, zn. Byspecifying zn, ∆p0 is specified by equation 4.11 (with windterms omitted, i.e. wind speed set to zero) by putting∆pi = 0 and zi = zn , i.e:

∆p0 = ∆ρ0 g zn (4.15)

It then follows that ∆pi is known and is given by:

∆pi = ∆ρ0 g zn – ∆ρ0 g zi (4.16)

The required areas can then be found using a rearrange-ment of equation 4.10, i.e:

(4.17)C Aq

S pi i

i

i id = ρ0

2 |∆ |

Note that the equations are solved for the product Cd i Ai.There is no need to carry out separate calculations wheneither Cd i or Ai is changed, provided the product remainsthe same.

Table 4.2 shows the calculations for a summer designcondition with ∆T0 = 3 K and with the neutral heightspecified as being at the top of the occupied spaces, i.e.zn = 10 m.

The value of ∆p0 is first calculated from the knowninternal and external conditions, as follows:

— external temperature: TE = 25 °C

— internal temperature: TI = 28 °C

— temperature difference: ∆T0 = 3 K

— wind speed: U = 0 m·s–1

— dynamic pressure: 0.5 ρ U2 = 0 Pa

— reference density: ρ0 = 1.2 kg·m–3

— density difference (eqn. 4.33): ∆ρ0 = 0.01208 kg·m–3.

The pressure difference is then obtained from equation4.15, thus:

∆p0 = 1.851 Pa

Equations 4.16 and 4.17 are then used to determine ∆piand Cd i Ai. The results are given in Table 4.2.

Figure 4.13 Examples of single-cell buildings

Case 5 Case 6

Case 7 Case 8


Although using any value of zn between 8.35 and 11.5 mwould satisfy the flow pattern, it is important to note thata value close to one of these two extremes will lead to avery large area, which is unrealistic. An alternative way ofspecifying ∆p0 is to set the values of ∆pi for openings 3 and4 as opposite and equal. This leads to ∆p0 = 1.1762 Pa andthe resulting calculations are shown in Table 4.3. It will beseen that the calculated areas do not differ greatly fromthose in Table 4.2.

Specifying ∆p0 completely determines all the values of ∆pi.It follows that the height of any opening can be changedwithout affecting the results for other openings. Also, theflow rate through any of the inlet openings can be changed

without affecting any of the other inlet areas. The area ofthe outlet opening will change because the outlet flow ratechanges when any inlet flow rate changes. In reality,changing an inlet area will lead to changes in the neutralheight and the flow rates through all of the openings. Thisis what is predicted by an implicit method and thisdemonstrates the basic difference between the explicit andimplicit methods.

With uniform temperature the flow pattern in Figure 4.14is achievable only when the internal temperature is greaterthan the external temperature. The building may actuallybe designed to make use of night ventilation with the aimof keeping the internal temperature below the external

z4 = 11·5 m

z3 = 8·35 m

10 mz2 = 5·1 m

z1 = 1·85 m

TE = 25 oC

q3 = 0·448 m3·s–1

q2 = 0·448 m3·s–1TI = 28 oC

q1 = 0·448 m3·s–1

U

Figure 4.14 Examples 4.1, 4.2, 4.3and 4.4: properties of building forcase 6

Table 4.2 Example 4.1: opening areas — buoyancy alone, uniform temperature

Opening zi / m Cp i qi / m3·s–1 Flow Si ∆pi / Pa Cd i Ai Cd i Ai / m2

pattern / m2

1 1.85 — 0.448 Inward +1 0.9659 0.3531 0.61 0.579

2 5.1 — 0.448 Inward +1 0.5807 0.4554 0.61 0.747

3 8.35 — 0.448 Inward +1 0.1955 0.7848 0.61 1.286

4 (outlet) 11.5 — –1.344 Outward –1 –0.1778 2.4692 0.61 4.048

Table 4.3 Example 4.1: opening areas — buoyancy alone, alternative specification of ∆ρ0


pattern / m2

1 1.85 — 0.448 Inward +1 0.9570 0.3547 0.61 0.582

2 5.1 — 0.448 Inward +1 0.5718 0.4589 0.61 0.752

3 8.35 — 0.448 Inward +1 0.1867 0.8032 0.61 1.317

4 (outlet) 11.5 — –1.344 Outward –1 –0.1867 2.4097 0.61 3.950

Table 4.4 Example 4.1: opening areas — buoyancy alone, flow pattern reversed


pattern / m2

1 1.85 — –0.448 Outward –1 –0.9562 0.3549 0.61 0.582

2 5.1 — –0.448 Outward –1 –0.5749 0.4577 0.61 0.750

3 8.35 — –0.448 Outward –1 –0.1936 0.7887 0.61 1.293

4 (outlet) 11.5 — 1.344 Inward +1 0.1760 2.4816 0.61 4.068


temperature at all times. Although it is mathematicallypossible to obtain a solution, it will lead to the sign of ∆pibeing opposite to the sign of qi , which is physicallyimpossible. However, it is possible to achieve the requiredflow pattern when the internal temperature is less than theexternal temperature for cases where the density is notuniform, i.e. where internal stratification occurs (seeExample 4.4, below) and for the case with a chimney (seesection 4.3.3, Example 4.6).

To investigate the case where the internal temperature isless than the external temperature and the flow pattern inFigure 4.14 is reversed (as can occur as a result of nightcooling or downdraught cooling), it is necessary to reversethe signs of qi and Si. Thus, reversing these signs andputting TI = 25 °C, TE = 28 °C and ∆T0= –3 K in Example4.1, the results shown in Table 4.4 are obtained. Note: thesmall differences in the magnitudes of the areas comparedto those given in Table 4.2 are simply due to the manner inwhich ∆ρ0 is defined.

Example 4.2: case 6: wind alone (no temperaturedifference)

Here the aim might again be to achieve the same flowpattern as in Figure 4.14. This may be optimistic for somewind directions because low wind pressures on theleeward side will impede fresh air entry through openingson that side. An essential but not sufficient requirement toachieve the pattern is that the wind pressure coefficient onthe upper opening must be more negative than thecoefficients on the leeward side. For the present simpleexample, the wind direction is such that the openings areon the windward side with positive values of Cp i . Therequired flow pattern would be achieved if the internalpressure were such that the pressure difference across theupper opening is one half of the total pressure differenceacross the building (i.e. ∆p4 = –0.5 ρ0 U2 ∆Cp / 2) and thiscondition is used here to specify ∆p0. Note that the valueof ∆p4 is negative because the flow is outward.

Substituting ∆p4 = –0.5 ρ0 U2 ∆Cp / 2 into equation 4.11and setting ∆ρ0 = 0 (i.e. no buoyancy effect) gives ∆p0 asfollows:

∆p0 = –0.5 ρ0 U2 [(∆Cp / 2) + Cp4] (4.18)

Substituting for ∆p0 in equation 4.11 (again with ∆ρ0 = 0)gives the following expression for ∆pi:

∆pi = –0.5 ρ0 U2 [(∆Cp / 2) + Cp4] + 0.5 ρ U2 Cp i

(4.19)

The value of ∆p0 is first calculated. All the conditionsgiven in Figure 4.14 apply, except that TI = 25 °C. Thewind pressure coefficients Cp1 to Cp4 are taken as 0.25,0.25, 0.25 and –0.1 respectively. Thus ∆Cp is 0.35.

Thus the internal and external conditions are as follows:





— dynamic pressure: 0.5 ρ U2 = 5.4 Pa


— density difference: ∆ρ0 = 0 kg·m–3

Equation 4.18 then gives:

∆p0 = 1.851 Pa

Equation 4.19 is then used to obtain ∆pi and equation 4.17to obtain Cd i Ai. The results are given in Table 4.5.

The results shown in Table 4.5 may be compared withthose for Example 4.1 in Table 4.2. If the two sets ofweather conditions were considered to be appropriatedesign conditions for Ai, it would be logical to choose theareas in Table 4.2 to size the openings, because they are thelarger values; i.e., in principle, the values of Ai can then bereduced by the occupants or the control system to satisfythe conditions of Example 4.2.

Example 4.3: case 6: wind and buoyancy combined

In this example the weather conditions for Examples 4.1and 4.2 are combined. Again the same flow pattern isspecified, so the value for ∆p0 is obtained by specifyingthat the values of ∆pi for openings 3 and 4 are opposite andequal. Thus, applying equation 4.11 gives:

∆p0 – ∆ρ g z3 + 0.5 ρ U2 Cp3 =

– (∆p0 – ∆ρ g z4 + 0.5 ρ U2 Cp4) (4.20)

The internal and external conditions are as follows:





— dynamic pressure: 0.5 ρ U2 = 5.4 Pa


— density difference: ∆ρ0 = 001028 kg·m–3.

Equation 4.20 then gives:

∆p0 = 0.771 Pa

Equation 4.11 is then used to obtain ∆pi and equation 4.17to obtain Cd i Ai. The results are given in Table 4.6.

