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The Barbour Guides are intended to provide a short introduction to, or overview of, a given subject area. They should

not be viewed in any way as being complete or comprehensive. Barbour recommends further reading, starting with

the references listed in the Guide and other information held within the Barbour services.

GENERAL VENTILATION

GUIDANCE

This covers general ventilation systems not covered by the other, more specific systems suchas kitchen and toilet extract, which have their own guidance elsewhere.

Typical applications may be office fresh air ventilation (not providing air conditioning), generalmake-up air supply for local extract systems, boiler room ventilation, etc.

Ventilation requirements are not necessarily the same as fresh air requirements. Arecommended air change rate for a space can be based on a number of factors includingroom air diffusion requirements, extract requirements or provision of fresh air for combustion.Actual supply rates for offices may have to meet heating and cooling loads as well as fresh airneeds.

Select the fresh air rate to suit the use of the particular space, based on recommendedguidelines. Increase the supply rate above the standard recommended values only whererequired, such as in areas permitting smoking, or for process requirements. The greater thefresh air supply rate, the more energy required to temper the air to acceptable temperaturelevels before it is introduced into the space.

If the system is to be a balanced one i.e. not positive or negative pressure in the space, thenproduce an air flow diagram to make sure that all air paths balance. If there is an extract froman area but no source of air to make up or replace that being extracted, then the system willnot perform adequately.

Assess conditions in the occupied zones to check that reasonable air velocities can beachieved for each space without undue drafts or stagnant zones. This can be done byconsidering room air diffusion patterns, details of throws from diffusers, etc. together with useof space and other sources of air movement. Complex room air flow modelling can also bedone but is unlikely to be justifiable for most spaces. A quick cross check can be done foroffices by calculating the room air change rate and checking against published guidelines.

FIRE

When passing through fire barriers or compartments, ensure that adequate protectionmeasures are put in place, e.g. fire and/or smoke dampers. Details of the firecompartmentation should be shown on the architects drawings, as well as in thespecification. The proposed measures need to be discussed with the local authority,particularly in Section 20 buildings.

Make sure that any such dampers are accessible for maintenance and testing. Should adamper fail, it may need to be replaced with minimum disruption to the occupants. Adequatemaintenance should be discussed and agreed with the architect at the design stage.

PRESSURISATION

Some tall buildings, or those with internal escape stairs, may require pressurisation. This isintended to provide a protected route for fire fighters to access the building to fight a fire.Pressurisation provides positive pressure in the fire lobbies so that smoke cannot get in. Itshould be designed in accordance with BSEN 12101-6 Smoke and heat control systems

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specification for pressure differential systems kits. The local authority will ultimately decideon whether you have to provide pressurisation at all, so check with them at an early stage.

Make sure that upon activation of the pressurisation system, any other ventilation systemsthat may possibly have an affect on the pressurisation system are isolated.

PLANT

Air handling units and separate supply and extract fans can be heavy consumers of electricalenergy, so choose fans that operate at the maximum possible efficiency. Some designs of fanare inherently more efficient than others, so try to select these wherever possible.

When specifying air-handling units, make sure that the quality of construction is adequatelydescribed. A lower quality unit may save money but result in poor performance throughexcessive leakage, reduced operating life or poor reliability.

Terminal devicesThe correct selection of terminal devices is vital for successful air distribution. Factors to beconsidered include: volume of air distance the air is to be thrown temperature of the air height of the terminal device above the ground location relative to surrounding surfacesThe performance of the terminal device may vary considerably as the volume of the airpassing through it changes. This may be very important if an air supply temperature changesto provide cooling and heating at different times of the year.

Avoid short circuiting between supply and extract points in a space.

CoilsWhen selecting air-handling units, the face velocity across the various coils is an importantfactor. Excessive velocity across cooling coils may result in moisture carry-over into the airstream, and ultimately into the space. Typical velocities across coils are: Heating coil: 3.5m/s Cooling coil: 2.5m/sFiltersChoose the grade of filtration appropriate for the application. Some specialist systems orfacilities will have very particular requirements designed to address life safety issues. If indoubt, check if the client has any particular filtration requirements necessary for their businessoperations. Examples of businesses that are particularly sensitive to filtration levels includethe nuclear, medical and pharmaceutical industries and some manufacturing processes.

