Revised Jan 06 Page 1

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.

FAN COIL AIR CONDITIONING

GUIDANCE

This describes air conditioning systems serving a variety of spaces using pre-conditionedprimary or re-circulatory air, and passing it, by fan, over heating and cooling coils to adjust thefinal temperature. The fan coil units are supplied with water from the plant room fortemperature control purposes. The units may be wall, window or ceiling mounted. The airdistribution is usually medium or high pressure.

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.

Consider free cooling. Also consider omitting humidification, unless close environmentalcontrol is required.

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 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.

Barbour Guidein association withBarbour

Barbour Revised Jan 06 Page 2

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.

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.

Fan coil unitsBe aware of noise levels when selecting fan coil units. Normally select a low speed if servingoffice areas, or select additional attenuation or acoustic lining. Specify the maximum noiselevels that are required downstream of the unit.

When selecting fan coil units, make sure that adequate space is available around them formaintenance. Access will be needed to filters, fan decks, control valves and electricalservices and controls wiring. Be aware of the handing of the unit and specify this clearly in theschedule - whether the valves, controls, etc. are on the left-hand side of the unit, or the right.This may be crucial in providing adequate maintenance clearance or co-ordination with otherservices.

Ensure that adequate height is available for the condensate drainage. If it is not sufficient,specify local condensate pumps where needed, either in horizontal runs or at risers.

Fan coil units cannot generally generate large pressures on the discharge side, being typicallycapable of around 25-40Pa. Therefore flexible ductwork between the discharge plenum andthe supply grilles or diffusers should be kept to a minimum, as the pressure drop throughflexible ductwork can be considerable. In many cases the low volume flow readings fromgrilles or diffusers during commissioning is down to excessive pressure drop in the dischargeductwork.

Specify the exact discharge arrangement required on the unit. Units can be supplied withsingle or multiple discharge spigots incorporated onto the discharge plenum.

If the fresh air supply is to be connected directly onto the fan coil unit, be sure to specify thisclearly, as units do not generally come with intake plenums.

When detailing the controls, specify if the units are to be air-side or water-side control.

Barbour Revised Jan 06 Page 3

Intake/discharge positionsThe location of intake and discharge air streams is very important to avoid recycling vitiateddischarged air back into the system intake, and bringing contaminated air from other sourcesinto 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 oncomingwind 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 are within acceptable limits.Large variations between the static pressures at branch inlets will result in large pressurelosses across flow regulating dampers, thereby encouraging noise. Static regain duct sizingcan minimise 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.

Barbour Revised Jan 06 Page 4

REFERENCE DOCUMENTS

The following documents provide further guidance:

BS 4856 Methods for testing and rating fan coil units, unit heaters and unit coolers Parts 1-4:

1972-1997BS EN 12101: 2005 Smoke and heat control systems specification for pressure differentialsystems kitsBSRIA Application Guide 95/20 Commissioning of pipework systems designconsiderations 1996Butterworth Heinemann Air conditioning engineering 2001Butterworth Heinemann Faber and Kells heating and air-conditioning of buildings 2002CIBSE Guide B Heating, ventilating, air-conditioning and refrigeration 2005

Bookmarks