Table 4.5 Example 4.2: opening areas — wind alone


pattern / m2

1 1.85 0.25 0.448 Inward +1 0.945 0.3570 0.61 0.585

2 5.1 0.25 0.448 Inward +1 0.945 0.3570 0.61 0.585

3 8.35 0.25 0.448 Inward +1 0.945 0.3570 0.61 0.585

4 (outlet) 11.5 –0.1 –1.344 Outward –1 –0.945 1.0709 0.61 1.756


4.3.3 Buoyancy alone with stratification

In large vertical structures such as atria, the assumption ofuniform internal temperature may be unrealistic.Temperature stratification can be expected and it ispossible to determine its effect on the opening areas. Thehydrostatic equation still applies but in a modified form totake account of the variation of internal density withheight. It is first necessary to specify the variation oftemperature. Since the atrium is open to the internalspaces, it is assumed that there are no horizontal differ-ences in temperature (horizontal density differences canoccur with a chimney and are dealt with in section 4.3.4).

Any density variation can be chosen, but to be realistic itshould be non-linear, with the greatest changes occurringat the higher levels. For the following example, thevariation is specified in terms of the internal temperaturesat z = 0 and at the neutral height, zn, i.e. T0 and Tn. Thereason for this is that the neutral plane typically lies justabove the occupied zone and is a measure of the maximumtemperature in the occupied part of the building. Theform of equation chosen here is as follows:

(4.21)

where T is the temperature at height z (m), T0 is thetemperature at height z = 0 (°C), Tn is the temperature atthe neutral height zn (°C), z is the height above groundlevel (m), zn is the height of the neutral plane aboveground level (m) and N is a numerical index.

The value selected for the index N is important. A value of1 corresponds to a linear increase of temperature withheight. A value of 7 is taken in the following examples.This corresponds to a more realistic case where the bulk ofthe temperature change occurs at the higher levels. At theinitial stage of a design, choosing the values for Tn and Nis best based on experience from other buildings. In theabsence of such experience a range of values can bechosen, to gauge their importance. Figure 4.15 shows theeffect of N on the temperature variation. (Note that theuniform temperature case is closely approximated byusing a value of N that is close to, but not equal to, zero).

With the above temperature variation the equation for ∆piwith non-uniform density (equation 4.4) becomes:

(4.22)

TT

T

T

z

z

N+ =

+

− −++

273273

1 1273

273

0

0

n n

where ∆T0 and ∆Tn denote the internal and externaltemperature differences at z = 0 and zn respectively. Therequired areas can again be found using equation 4.17.

Example 4.4: case 6: buoyancy alone with stratification

For the following example, all of the values are the same asfor Example 4.1, except that, for stratification, Tn is takenas 31 °C, i.e. ∆T0 = 3 K and ∆Tn = 6 K.




— internal temperature at neutral height: Tn = 31 °C


— temperature difference: ∆Tn = 6 K




— density difference: ∆ρ0 = 0.01028 kg·m–3

The neutral height is taken as 10 m and substituting thisvalue into equation 4.22 enables ∆pi to be calculated.Equation 4.17 is then used to obtain Cd i Ai. There is noneed to calculate ∆ρ0 , because it is implicit in equation4.22 (by virtue of the fact that the temperature at theneutral height is specified). However, putting zi = 0 inequation 4.22 gives ∆ρ0 = 1.3303 Pa.

The corresponding temperatures at the heights of theopenings are shown in Table 4.7 and in Figure 4.14.

Comparing Tables 4.7 and 4.2, it is clear that, except forthe upper opening, the effect of the stratification on the

∆ ∆ ( )

∆ ∆∆

∆

p g z z

g z

N

T

T

T

T

z

z

i i

iN

N

= −

++

−

−

+

−

+

+

ρ

ρ0 n

0 n n

0

n

n n1

1 1273

11

1

Table 4.6 Example 4.3: opening areas — wind and buoyancy combined


pattern / m2

1 1.85 0.25 0.448 Inward +1 1.9017 0.2516 0.61 0.413

2 5.1 0.25 0.448 Inward +1 1.5165 0.2818 0.61 0.462

3 8.35 0.25 0.448 Inward +1 1.131 0.3262 0.61 0.535

4 (outlet) 11.5 –0.1 –1.344 Outward –1 –1.131 0.9785 0.61 1.604

z / z

n

1·2

1·0

0·8

0·6

0·4

0·2

03525 27 29 31 33

Internal temperature, T / °C

N = 0N = 1N = 2N = 5N = 7

Figure 4.15 Variation of internal temperature with z / zn for differentvalues of N (T0 = 28 °C, Tn = 31 °C)


openings is small. This is expected since the temperatureat the inlet heights is little different to the uniformtemperature case.

Stratification allows the case where the required flowpattern is achieved, even though the internal temperaturesof the occupied spaces are lower than the external temper-ature. This corresponds to the case, for example, wherenight cooling has successfully taken place. By settingTI = 24 °C in Example 4.4 (∆T0 = –1 K), with all otherquantities unchanged, the results shown in Table 4.8 areobtained.

4.3.4 Chimneys

Figure 4.16 shows the same space as in Figure 4.14, butventilated by a chimney of length L. The chimney extendsa distance h above the height of the occupied space, H.

Two conditions are considered, both with buoyancy alone;the first where the chimney temperature is uniform andequal to the room temperature and the second where thechimney temperature is greater than the room tempera-ture (for simplicity, both are still assumed to be uniform).

Example 4.5: case 7: chimney temperature equal tointernal temperature

For the case where ρc = ρI , the required area for thechimney can be obtained simply by substituting the valueof Cd i into the solution for case 6, Example 4.1. This isbecause, with no variation of density within the space, thepressure distribution is unchanged if zn is unchanged(Note that the flow direction is upward, so zi = H + h).

It should be noted that because the shape of the chimneyis very different to a sharp-edged opening, its dischargecoefficient can be considerably different to 0.6. If thechimney is straight with no internal obstructions and hasa streamline inlet and outlet, Cd i may exceed 0.6. Thus incase 6, Example 4.1, the use of a stack with Cd i = 0.8instead of the sharp-edged opening would reduce therequired area of the chimney to 3.02 m2. However, if thestack contained several bends and a restrictive outlet withCd i = 0.4, say, the area would be increased to 6.17 m2. Inpractice it might then be preferable to retain the simpleropening and modify its shape (e.g. a bellmouth inlet) toincrease Cd i . Another feature of chimneys is that becausethey tend to be long, their discharge coefficient will, in

Table 4.7 Example 4.4: opening areas — buoyancy only, with stratification


pattern / m2

1 1.85 0.25 0.448 Inward +1 1.1111 0.3292 0.61 0.540

2 5.1 0.25 0.448 Inward +1 0.7252 0.4075 0.61 0.668

3 8.35 0.25 0.448 Inward +1 0.3064 0.6269 0.61 1.028

4 (outlet) 11.5 –0.1 –1.344 Outward –1 –0.4768 1.5077 0.61 2.472

Table 4.8 Example 4.4: opening areas — buoyancy only, with stratification following night cooling


pattern / m2

1 1.85 0.25 0.448 Inward +1 0.0169 2.6710 0.61 4.379

2 5.1 0.25 0.448 Inward +1 0.1437 0.9154 0.61 1.501

3 8.35 0.25 0.448 Inward +1 0.1936 0.7887 0.61 1.293

4 (outlet) 11.5 –0.1 –1.344 Outward –1 –0.6384 1.3029 0.61 2.136

z4 = 11·5 m

z3 = 8·35 m

H = 10 m

L = 8 m

h = 1·5 m

z2 = 5·1 m

z1 = 1·85 m

TE

q3 = 0·448 m3·s–1

q2 = 0·448 m3·s–1TC

TI

q1 = 0·448 m3·s–1 Figure 4.16 Examples 4.5 and4.6: properties of building forcase 7


principle, vary with flow rate (strictly speaking withReynolds number), due to friction at the walls. In mostcases this variation is not great and a simple iterationprocedure can be used to account for it. This procedure isdescribed in section 4.4.3. Chimneys offer a further benefitwhen their internal temperature is greater than TI, asshown in Example 4.6.

Example 4.6: case 7: chimney temperature greater thaninternal temperature

The expression for the pressure difference for the chimneyis now different to that for the other openings (whichremains unchanged as equation 4.11) and is given byequation 4.4. The integral is evaluated in two stages: (a)up to the height of the inlet to the chimney and (b) overthe height of the chimney. This gives:

∆pc = ∆p0 – ρE g (H + h) + ρI g (H + h – L) + ρc g L

(4.23)

which can be re-arranged to give:

∆pc = ∆p0 – ∆ρ0 g (H + h) – ∆ρc g L (4.24)

where:

∆ρc = ρI – ρc (4.25)

In the present example the temperature in the chimney(Tc) has been set to 31 °C and the length of the chimney(L) has been taken as 8 m, but all other values are the sameas Example 4.1. The flow pattern required is the same asthat shown in Figure 4.14 and the value of ∆p0 is obtainedby specifying that the values of ∆pi for openings 3 and 4(the chimney) are opposite and equal, i.e:

∆p0 – ∆ρ0 g z3 = –(∆p0 – ∆ρ0 g (H + h) – ∆ρc g L)

(4.26)





— chimney temperature: Tc = 31 °C

— chimney temperature difference: ∆Tc = 3 K




— density difference: ∆ρ0 = 0.01028 kg·m–3

— chimney density difference: ∆ρc = 0.01196 kg·m–3

— building height: H = 10 m

— chimney length: L = 8 m

— height of top of chimney above building: h = 1.5 m.