The inclusion of panel filters will provide a relatively coarse filtration, which may be used ontheir own in many situations. They may also extend the life of bag filters, when used together,to provide a higher level of air cleanliness. Panel filters are also less expensive to replacethan bag filters.

Intake/discharge positionsThe location of intake and discharge air streams is very important. This must avoid recyclingvitiated discharged air back into the system intake and bringing contaminated air from othersources into the system, such as boiler flues and factory chimneys.

High air velocities across intake and discharge grilles or louvres can generate excessivenoise levels. In the case of intake applications, it can also pull moisture and other air bornedetritus such as leaves and dust into the ductwork system.

In areas that experience particularly gusty wind conditions, intakes facing into the oncoming

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wind can cause excessive positive/negative system pressurisation problems. Check the localprevailing wind direction and locate the intakes away from any strong air currents.

COMMISSIONING

Ensure that the division of responsibilities between the designer and the commissioningcontractor/specialist is clearly defined in the specification and other contract documents.

Select ductwork components and layouts to ensure that the system is as inherently stable andself-balancing as possible. This will reduce the time taken to regulate branch flow rates andwill often make it unnecessary to balance flows through terminal units.

Check that static pressure at the inlets to terminal branches is within acceptable limits. Largevariations between the static pressures at branch inlets will result in large pressure lossesacross flow regulating dampers, thereby encouraging noise. Static regain duct sizing canminimise this problem.

Design pressure losses for terminal branches fed from the same main branch should ideallyhave similar resistances to aid system balancing. For example, mixing ceiling diffusers withcombined air/light fittings should be avoided.

Check that the fan is selected to operate under the stable region of its curve. The fan dutypoint must be well away from reverse slopes or stall points.

Where applicable, check that the fan has been selected on total pressure, including both inletand outlet paths.

Ensure that straight lengths of ductwork are provided at key flow measurement points. Theequivalent of 10 duct diameters of straight duct length may be required upstream of flowmeasurement points.

Provide flow-regulating dampers at key positions such as the discharge from the fan and in allbranches, sub-branches and terminal branches. Select these to achieve design flow ratewithout the need to close the device below the 25% open position.

Terminals and their associated dampers should be located as close together as possible tofacilitate simultaneous flow measurement and regulation. Assess whether this would create anoise problem.

Flow measurement locations should be indicated on schematic design drawings and oninstalled drawings produced by the contractor, with commissioning data identified. In-duct flowmeasurements may be required at all main branches and sub-branches serving multipleoutlets.

Check that the accuracy of the specified flow measurement is adequate to achieve therequired flow balance tolerance. Depending on the method used, accuracy can vary between+/-5% and +/-20%.

Ensure damper positions are clearly marked.

REFERENCE DOCUMENTS

The following documents provide further guidance:

BS 5925: 1991 Code of practice for ventilation principles and designing for natural ventilationBS EN 12101: 2005 Smoke and heat control systems specification for pressure differentialsystems kitsBS EN 13779: 2005 Ventilation for non-residential buildings performance requirements forventilation and room-conditioning systems

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PD CR 1752: 1999 Ventilation for buildings design criteria for indoor environmentAir Infiltration and Ventilation Centre Guide to energy efficient ventilation 1996BRE Digest 398 Continuous mechanical ventilation in dwellings design, installation andoperation 1994BSRIA Technical Memoranda 1990/2 Displacement ventilation 1990BSRIA Technical Note 2000/12 Location of fresh air intakes 2000Butterworth Heinemann Faber and Kells heating and air-conditioning of buildings 2002CIBSE Application Manual 13 Mixed mode ventilation 2000CIBSE Guide B Heating, ventilating, air-conditioning and refrigeration 2005HVCA Guide to air distribution technology for the internal environment 2000ODPM Building Regulations Approved Document F Ventilation 2000 (or national equivalents) SI 2000/2531 Building Regulations, England and Wales (or national equivalents)

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