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CPD – Tutorial

This article was originally produced as a contribution to CIBSE Continuing Professional Development (CPD) and published in Building Services Journal 2001

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Under Floor Air Conditioning Tutorial

Since raised access floors have become standard in offices, under floor air conditioning has become more popular.  Here we examine the different types available


1. Definition

2. Comfort

3. Zonal Displacement Ventilation

4. Problems with such Systems

5. A More Flexible Solution

6. Floor Void Supply – Ceiling Return

7. Floor Void Supply – Floor Void Return

8. The Egan Initiative

9. The Benefits

10. A Disadvantage




Air-conditioning is defined as the simultaneous control of temperature, humidity, air movement and the quality of the air in the space.

(source: The Trane Company)

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The Ideal Conditions

For optimum comfort one should have warm feet and a cool head without noticeable draught. Air velocity in the workspace should not exceed 0.18-0.2 m/s and good air movement should be maintained to avoid stuffiness and dilute pollutants such as ozone and formaldehyde.

The FM Dilemma

Air-conditioning systems have tended to be either central station systems in a variety of configurations or smaller diversified plants such as fan coil systems.  Facilities managers have been experiencing high costs associated with the maintenance and reconfiguration of systems above workstations and, on occasions, costly leaks of water have seriously damaged computer networks. In the drive to achieve economy, systems have tended towards higher volume/higher temperature differential solutions but this has led to increased complaints of draughts and noise. Many aspects of office design are now affected by legislation and operators have the task of adapting systems to meet changing legislation and office layouts.

The Options

Building services engineers have been charged with addressing these issues. As a result, they have had to reconsider the options available, investigate new systems and use computers for simulation of air movement patterns, stratification etc. It has been found that air introduced from a low level offers improved conditions in the workspace while offering additional benefits in cost and time.

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Zonal displacement system – air circulation diagram
zonal displacement system









Early developments in Sweden led to the system called displacement ventilation. But as it is essentially a ventilation system with limited cooling (40-60 W/m 2 ), it does not fully meet the definition of air-conditioning. Displacement systems, because of the large areas they serve, often have a demand for heating and cooling within that large area. Additional equipment in the form of underfloor fan coils or electric trench heating is often applied at perimeters to reheat the cool air and introduce it into the space.  However, because this warm air is naturally buoyant it rises to a high level and is exhausted before completely heating the space. The heating efficiency is impaired. Zones are usually large and therefore can provide only an average humidity which often leads to complaints from users.

Swirl Grills in the floor

Air is introduced approximately 3°C below room temperature to avoid complaints of draught, through “swirl” grilles or “perforated plate” outlets across the floor. This air picks up heat from the occupants and machinery, along with solar and fabric gains, and rises. Ceiling extract grilles remove this air, avoiding recirculation.


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Chilled Ceilings

Displacement ventilation systems are of limited use in spaces with high heat loads because of the low cooling capacity available, so they are often teamed with a secondary system of chilled ceilings. This offers a total cooling capacity in the order of 120-150 W/m 2 but the cold ceiling may cause some “displaced” air to fall back into the room carrying with it some of the contamination.

Ceiling Condensation

Where there is a high latent load causing high humidity, the possibility of condensation occurring on cold surfaces of the metal ceiling needs to be checked. Controls can be incorporated to raise the surface temperature in such conditions, but this in turn reduces the cooling capacity often at the time it is most needed.

Central Station Plants and Ducting

Using large central station plants for the treatment of air supply requires floor voids of approximately 600mm deep for the supply ductwork, room heights in the order of 2.7-3.2 m and a ceiling void of about 600 mm to accommodate the exhaust air and chilled ceiling equipment. Of course, the extra space needed to house the system means an increase in the cost of construction. And although users find that such systems are quiet in operation, they restrict flexibility, ceiling design and lighting design.

Lack of Personal Control

The environment created by displacement ventilation is reasonable but offers little or no personal control over conditions. A large number of swirl grilles are needed and care should be taken in their selection and location. This system has only limited application in offices and other spaces of low height as the desired stratification cannot be achieved.

Energy Costs

Displacement ventilation offers a high level of free cooling in mid season. But in mid winter, when the supply air requires tempering to about 18°C, it can be energy demanding. Restricting the volume to reduce the energy requirement further limits the cooling capacity.

Possible Pollution Levels

Mixing systems ensure good dilution whereas true displacement can have quite high levels of pollution carried in the ascending air as previously demonstrated by cigarette smoke trails in offices.




Zonal System Options

Zonal systems are now receiving a lot of attention from developers and end users. Generally there are two types: floor void supply – ceiling return (zonal displacement systems); and floor void supply – floor void return (zonal mixing systems).Both have their advocates, but in low height office space (2.35-2.6 m) the notion that high level fume should be exhausted from a high level is somewhat flawed because the amount of mixing that takes place in such space, caused by occupant movement, door swings, etc, can render such design intention almost impossible.  Both solutions rely on similar central plant to that of fan coil systems, but have less on-floor engineering.  Reduced need for duct work compared with central displacement ventilation systems they have much smaller fresh air ductwork and when well designed, lateral ductwork can be almost, if not completely, eliminated, freeing up the floor for use by the tenant for data cabling and other systems. Improved control and air quality.  Both types of systems are served with chilled water operating at 6°C flow/11°C return to achieve good levels of dehumidification in high occupation spaces. They can provide closer control of humidity to smaller zones and can ensure individual zones maintain high indoor air quality.