Equation 4.26 gives:

∆p0 = 1.6455 Pa

Equation 4.11 (with U = 0) is then used to obtain ∆pi andequation 4.17 to obtain Cd i Ai. The results are given inTable 4.9.

With Cd i = 0.8 the required chimney area is 1.607 m2.(With Cd i = 0.4, the required chimney area is 3.213 m2.)

Evaluating the neutral height for the occupied spacereveals that it is greater than the height of the building,i.e. zn = 13.88 m. The areas required for the inlet openingsare correspondingly reduced compared to Example 4.1.This is now possible because setting the value of ∆p0 andthe neutral height does not have the same physical inter-pretation that it had in the examples for case 6 (Examples4.1 to 4.4).

As with case 6, it is also possible to treat the case of a spacewith TI less than TE while retaining the same flow pattern.By setting TI = 24 °C (∆T0= –1), with all other quantitiesunchanged, the results shown in Table 4.10 are obtained.This means that ∆pi increases with height from its value of∆p0 at ground level. The areas can be compared with thosein Example 4.1 (Table 4.4).

Table 4.9 Example 4.6: opening areas — chimney; chimney temperature greater than internal temperature


pattern / m2

1 1.85 0.25 0.448 Inward +1 1.4263 0.2906 0.61 0.476

2 5.1 0.25 0.448 Inward +1 1.0411 0.3401 0.61 0.558

3 8.35 0.25 0.448 Inward +1 0.6560 0.4285 0.61 0.702

4 (outlet) 11.5 –0.1 –1.344 Outward –1 –0.6560 1.2854 0.8 1.607

Table 4.10 Example 4.6: opening areas — chimney; internal temperature less than external temperature


pattern / m2

1 1.85 0.25 0.448 Inward +1 0.7908 0.3902 0.61 0.640

2 5.1 0.25 0.448 Inward +1 0.9192 0.3619 0.61 0.593

3 8.35 0.25 0.448 Inward +1 0.3390 0.3390 0.61 0.556

4 (outlet) 11.5 –0.1 –1.344 Outward –1 –1.0476 1.0171 0.8 1.271


Example 4.7: case 8: combined chimneys for inlet andoutlet (buoyancy alone)

Figure 4.17 illustrates a system whereby a space isventilated by a roof mounted pair of identical chimneys,with one acting as the inlet and the other as the outlet.There are no other openings. Note that the results applyequally to the case where the chimneys are separate.

When there is no wind, the values of ∆pi for the inlet andoutlet chimneys are given respectively by equation 4.2 as:

∆p1 = ∆p0 – ∆ρ0 g (H + h – L) (4.27)

∆p2 = ∆p0 – ∆ρ0 g (H + h) (4.28)

Since there are no other openings, q1 = –q2 and, when thechimneys are identical, ∆p1 = –∆p2 . It then follows that∆p0 is fixed and given by:

∆p0 = ∆ρ0 g (H + h – L/2) (4.29)

This leads to the same result as for case 1, i.e:

(4.30)

With TE = 25 °C, ∆T0 = 3 K, qi = 0.448 m3·s–1 and L = 3 m:

Cd i Ai = 0.823 m2

Example 4.8: case 8: combined chimneys for inlet andoutlet, wind alone

For this example, the building is the same as that used forExample 4.7 (Figure 4.17).

The result for the ‘wind alone’ case is the same as that forcase 4, i.e:

(4.31)

where ∆Cp = Cp1 – Cp2.

With Cp1 = 0.25, Cp2 = –0.1, qi = 0.448 m3·s–1 and U = 3 m·s–1:

Cd i Ai = 0.357 m2.

C Aq

U Ci i

id

p

= 2

∆

C A qg L

qT

T g Li i i id

E= = +ρρ

0

0 0

273

∆ ∆

4.3.5 Non-dimensional graphs

Figure 4.18 shows the non-dimensional pressure differ-ence, ∆pi / ∆ρ0 g zn, as a function of non-dimensionalheight, zi / zn for three different cases of stratification, i.e.∆Tn / ∆T0 = 1, 2 and 3. The first case is the uniformdensity case and equation 4.16 appears as a straight line.

It is a simple matter to calculate the open area required foran opening. Taking the opening at height 5.1 m in case 6with the neutral height at 10 m, the value of zi / zn is 0.51.The corresponding value of ∆pi / ∆ρ0 g zn is read fromFigure 4.18 for uniform temperature as 0.49. Since ∆ρ0 g zn = 1.185 Pa, ∆pi = 0.58 Pa. Substituting into equa-tion 4.17 gives Cd i Ai = 0.46. The same procedure can beused for the cases with stratification to give the areasshown in Table 4.11.

The results for ∆Tn / ∆T0 equal to 1 and 2 can be comparedwith the corresponding values in Tables 4.2 and 4.7respectively. Although less precise, the agreement isadequate.

As noted in section 4.3.2, the effect of stratification with∆Tn / ∆T0 = 2 is small, with a reduction in area of about10%. With ∆Tn / ∆T0 = 3, the reduction increases to 20%.For the upper opening, zi / zn = 1.15, the reductions aremore substantial.

4.4 Input data requirements and selection

Before any of the design methods described in section 4.3can be used, appropriate input data must be identified. Inthis section, the data items are reviewed and guidanceprovided on sources of useful information.

Figure 4.17 Examples 4.7 and 4.8: properties of building for case 8, spaceventilated by two chimneys

IH

Lh

ρ

Eρ

Eρ

Iρ

U

∆pi /

∆p 0

g z

n

1·4

1·2

1·0

0·8

0·6

0·4

0·2

0

–0·2

–0·4

–0·6

–0·8

–1·01·20 0·1 0·2 0·3 0·4

z / zn

0·5 0·6 0·7 0·8 0·9 1·0 1·1

Ratio = 2, N = 7

Ratio = 3, N = 7Uniform

Figure 4.18 Non-dimensional results of explicit model showing effect ofstratification

Table 4.11 Example 4.1: determination of inlet area for three cases ofstratification

zi / zn ∆Tn / ∆T0 ∆pi / ∆ρ0 g zn ∆pi Cd i Ai Cd i Ai/ Pa / m2 /m2

0.51 1 (uniform) 0.49 0.58 0.46 0.61 0.75

0.51 2 0.61 0.72 0.41 0.61 0.67

0.51 3 0.75 0.89 0.37 0.61 0.60


4.4.1 External wind pressures

Wind pressures are usually expressed in the form of apressure coefficient, Cp i , defined by equation 4.6. Thereason for this is that, unlike pw i, Cp i does not vary greatlywith wind speed. This is primarily due to the fact thatsharp edges on the building fix the positions of flowseparation. Thus for a given building shape and surround-ing environment (which determines the characteristics ofthe approaching wind) the Cp i distribution depends onlyon wind direction and the position of the point wherewind speed (U) is measured. Surface pressure coefficientsfor different generic building forms are available(63,64).Pressure coefficients for specific buildings can beestimated from these generalised data sets or they can begenerated from wind tunnel studies (see section 4.2.7) or,possibly, by CFD (see section 4.2.5).

In general terms, the pressure coefficient of an unobstruc-ted building surface is positive on facades facing the windand negative on other surfaces. The negative pressures areoften associated with flow separation. This means thatroofs with slopes less than 30° are usually in a low pressureregion, irrespective of wind direction. Positive pressures

may be experienced on roofs of greater pitch, or on deepflat roofed buildings where flow re-attachment may occur.

The data on wind pressure coefficients are extensive andreference must be made elsewhere for further informa-tion(63,64). Figure 4.19(63) shows the sort of informationthat is available.

Stacks and chimneys usually extend above roof level. It isimportant to ensure that they are in a low pressure area toeliminate the possibility of reverse flow in strong winds.Guidance for this is available elsewhere(64). The recom-mendations for siting flue terminals for gas appliances(65)

may also be of some help, although it must be remem-bered that temperatures are much higher than thoseencountered with natural ventilation.

It is important that the correct value of wind speed (U) isused when evaluating pw i , because it is proportional to U2,and, in the absence of buoyancy, flow rates are propor-tional to U. The Cp i data set should specify the height atwhich the reference wind speed was measured and thenature of the terrain surrounding the building. Thedesigner will probably not have wind speed records for the

H/6

0·65

0·62

0·59

0·43

0·42

0·45

0·65

0·62

0·59

0·43

0·42

0·45

0·60

0·72

0·64

0·56

0·57

0·67

0·25 W 0·25 W0·5 W

Ground levelD A

BC

C

c b

d a

C

D A

C

Wind direction

Outside walls

Roof plan

Roof level

Ground level

Lightwellwalls

Roof level

–0·06

–0·23

–0·79

–0·06

–0·23

–0·79

–0·07

Lightwell

–0·65

–0·09

–0·08

–0·05

–0·06

–0·10

–0·09

–0·09

–0·08

–0·07

–0·06

–0·10

–0·09

–0·06

–0·01

–0·08

–0·07

–0·08

–0·07

–0·56

–0·53

–0·50

–0·49

–0·47

–0·42

–0·06

–0·08

–0·05

–0·04

–0·04

–0·02

–0·23

–0·23

–0·21

–0·20

–0·19

–0·15

0·3 W

0·3 W

0·4 W

–0·06

–0·08

–0·05

–0·04

–0·04

–0·02

–0·56

–0·53

–0·50

–0·49

–0·47

–0·42

–0·23

–0·23

–0·21

–0·20

–0·19

–0·15

H/6

H/6

H/6

H/6

H/6

–0·21

–0·30

–0·30

–0·34

–0·27

–0·28

–0·35

–0·32

–0·33

–0·30

–0·28

–0·26

–0·13

–0·36

–0·31

–0·34

–0·29

–0·29

–0·13

–0·36

–0·31

–0·34

–0·29

–0·29

d

c b

a d

Figure 4.19 Examples of wind pressure coefficient data(62)


reference position and it is then necessary to estimate thevalues of U from the data available, e.g. from Met Officerecords for a site that could be many kilometres from thebuilding. The procedure(62) described below may be used,although it is subject to uncertainty associated withchanges of terrain between the building and the MetOffice site.

A more recent procedure is that used for evaluatingstructural loads on buildings(34). This would need to beadapted for ventilation calculations, because the aim is todetermine the magnitude of a 3-second gust, which is notrelevant to ventilation. However if the procedure wereapplied to the building as part of the structural design, theeffort involved in applying it to ventilation design may bejustifiable.

The uncertainty associated with determining windpressures is another reason why the treatment of venti-lation due to wind is less precise than for buoyancy alone.It may be worthwhile to assess how important the effectsof wind are to the design compared to buoyancy. In thisconnection it can be noted that, for a summer designcondition, the wind is likely to be less important thanbuoyancy because the main problem is to achieve adequateventilation under no-wind conditions. For a winter designcondition, when the temperature difference is large,buoyancy can be the dominant driving force at relativelyhigh wind speeds.

4.4.1.1 Determination of wind speed for evaluating wind pressure coefficient(62)

The wind speed (measured at a height of 10 m in opencountry) at the Met Office site is denoted by Umet. The Cp ivalues use wind speed measured at height z. For a givenvalue of Umet , the wind speed at height z is given by:

U = Umet k za (4.32)

where U is the wind speed at height z (m·s–1), z is heightabove ground (m), k and a are coefficients determined bythe terrain in which the building lies.

Values for the terrain coefficients are given in Table 4.12.For example, if the building is situated in urban terrainand the wind speed at height z = 20 m is required, then:

U = Umet 0.35 z0.25

Thus, for Umet = 4 m·s–1, say, the wind speed to be usedfor calculating pressures is U = 2.9 m·s–1.

The designer will need to choose a value for Umetcorresponding to a probability that it will not be exceeded;this can be done using tabulated data derived frommeteorological records(62).

4.4.2 Specification of densities

Buoyancy is much easier to treat than wind because tem-peratures are subject to much less uncertainty. Accuraterecords of external temperature are widely available andthe appropriate values will have been defined by theweather data used for the design. The internal tempera-tures can be taken as the design temperature specified foreach of the internal spaces.

It is important to remember that all calculations of densityshould be based on air temperature rather than, say, dryresultant temperature. When carrying out sizing calcu-lations such as those in section 4.3 it is important to use arelatively accurate value for ∆ρ, but the value used for ρ isless important. This is consistent with the neglect of ρ inequation 4.9 for the explicit method. Consequential errorsin Ai should usually be less than 10 %.

For the worked examples given in sections 4.3 and 4.4, thefollowing approximations have been used:

∆ρ0 TI – TE—– = ———— (4.33)ρ0 TE + 273

∆ρc Tc – TI—– = ———— (4.34)ρ0 TI + 273

The reference density ρ0 has been taken as 1.20 kg·m–3.The reference density could equally well have been takenas the average of the internal and external values, in whichcase the calculated values of Ai would have differed by afew percent.

These approximations are usually adequate. If the absolutepressure and relative humidity are specified as well as theair temperature, a precise value of ∆ρ can be obtained bysubtracting the densities evaluated in the mannerdescribed in CIBSE Guide C(66). In general, the sizing ofopenings does not warrant such precision, partly becausethe sizes of commercially available air vents etc. areunlikely to match precisely the calculated values.Moreover, the uncertainties associated with the calcu-lation procedure, particularly when wind is present, willgenerally exceed those in the evaluation of ∆ρ.

4.4.3 Discharge coefficients and areas of openings

Explicit methods give the required value of Cd i Ai for eachpurpose-provided opening. It is then necessary to select anopening that has this value, either from manufacturers’data or by specifying the dimensions of a bespokeopening. It is the product Cd i Ai that matters, but it isnecessary that the values of Cd i and Ai are consistent withone another. Procedures for determining these values forpurpose-provided openings are described below. Theseprocedures can also be used for implicit methods, whichrequire the specification of Cd i Ai. Implicit methods canalso account for adventitious openings and these are dealtwith separately.

4.4.3.1 Purpose-provided openings

Ideally manufacturers’ data for an opening will providethe value of Cd i and the value of Ai on which Cd i is based.It is then simply a matter of multiplying these two valuesand selecting the opening which gives the closest value to

Table 4.12 Terrain coefficients(18)

Terrain k a

Open flat country 0.68 0.17Country with scattered wind breaks 0.52 0.2Urban 0.35 0.25City 0.21 0.33


the required value of Cd i Ai. With small openings such asair vents it is not uncommon for the ‘effective area’ (Aeff)of the vent to be quoted. The effective area is usuallydefined as the area of a sharp-edged circular orifice thatgives the same flow rate as the opening at a given pressuredifference. Since the discharge coefficient of a circularorifice is taken as 0.61, the product 0.61 Aeff is used toselect the opening. Sometimes a geometric area, Ag, isquoted with a discharge coefficient Cdg. In this case theproduct Ag Cdg should be used. The geometric area is oftencalled the ‘free area’ or the ‘open area’.

Manufacturers’ data will almost certainly have beenobtained from still-air tests. This corresponds to thebuoyancy-alone condition and the data can be appliedwith confidence to this case. The discharge coefficient Cd ican be expected to change in the presence of wind butthere are few data available so the still-air value willusually have to be used as a default. For openings that lieflush with the envelope, the effect of wind is likely to be arelatively small reduction in Cd i . For openings thatprotrude into the flow, the effects of wind may be largeand can reduce or increase Cd i depending on the local flowdirection relative to the opening. Basically, the protrusioncan deflect flow into or out of the opening and the conceptof the discharge coefficient becomes less tenable. This isparticularly the case for windows with openable vanes.

Data provided by the manufacturer should always be usedwhere possible. If such data are not available, the follow-ing procedures may be used.

(a) Simple estimation

The shape of open windows is usually such that Cd i can betaken as 0.6 with Ai taken as the minimum geometric openarea of the device, measured in a plane normal to the flowpath through the opening. This is illustrated in Figure4.20 and the dimension d should be used to evaluate thearea. The same applies to a single grille. Where two grillesare used in series, as shown in Figure 4.20(b), the value ofCd i for the combination should be reduced to 0.4, with Aibeing taken as the area of the single grille.

(b) Use of tabulated loss coefficients

For long openings, such as chimneys and ducts, Cd i can beevaluated from tabulated loss coefficients (CL) of thecomponents. The procedure is essentially the same as thatused for pressure losses in mechanical ventilation ducts.The results are likely to be less accurate, however, partlybecause natural ventilation ducts tend to be short, so therequirement for fully developed flow may not be satisfied.Also, flow rates, and hence Reynolds numbers, tend to below and may lie outside the range of tabulated data. Figure4.21 shows a simple circular chimney with diameter d andtotal length L from inlet to outlet. The inlet and outlet areplain and there is one bend.

The discharge coefficient can be obtained from the losscoefficients for the inlet, the bend and the outlet and thefrictional loss coefficient for the length L(51), i.e:

(4.35)

where CL(in) is the loss coefficient for the inlet, CL(bend) isthe loss coefficient for the bend, CL(out) is the losscoefficient for the outlet, L is the total length of thechimney (m), d is the diameter of the chimney (m) and cfis the friction coefficient.

The fourth term on the right-hand side is the skin frictionterm and this will lead to a dependence of dischargecoefficient on Reynolds number. The Reynolds number(Re) is defined by:

ρ q d 4 ρ qRe = —— = ––––– (4.36)

A µ π d µ

CC C C Lc d

idL(in) L bend L(out) f

=+ + +

1

4( ) ( / )

Figure 4.20 Estimation of Cd i and Ai for windows and air vents; (a) top hung window, (b) two grilles in series

Two grillesin series

Windowopening

Not to scale

d

d

(a) (b)

Figure 4.21 Estimation of Cd i for a simple chimney

Total length L

Diameter d

CL(in)=0·67

CL(out)=1·0

CL(bend)=0·3


where Re is the Reynolds number, ρ is the air density(kg·m–3), q is the flow rate (m3·s–1), d is the diameter (m), Ais the area (m2) and µ is the viscosity of air (kg·m–1·s) andcan be taken as 1.8 × 10–5.