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Zonal Displacement Systems

Zonal displacement systems use the floor void as a supply channel and the ceiling void or office space as a return channel. Zones may be 100-250 m2.  They usually work with minimum fresh air rates offering savings in fan power and seasonal peak energy demand. The supply air velocity under the floor may be low and designers should take care to check temperature pick up. Simulation shows that lengths of paths under the floor should be less than 15 m.  Zone units are optimally positioned near the centre of the zone served.

Underfloor Requirements

Such systems can make use of swirl grilles when the cooling load is small (60-80 W/m 2 ) and the supply temperature can be limited to 18°C, but with higher loads, swirl grilles should be replaced by fan assisted terminals with vertical distribution. The vertical distribution avoids draughts as warmer room air is entrained into the air stream instead of cooler air being blown across the floor at a low level as in the case of swirl.

Good for High Load Areas

These systems are suitable for machine rooms where air can be introduced directly into the base of the equipment at a low temperature. Such systems are used for internet hotels and other high load areas where cooling loads can exceed 1200-1500 W/m2.  Fresh air is usually ducted to the top inlet of the zonal unit. This system has a more complicated ventilation distribution and exhaust system than zonal mixing systems.  System noise levels are in the region of 40 NR.

 Zonal mixing system












Zonal Mixing Systems

Zonal mixing systems make use of the floor void as both supply and return channels. Metal ducts are not used, but by the application of air segregation baffle usually fitted to the raised floor substructure. Zones may range up to 300 m 2 . Filtration may be to EU7 standard which reduces or removes pollen and other allergens, and thus reduces hay fever and other similar complaints.

Fan Assisted Terminals

The individual fan assisted terminals introduce the conditioned air into the space. Users have freedom to adjust both the temperature set point and fan speed on the unit. The space temperature is controlled by the flow rate of air introduced, which in turn is usually controlled by a damper, operating under the dictates of onboard sensors. The need for wall-mounted thermostats is eliminated.

Return Air

Air returns to the zonal unit via simple return air grilles positioned above the return air plenum in the raised floor. Fresh air may be introduced into the
return plenum or directly to the unit and exhaust air may be extracted through toilets, kitchens and additionally through exhaust air grilles at high level if required.

Flexibility with Efficiency

In offices where cooling loads are high, the zonal mixing solution becomes more attractive than the zonal displacement system as it can achieve cooling loads up to and in excess of 250 W/m 2 without undue draught, while offering the possibility for effective
warm air heating in perimeter zones in winter and cooling in summer.





Reduction in Floor Heights

Office heights can be kept within “pension fund” limits of about 2.5-2.6 m and thus the overall floor-to-floor height of the building can be in the region of 3.2-3.6 m overall compared with 5 m for conventional displacement systems. This offers dramatic savings in cost and time of construction. Such solutions support the Egan initiative.

Efficient Cooling, Heating and Ventilation

High levels of cooling can be achieved through the use of fan assisted terminals which introduce about 500 m 3 of conditioned air/hour into the space vertically instead of horizontally, thus permitting high induction and lower supply air temperatures (eg. 14°C) to be applied. This allows about three times more cooling to be achieved for the same volume of air distributed. Ventilation efficiency is high because of reduced shortcircuiting compared with ceiling-based systems. These terminals, while offering about 1.3-1.5 kW of sensible cooling and generally located every 12-15 m 2 , often offer personal control of fan speed and temperature set point and leading solutions have fully intelligent links to the central building management system.

Care in Design

With low supply temperature conditions it is advisable to run the computer calculation for condensation and possibly apply insulation and a vapour seal to the underside of the slab.  Maximum supply air to room temperature differentials should not usually exceed 10°C.  The velocity of the air should be 1 m/s to minimise temperature pick-up.  Paths can be extended under the floor to 30-35 m.  System noise levels are generally in the region of 32-35 NR.

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Floor voids need be no more than 300-350 mm high, offering a saving in building height. In new-build projects, this overall reduction in the height of the building (10-15%) can amount to a 5-7% saving in construction cost  (source: Swedish National Pension Fund)

Construction time can be cut by 15-25%  (source: BAA Lynton)

Zonal mixing systems do not require ceiling voids and thus can be used where head room is limited.

The low-pressure plenum (usually 5-15 Pa) reduces fan power and noise.

Computer simulations indicate energy savings in the order of 25% compared with VAV systems.
(source: Building Simulation)

Computer simulations indicate night-time cooling can achieve energy savings of about 9% and lower carbon dioxide emissions.
(source: J Roger Preston)

Multi-tenanted buildings or large single occupant buildings can benefit from diversified operational hours.

Humidity control can be installed on a zone-by-zone basis offering further energy saving.

Floor voids are easily accessed for cleaning whereas ductwork is difficult to access and expensive to clean.

The extent of diversification reduces the impact of equipment failure, maintenance and reconfiguration.

The systems are quick and easy to design.

Relocation of equipment is fast, offering savings in the cost of change – £100-170/m 2 compared with ceiling systems.
(source: Baker Wilkins and Smith)

Capital allowances/depreciation can be granted when the floor void is used as a ventilated plenum.

High levels of user satisfaction have been recorded in many countries.
(source: Swedish Clinic for Occupational Medicine)



Zonal units may take up 0.5% of floor space served, but riser sizes are usually much smaller than VAV and central displacement systems and floor-by-floor plant rooms can be reduced or even eliminated.  Ceiling-based fan coils have no need for floor-based equipment but require increased building height and more costly relocation and maintenance.

Air handling unit