For turbulent flow in the chimney, the skin friction termin equation 4.35 can be taken as (0.316 L / Re0.25 d). Sincethe purpose of the explicit calculation is to determine d, itis necessary to make an initial guess of its value to evaluatethis term.

For example, in Example 4.6 above the flow in thechimney is 1.344 m3·s–1. Taking d = 1.0 m as the initialguess gives Re = 1.14 × 105 and the friction term as 0.137.Using the loss coefficients given in Figure 4.21, equation4.35 gives Cd i = 0.673. From the results for Example 4.6,Cd i Ai = 1.2854, so the value of Cd i = 0.673 corresponds toan area of 1.91 m2, i.e. d = 1.56 m. This is considerablylarger than the initial guess. Recalculating using this newvalue for d gives Cd i = 0.679. This is only slightly differ-ent to the original value of 0.673, so we can take this valueas being of sufficient accuracy. Using this value gives thearea as 1.89 m2. Basically the results are not very sensitiveto Reynolds number because the chimney is not very longand friction is not a major loss. In comparison, if the plaininlet were replaced by a bellmouth inlet (for whichCL(in) = 0), Cd i increases to 0.84 and Ai reduces to 1.54 m2.

(c) Use of computational fluid dynamics (CFD)

A potentially accurate procedure is to determine Cd i usingCFD. This procedure may be justifiable for large com-ponents such as chimneys or complicated outlets, .

(d) Direct use of tabulated data

Data compilations of discharge coefficients are sometimesincluded with loss coefficients(67) for certain genericshapes of opening.

(e) Use of scale models

For large sharp-edged openings such as windows, Cd i canbe obtained by measurements on scale models. Thistechnique can reveal the effects of wind.

It is important to note that in certain situations the simpledischarge coefficient cannot accurately describe the flow.As mentioned in (a) above, this can occur due to thepresence of wind. It can also occur when buoyancypredominates if the vertical dimension of the opening islarge in relation to the height of the ventilated space. Insuch cases there may be inflow at the bottom and outflowat the top of the opening due to the variation in pressuregradient either side of the opening. For an isolated room,bi-directional flow through an opening may form part ofthe design, see case 2 in section 4.3.1. In such cases thedischarge coefficient is not appropriate and is replaced bya coefficient C defined in terms of ∆ρ. Empirical data areavailable for such cases, including those where buoyancyand wind are combined. In this way they can be includedin envelope flow models(51).

4.4.3.2 Adventitious openings

Adventitious openings are those openings that are notpurpose-provided such as cracks in window frames, pen-

etration of walls by services, junctions between envelopecomponents and porosity of walls. Their magnitude isusually expressed as a leakage flow rate rather than as anopening area. The leakage of an envelope is the flow ratethat occurs when the envelope is subjected to a specifiedpressure difference (often 50 Pa). The desirability ofreducing leakage through adventitious openings has longbeen recognised and it is now incorporated in Part L2 ofthe Building Regulations(5).

In many cases, adventitious openings cannot be identifiedand they are then referred to as ‘background openings’.Openings in doors and windows can be identified (interms of their position) and are known as ‘componentopenings’. Component openings can be treated in animplicit envelope model in the same way as purpose-provided openings, provided their size can be estimated.CIBSE Guide C(66) gives advice on this; BS 6375(68) givesinformation on air leakage classification of openablewindows and doors. Information on the measured per-formance of real components is available in the ventilationliterature.

For background openings it is best to deal in terms oftheir leakage characteristics. At the early stage of a designit may be justifiable to treat component openings in thesame way and to consider the total adventitious leakage ofthe envelope.

Adventitious leakage is of more concern for the wintercondition, when the areas of purpose-provided openingsare at a minimum to conserve heating energy. In thesummer, assuming windows are open, the adventitiousleakage can be neglected. With a leaky envelope, the flowthrough adventitious openings (‘infiltration’) may exceedthe flow through the air vents. This is clearly undesirableand the designer may wish to check the effect of adventi-tious leakage. Envelope flow models are the only feasibleway of calculating the effect of adventitious leakage, forwhich it is necessary to specify the leakage characteristicof the envelope.

4.4.4 Calculating effect of adventitious leakage

The leakage characteristic of an envelope is therelationship between the flow rate (QL) and the uniformpressure difference (∆pL) across the envelope generated bya fan pressurisation test. The relationship is usually takento be of a power law or quadratic form, i.e:

QL = α (∆pL)β (4.37)

or

∆pL = a QL2 + b QL (4.38)

where α, β, a and b are leakage coefficients.

The best way of specifying the adventitious leakage of abuilding is by measurement for which there are recog-nised pressurisation techniques, see CIBSE TM23(24). Atthe design stage measurement may be impossible, inwhich case the leakage can be estimated using tabulatedvalues for different types of components and construction,as mentioned above. Such estimates are subject toconsiderable uncertainty. The designer may therefore wish


to carry out calculations for a range of adventitiousleakage rates to ascertain its potential importance. Suchcalculations can be carried out using an envelope flowmodel. A quicker procedure is to make use of calculationresults that have been plotted in non-dimensional form.

Figure 4.22(54) is a plot of the non-dimensional infiltrationrate (i.e. ventilation due solely to adventitious leakage)against the non-dimensional leakage parameter. The non-dimensional infiltration rate and the non-dimensionalleakage parameter are defined thus:

(4.39)

(4.40)

Figure 4.22 is for the case of buoyancy alone with theneutral height chosen such that the flow pattern is thesame as in Example 4.1, i.e. all outward flow occursthrough adventitious openings above the occupied spaces.By calculating the infiltration rate and comparing it withthe design ventilation rate provided by the purpose-provided openings alone, the designer can assess theimportance of the adventitious openings. If the infiltrationrate exceeds the design rate, it may be advisable toconsider remedial work on reducing the adventitiousleakage.

To calculate Qinf it is first necessary to determine a and bfrom the leakage characteristic. If the characteristic hasbeen measured it is simply a matter of fitting equation4.38 to the measured points.

It is likely that only a value for the leakage at a pressuri-sation of 50 Pa will be known (Q50). In this case, the valuefor a / b2 can be taken as 0.1 (this corresponds to a powerlaw index β = 0.67). The value of a is then given by:

a = 32.1 / Q502 (4.41)

where Q50 is the leakage rate at a pressurisation of 50 Pa(m3·s–1).

Non dim leakage parameter

n

− = 1

2∆ /ρ g z a b

Non dim. infiltration rate inf

n

− = Q

g z a∆ /ρ 2

Using the same information as for Example 4.1:

∆ρ0 g zn = 1.185 Pa

Suppose that the value of Q50 is 2.0 m3·s–1. Then, fromequation 4.41, a = 8.025. Thus, from equation 4.40, thenon-dimensional leakage parameter is 2.05. Thereforefrom Figure 4.22, the non-dimensional infiltration rate is0.21. Hence from equation 4.39, Qinf = 0.057 m3·s–1. Thisis the total infiltration rate and can be compared with thedesign rate of 3 × 0.448 = 1.344 m3·s–1. Not surprisingly,the infiltration rate is negligible. However, for the winterdesign condition, the design ventilation rate will be muchlower and ∆ρ0 g zn will be higher. For example with∆ρ0 g zn = 8 Pa, Qinf = 0.32 m3·s–1, which may exceed thedesign ventilation rate.

4.5 Reservoir effect

It is quite common to specify the design flow rate of air asthat required to limit the concentration of a pollutant to afixed value. For example, the aim is often to limit theconcentration of CO2 in a space to 1000 ppmv (volumetricparts per million). When the source of pollution is theoccupants alone, this requirement is deemed to maintainbody odours at an acceptable level. It takes time for theconcentration to increase from its initial value (thereservoir effect) and if the occupancy period is known,allowance for this can be made in specifying the design airflow rate.

For a well-mixed space, the change in CO2 concentrationwith time is given by:

(4.42)

where C(t) is the concentration at time t (ppmv), t is time(s), Cext is the concentration in the external air enteringthe space (ppmv), qCO2 is the volumetric emission rate ofCO2 (m3·s–1), Q is the volume flow rate of air entering thespace (m3·s–1), C(0) is the concentration at time 0 (ppmv)and V is the volume of the space (m3).

Equation 4.42 follows from the mass balance for CO2. Itassumes that the amount of CO2 present in a given volumeis always very much less than the amount of air, so Q canbe taken as the volume flow rate of the mixture.

In the following, the assumption is made that C(0) = Cext.The steady state concentration, Css, is obtained by makingt tend to infinity and is given by:

(4.43)

where Css is the steady state concentration (ppmv).

The flow rate of air required to ensure that Css does notexceed a certain value is thus given by:

C Cq

Qss ext

CO2= +106

C t Cq

Q

C Cq

Q

Q t

V

( )

( )

= + ×

− − + ×

−

extCO2

extCO2 exp

10

010

6

6

Figure 4.22 Graph for calculating infiltration rate, buoyancy alone

Non

-dim

ensi

onal

ven

tila

tion

rat

e

1·0

0·8

0·6

0·4

0·2

050 1 2 3 4

Non-dimensional leakage parameter


(4.44)

where Qss is the steady state flow rate (m3·s–1).

It is this value that is commonly used for the design flowrate.

Although Css can in principle only be reached after aninfinite time, the concentration will reach 95% of Css aftera finite time, t95. If the occupancy period is known to beless than t95, the air flow rate can be reduced by specifyingthat Css be reached at the end of the occupancy period.

Equations 4.42 and 4.44 can be combined to give:

(4.45)

where Qoc is the flow rate required to give Css at the end ofthe occupancy period (m3·s–1) and toc is the occupancyperiod (s).

Example 4.9: Determination of flow rate to limit CO2concentration

A lecture theatre with a volume of 3000 m3 is occupied by100 people. The emission rate of CO2 per person is5.56 × 10–6 m3·s–1 (20 litres per hour). The designconcentration is 1000 ppmv and the external and initialconcentrations are both equal to 350 ppmv. Equation 4.44gives the steady state flow rate as Qss = 0.854 m3·s–1

(3075 m3·h–1). The corresponding form of equation 4.45 isshown in Figure 4.23. It can be seen that if the occupancyperiod does not exceed 1.5 hours, the air flow rate can bereduced to about 0.6 × Qss, i.e. 0.512 m3·s–1 (1845 m3·h–1).

Figure 4.23 also implies that with an occupancy periodless than about 1 hour, no ventilation is required. This isdue to the fact that even with no ventilation it takes nearly1 hour for the concentration to reach 1000 ppmv. This canbe seen by making Q tend to zero in equation 4.42 andexpanding the exponential term to give:

(4.46)

Using the values in Example 4.9, equation 4.46 gives thetime to reach 1000 ppmv as 3508 s.

C t Cq

Vt( ) ( )= +0

106CO2

Q

Q

Q t

Voc

ss

oc ocexp= − −

1

Qq

C Css

CO2

ss ext

=−

106

The reservoir effect offers significant reductions inventilation rates, but it must be remembered that after theoccupancy period there needs to be a period to allow theconcentration to decrease prior to the next occupiedperiod. Low ventilation rates could also of course lead toother problems, e.g. overheating of occupants.

Equation 4.42 can be used to calculate the time requiredfor the concentration to drop to an acceptable level (byrestarting the calculation with t = 0, C(0) = 1000 andqCO2 = 0. This is illustrated in Figure 4.24 for the lectureroom where the ventilation rate has been taken as0.512 m3·s–1 and the occupancy period as 5400 s (1.5 h). Itcan be seen that with this ventilation rate, several hoursare required for the concentration to fall to the externallevel.

Note: the above equations assume that the pollutant is wellmixed. This is likely to be a reasonable assumption whenthe ventilation rate is high, but less so with a lowventilation rate and/or a high ceiling, where stratificationmay be significant. Stratification will however tend to actin a beneficial way, by leading to higher concentrationsabove the occupied zone.

Equation 4.42 differs from the equation given in BS5925(62) in two respects. First, it includes the initialconcentration. Secondly, the treatment of the mass flowassociated with the pollutant is different and is moreappropriate for carbon dioxide, which is formed from theair within the space and does not correspond to an extramass flow into the space, unlike some other pollutants or atracer gas. This only leads to differences when Q is verysmall, i.e. of similar magnitude to qCO2. Note: equation4.42 is included in the spreadsheet* that may be obtainedfrom the CIBSE website.

4.6 Complementary design techniques

As has been stressed throughout this publication, thedesign of the natural ventilation system has to be carefullyintegrated with the other elements of the design. Abuilding will not be regarded as acceptable if the lightingor acoustic environments are poor, even if the ventilationstrategy is working perfectly. Guidance on design methodsis available in other CIBSE publications.

Qoc

/ Q

ss

1·0

0·8

0·6

0·4

0·2

080 1 2 3 4

Occupancy period / h5 6 7

Figure 4.23 Example 4.9: effect of occupancy period on ventilation raterequired to achieve 1000 ppmv CO2

Ci(t

) / p

pmv

60 21 3 4 5Time / h

1200

1000

800

600

400

200

0

Figure 4.24 Example of concentration decay following a period ofoccupancy

* The spreadsheet may be downloaded from the CIBSE website(www.cibse.org/venttool)


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62 BS 5925: 1991: Code of practice for ventilation principles anddesigning for natural ventilation (London: British StandardsInstitution) (1991)

63 Orme M, Liddament M W and Wilson A Numerical data for airinfiltration and natural ventilation calculations AIVC TN 44(Coventry: Air Infiltration and Ventilation Centre) (1998)

64 Liddament M W A guide to energy efficient ventilation (Coventry:Air Infiltration and Ventilation Centre) (1996)

65 BS 5440-1: 2000: Installation and maintenance of flues andventilation for gas appliances of rated input not exceeding 70 kW net(1st, 2nd and 3rd family gases). Specification for installation andmaintenance of flues (London: British Standards Institution)(2002)

66 Reference data CIBSE Guide C (London: Chartered Institutionof Building Services Engineers) (2001)

67 Idelchik I E Handbook of hydraulic resistance (Washington DC:Hemisphere) (1986)

68 BS 6375-1: 2004: Performance of windows. Classification forweathertightness and guidance on selection and specification(London: British Standards Institution) (2004)

ForewordThe need for the Institution to provide professional guidance on the design and applicationof natural ventilation in buildings was first identified when I was CIBSE President in 1992.The resulting Applications Manual was first published in 1997 with the aim of ‘providingmore guidance on energy related topics in order to realise quickly the improvements inenergy efficiency which should arise from the application of the guidance presented’.

Much has happened since 1997 in relation to energy use in buildings. The EnergyEfficiency Best Practice Programme which sponsored the first edition has been replaced bythe Carbon Trust, which has become very widely recognised for its high profile campaignsraising awareness of business use, and waste, of energy. Part L of the Building Regulations,Conservation of Fuel and Power, has been transformed and will shortly be revised once moreas Part L (2005). The Energy Performance in Buildings Directive has been adopted by theEU, and will be implemented in the UK from the start of 2006. And, late in 2004, theSustainable and Secure Buildings Act reached the statute book, to enable BuildingRegulations to address these two, sometimes conflicting, themes.

In the light of all these changes, as well as the growing practical experience of advancednaturally ventilated buildings, it is timely to issue a revised edition of this guidance. Theprinciples remain largely unchanged — as do the laws of physics on which they depend.However, experience in their application has advanced, and new examples have appeared.As a result, the material has been re-ordered, and the examples, instead of standing alone atthe end, are now incorporated within the guidance at appropriate places. This edition alsodraws extensively on work funded by the Partners in Innovation scheme of the DTI onautomatic ventilation devices. The guidance contained within this edition will enablepractitioners to apply the principles of natural ventilation based on a sound understandingof their underlying basis. In so doing further improvements in energy efficiency will beachieved.

The revision has been undertaken by one of the original authors, Steve Irving, aided byDavid Etheridge and Brian Ford of Nottingham University. The revision has again beensteered by a small group of leading practitioners from a range of professional backgrounds,with the aim of producing guidance that is as far as possible accessible to architects andengineers alike, and will assist them in adopting an integrated approach to building design.The Institution would like to thank the Steering Group, listed below, for their contributionto the project, and also to acknowledge the support of the Carbon Trust for the work.

Brian MossChairman, CIBSE Publications, Research and Outputs Delivery Committee

AcknowledgementsThe Chartered Institution of Building Services Engineers gratefully acknowledges thefinancial support provided by the Carbon Trust in the preparation of this publication.However, the views expressed are those of the Institution and not necessarily those of theCarbon Trust. The Carbon Trust accepts no liability for the accuracy or completeness of, oromissions from, the contents of the publication or for any loss arising from reliance on it.Any trade marks, service marks or logos relating to the Carbon Trust used in this publica-tion are the property of the Carbon Trust and must not be used or reproduced without theCarbon Trust’s prior written permission.

Principal authorsSteve Irving (FaberMaunsell) (Sections 1 and 2)Prof. Brian Ford (School of the Built Environment, University of Nottingham) (Section 3)David Etheridge (School of the Built Environment, University of Nottingham) (Section 4)

AM10 Steering GroupHywel Davies (CIBSE) (chairman)Derrick Braham (Derrick Braham Associates Ltd.)Prof. Derek Clements-Croome (School of Construction Management & Engineering,University of Reading and CIBSE Natural Ventilation Group)David Etheridge (School of the Built Environment, University of Nottingham)Prof. Brian Ford (School of the Built Environment, University of Nottingham)Prof. Michael Holmes (Arup)Gary Hunt (Department of Civil and Environmental Engineering, Imperial College)Steve Irving (FaberMaunsell)Tony Johnson (The Carbon Trust)Chris Twinn (Arup)

EditorKen Butcher

CIBSE Research ManagerDr Hywel Davies

CIBSE Publishing ManagerJacqueline Balian

This page has been reformatted by Knovel to provide easier navigation.

INDEX

Index Terms Links

A

Acoustic baffles 7

Acoustic environment 2 7 19 21

36 59

Actuators 20 29–32

control 30 34

dampers 23 29

fixings 30 31 32

free area and 30 33

IP rating 31

location 31

number of 31–32

reliability 32

responsibility for 21–22 24 30

rooflights 30 31 34

safety 31

security 31

sizing 30

travel 29 30 31 35

ventilation rates 33 34

ventilators and 20

windows 22 23 26

Adventitious openings 41 45 55 57–58

Air flow paths 8–10 11–12 13 14–15

17 18 37

see also Air flow resistances

Air flow patterns 8 9 10 14–15

18 48–49 51

see also Air flow resistances; Envelope

flow models

Air flow rates — see Leakage; Ventilation rates

Air flow resistances 10 11 16 17

27–28

see also Obstructions; Short circuits

Air infiltration — see Adventitious openings Leakage

Air movement — see Air speeds; Draughts; Ventilation rates

Index Terms Links


Air permeability — see Adventitious openings; Leakage

Air quality

air flow paths 10

calculation techniques 3 36–37 58–59

commissioning 36

cross ventilation 16

energy costs and 3

heat loads and 6

mechanical ventilation 19

objective 1 2

sensors 34

spreadsheet tool 3

stack effect 59

strategies for 2–4

ventilation control 33

ventilation rates and 3–4 8 36–37 58–59

zones 10

Air speeds

cooling 6

see also Draughts; Wind speeds

Air supply — see Supply points; Ventilation rates

Airtightness 12 31

see also Adventitious openings; Leakage; Sealing

Atria 12 18–19

modelling 40 41 44 46

50–51

on/off control 33

see also Solar chimneys

Automatic control 2 17 20 21

22 23 34

see also Actuators

B

Back draughting 27

Background openings 57

Background ventilation 23

Bank control 33

Blinds 4 5 6 7

18 19 22 33

see also Shading

BMS (building management systems) 20 32–33 34 36

BRE Digest: Selecting lighting controls 5

Index Terms Links


BRE Digest 399: Natural ventilation in non-domestic buildings 15

BRE Digest 454: Thermal mass in office buildings 20

BRE Environmental Building 18

Brise–soleil 4

BS 5925 46 59

BS 6375 57

BS 6399 27

BS 7346: Part 1 30

Building Bulletin BB93: Acoustic design of schools 7

Building management systems 20 32–33 34 36

Building Regulations 1991: Approved Document F 23 45

Building Regulations 1991: Approved Document F1 1 2–3 4

Building Regulations 2000: Approved Document L2 4 29 35

Building Regulations 2000: Part L2 35 57

Building ventilation theory and measurement 39

Buoyancy — see Density differences; Stack effect; Stack ventilation

C

Cable–driven actuators 29 30 32

Calculation techniques 2 3 15 16

17 36–59

see also Combined models;

Computational fluid dynamics; Envelope

flow models; Input data

Carbon dioxide levels 3 36 58–59

Catches 31

Cellular spaces 9 15 27–28

see also Multi–cell envelope flow

models; Single–cell envelope flow models;

Single–cell spaces

Chain–driven actuators 29 30 31 32

Changeover mixed–mode ventilation 8

Chimneys


discharge coefficients 51–52 56–57

mechanical supply systems 19


49 51–53

stack ventilation 17–18

tabulated loss coefficients 56–57

wind pressure data 54

see also Outlets; Solar chimneys; Stack

heights; Stack ventilation

Index Terms Links


CIBSE AM11: Building energy and environmental modelling 37

CIBSE AM13: Mixed mode ventilation 8

CIBSE Guide A: Environmental design 1

climate change data 5

comfort 38

heat island effect 5

small power load gains 6

thermal capacity 7

ventilation rate calculations 37 46

whole–building ventilation 3

wind speeds 14

CIBSE Guide B2: Ventilation and air conditioning 1 2

CIBSE Guide C: Reference data 55 57

CIBSE Guide J: Weather, solar and illuminance data 14 38

CIBSE TM23: Testing buildings for air leakage 57

CIBSE TM31: Building log books 35

CIBSE TM37: Design for improved solar control 4 5

Circulation areas 10

Cleanliness 10 27 36

Climate change 1–2 4–5 38

see also Weather data

Closed loop control 34

CO2 levels 3 36 58–59

Coatings 4 6

Code for lighting 5

Combined models 2 41 43

Combined occupant and automatic control 3–5 23

Combined openings 23 26–27

Combined wind and stack ventilation 14–15 18 49–50 55

57

Combined window types 23 26

Comfort 6–7

air flow paths 10

air flow pattern design 9

cooling and 4

global warming and 1–2

heat loads and 5 6

night ventilation 19 20

purge ventilation 3

stack effect 12

ventilation control 33 34–35

ventilation rates and 8 37 39

Index Terms Links


Comfort (Cont.)

weather data 38

windows 23

see also Occupants

Commissioning 21 24 30 35–36

Completion 36

Component openings 57

Components 20–36

see also Actuators; BMS; Dampers;

Louvres; Rooflights; Sensors; Ventilators Windows

Computational fluid dynamics (CFD) 2 39 41–43 54

57

Concrete — see Hard surfaces

Concurrent mixed–mode ventilation 8

Conservation of mass 11 12 40

Contaminants — see Air quality

Contingency mixed–mode ventilation 8

Control 8 10 20 32–35

combined windows 26

commissioning 36

façade design 4

integration 21–22 23 34

night ventilation 20 23 33

responsibility for 21–22 24

see also Automatic control; Bank

control; BMS; Closed loop control;

Combined occupant and automatic

control; Components; Fully–modulating

control; Occupant control; On/off control;

Open loop control; Responsibilities; Solar

control; Stepped control; Summer overheating

Control schedules 34 35

Cooling 4 6 8 33

34 39 49

see also Night ventilation

Cooling demand 8

Cooling potential 4 10

Cooling strategies 1

Corridors 10

Courtyards 16

Cowls 27 28

Index Terms Links


Cross ventilation 16–17 28 44 45

46

D

Dampers 17 22 23 27

29 35

Data — see Input data

Daylighting — see Lighting

Deep plan spaces 19

Densities

specification 55

see also Occupancy; Stack effect; Stack

ventilation; Temperatures

Density differences

modelling 40 44 46 50–51

see also Stack effect

Design charts 37

see also Non–dimensional graphs

Design Summer Year 38 39

Discharge coefficients 39 40 41 42

45 46 51–52 55–57

Displacement ventilation 3–4 19

Documentation 35 36

Doors 22

Double–skin façade ventilation 19

Downdraughts 17 49

Draughts

changeover mixed–mode ventilation 8

design considerations 6 45

night ventilation 19

single sided ventilation 16

solar chimneys 17

stack ventilation 12

ventilation commissioning 36

ventilation control 32 33 34

ventilator location 4 23

windows 23

see also Back draughting; Downdraughts

Driving forces 8 10–15 17 38

see also Fans; Stack effect; Temperature

differences; Wind driven ventilation

Index Terms Links


Ducted ventilation 17 26–27

see also Double–skin façade ventilation

Ducts 26–27 56–57

E

Electrical safety 31

Energy 1–2 3 4 8

37

Envelope flow models 39–41 43 44–53 57

see also Explicit envelope flow models;

Implicit envelope flow models

Environmental Design Manual 37

European Directive, energy performance 4

Exhaust outlets — see Outlets

Explicit envelope flow models 2 41 44–53 55

Exposed concrete — see Hard surfaces

Extract ventilation 3 6 10 17

18 19

see also Outlets

F

Façade design 4

see also Double–skin façade ventilation

Fanlights 29 31

Fans 7 8 17 19

29

Films 4

Fine–tuning 21 36 37

Fire safety 2 31

see also Smoke ventilation

Fixings 20 21 30 31

32

Flow equation 12 15

Flow path — see Air flow path

Flow rates — see Ventilation rates

Flues — see Chimneys; Outlets

Free area 21 22 30 33

53 55–57

Fully–modulating control 34

Index Terms Links


G

g–value 4

Gas strut with cable actuators 30

Glazing — see Shading; Solar chimneys; Solar control; Windows

Global warming — see Climate change

H

Hard surfaces 2 7

Health and safety 2 31

Heat gains 4 5–6

objective 1 2

purge ventilation and 3

solar chimneys 17


ventilation rate calculation 37

see also Solar control; Stack effect;

Summer overheating; Temperature differences; Temperatures

Heat island effect 5

Heat loads 4 5 6

Heavyweight buildings 7

Hydrostatic pressures 11

I

Ice loads 30 31

Implicit envelope flow models 2 41 45 48

55 57

Indoor air quality — see Air quality

Infiltration — see Adventitious openings; Leakage

Ingress protection 21 31

Inlets 10

commissioning 36

control 33 34

cross–ventilation 16 17

duct and shaft ventilation 27

envelope flow models 41 46 48 51

53

location 4 7 10 16

23 30

noise control 7

single sided ventilation 15–16

sizing 10 30

Index Terms Links


Inlets (Cont.)

solar chimneys 17

tabulated loss coefficients 57

see also Openings; Supply points; Ventilators; Windows

Input data 5 37–39 43–58

see also Discharge coefficients;

Temperatures; Weather data; Wind

pressures; Wind speeds; Wind tunnels

Installation 24 35–36

Integration 21 23 34

IP (ingress protection) rating 31

Isolated rooms and spaces 44 45–46 57

see also Zones

L

Large eddy simulation models 43

Latches 31

Lead screw actuators 30

Leakage 23 29 36 41

57–58

see also Adventitious openings; Airtightness; Sealing

Lever arm actuators 30

Lighting 5–6 16 18 59

see also Heat gains; Rooflights; Windows

Lightshelves 6

Lightweight buildings 7

Linear push–pull piston actuators 29 30

Linear sleeved cable or rod actuators 29

Linkages 20 21–22 24 30–31

Liquids, modelling with 43–44

Loads 30 31 32

see also Heat loads; Small power loads

Location

actuators 31

inlets 4 7 10 16

23 30

openings 8 30

outlets 8 9

sensors 34

Log books — see Documentation

Louvres 6 23 35

Index Terms Links


M

Maintenance 24

Mass — see Conservation of mass; Thermal capacity

Mechanical ventilation 19 33 34

see also Mixed–mode ventilation

Medium–weight buildings 7

Mixed–mode ventilation 8 33

Modelling — see Combined models;

Computational fluid dynamics; Envelope

flow models; Large eddy simulation

models; Scale models

Multi–cell envelope flow models 41 45

N

NatVent project 7

Neutral pressure level 12

Night ventilation 19–20

air flow patterns 9 48–49 51

changeover mixed–mode ventilation 8

control 20 23 33

heat island effect 5

security 8 12 20

summer overheating 37

ventilation strategies 6 12

windows 20 23

Noise — see Acoustic environment

Non–dimensional graphs 41 53 58

see also Design charts

O

Obstructions 27–28 31 36 41

43

see also Air flow resistances

Occupancy 5 58–59

Occupant control 2 6–7 20 21

23 30 34–34

Occupants

education 21

heat gains from 5

see also Comfort

On/off control 33 35

Index Terms Links


Open area — see Free area

Open loop control 34

Open plan spaces 8

Openings 22–23 26–27

areas and ventilation rates 33 55–57


isolated rooms and spaces 44

location 8 30

responsibility for 21 24

security 12


single–cell spaces 44–45

sizing 8 12 14 28

39–53

stack effect 12

ventilation modes 10

ventilation rates 33 55–57

wind driven ventilation 14

see also Adventitious openings;

Background openings; Combined

openings; Component openings;

Dampers; Doors; Envelope flow models;

Inlets; Louvres; Outlets; Rooflights;

Security; Single openings; Supply points;

Ventilators; Windows

Operation, responsibility for 24

Outlets

atria 18–19

buoyancy and 9

commissioning 36

control 33 34

discharge coefficients 57

location 8 9


51 53

sizing 8 17

solar chimneys 17 18

stack ventilation 12 17 18

temperature differences and 9

wind pressure data 54

zones 10

see also Extract ventilation; Openings;

Stack heights; Supply points; Ventilators

Index Terms Links


Overhangs 4

Overheating — see Heat gains; Summer overheating

P

Physical models — see Scale models

Pollutants and pollution — see Air quality

Practical completion 36

Pre–cooling 5 8 33

see also Night ventilation

Pressure gradients 11–12 14 17

see also Stack effect; Temperature differences

Pressures — see Hydrostatic pressures; Neutral pressure

level; Wind pressures

Purge ventilation 3

R

Rack–and–pinion actuators 29 30 32

Records — see Documentation

Reservoir effect 37 58–59

Response factor 6–7

Responsibilities 2 20–22 24–25 30

32–33

Reynolds number 56–57

Roof ventilators 17 18–19 27

Rooflights 20 22–23 29 30

31 34

Room outlets — see Supply points

Rotary actuators 29–30

S

Safety 2 31

Saltbath method 44

Scale models 2 39 41 43

57

Sealing 20 23 29 31

see also Airtightness; Leakage

Security 8 12 20 21

23 31

Sensors 33 34

Shading 4 5–6 33

see also Blinds

Index Terms Links


Shaft–box double–skin façade ventilation 19

Shafts 26–27 28

Short circuits 10

Side fins 4

Simulation 37 39

see also Large eddy simulation models

Single–cell envelope flow models 39–41

Single–cell spaces 44–45 46–49

Single openings 14 15 45–46

Single sided ventilation 15–16 17 44 45–46

Sizing

actuators 30

calculation techniques 39–53

inlets 10 30

openings 8 12 14 28

39–53

outlets 8 17

shafts and ducts 26

Skin friction term 56–57

Small power loads 5–6

see also Heat gains

Smoke ventilation 2 19 30 31

Snow loads 30 31

Solar baffles 18

Solar chimneys 12 17 18

see also Atria; Double–skin façade ventilation

Solar control 4–6 7 12 18

19 38

see also Heat gains

Solar loads 4 5

Sound — see Acoustic environment

Specifications 20 21–22 55

Spreadsheet tool 2 3 46 59

Stack effect 11–13


modelling 41 44 45–46 47–49

50–53

pollutant concentrations 59

seasonal variations 8


wind scoops 16

Index Terms Links


Stack heights 10 12 17 18

27

Stack ventilation 10–13 17–19 46 54

see also Chimneys; Combined wind and stack ventilation

Stairwells 10

Static completion 36

Status lamps 35

Status lights 30

Stays — see Window stays

Stepped control 34

Strategies 1–20 21 24 28

37

Stratification — see Temperature differences

Summer overheating 4–7 33 37 38

Suppliers’ responsibilities 21 22 30 32–33

Supply points 10

see also Inlets

T

Tabulated loss coefficients 56–57

Temperature differences

driving force 8 11


50–53

outlets and 9


sizing of openings 39

see also Density differences;

Hydrostatic pressures; Night ventilation;

Stack effect

Temperatures

combined thermal and ventilation models 43

heat gains and 5

solar chimneys 18


ventilation rate calculation 37 38 48–49 55

see also Climate change; Comfort; Densities; Heat gains; Summer

overheating; Temperature differences

Terrain coefficients 55

Test Reference Year 38

Testing, leakage 36

Thermal capacity 6 20

Index Terms Links


Thermal comfort — see Comfort

Thermal mass — see Thermal capacity

Thermal models 2 41 43

Tints 4

Total solar energy transmittance 4

Transfer grilles 28

Travel, actuators 29 30 31 35

Trickle ventilators 23 45

Turbulence — see Wind turbulence

U

Underfloor ventilation 16 17 19 23

Unsteady envelope flow models 41

Users — see Occupants

V

Ventilation rates 3–4 32 33

actuators 33 34

air quality and 3–4 8 36–37 58–59

calculation techniques 36–39 41 45–49 58–59

combined wind and stack ventilation 15

comfort and 8 37 39


driving forces and 10

night ventilation 19

openings 33 55–57

reservoir effect 58–59

roof–mounted ventilators 17


sizing of openings 8 10

stack ventilation 11–12

thermal mass and 6

ventilator geometry and 21

windows 22 33

Ventilators 4 20 21

see also Openings; Roof ventilators;

Trickle ventilators; Windows

W

Weather data 38–39

Weather effects 9

Index Terms Links


Weather protection 21

Weight, ventilators 30 31

Whole–building ventilation 3 4

Wind direction 9 11 13 14

17 35 54

Wind driven ventilation 13–15 46 49 54–55

see also Combined wind and stack

ventilation; Cross ventilation; Single sided

ventilation; Wind pressures

Wind loads 30 31 32

Wind pressures

cross ventilation 16 17

driving force 8 10–11 13–15

input data 54–55


shaft outlets 27

stack ventilation 17 18 54

see also Wind tunnels; Wind turbulence

Wind scoops 16 17 27

Wind speeds 13–14

data 14 38 54–55

pressure differences 11

ventilation commissioning 36


ventilation rate calculation 39 41

see also Wind pressures; Wind turbulence

Wind tunnels 14 40 41 43

54

Wind turbulence 14 15 41 43

see also Wind pressures

Window stays 22 29–32

see also Actuators

Windows 20 22–23

concurrent mixed–mode ventilation 8


discharge coefficients 56 57

ducted ventilation 17

envelope flow models 40

free area 23 33

night ventilation 20 23

obstructions 27–28

occupant control 7 23 30

Index Terms Links


Windows (Cont.)

responsibility for 20 21 22

security 8 23

shading 4 5–6


stack effect 13

ventilation rates 23 33

ventilation status indication 35

weight 31

see also Actuators; Combined window types

Work at Heights Regulations 2

Z

Zonal envelope flow models 41

Zoned mixed–mode ventilation 8

Zones 10 34

see also Isolated rooms and spaces

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