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Article Date: 12th April 2018

AET Flexible Space - UfAC vs Displacement Ventilation

Displacement Ventilation - Underfloor Air Conditioning - Raised Access Floors

AET - To air-condition 180 Great Portland Street, AET Flexible Space supplied an underfloor zonal mixing system

It is claimed that underfloor air conditioning can offer improved working conditions. This CPD module considers the difference between UfAC and Displacement Ventilation.

In the first Trane Company Air Conditioning Manual, published in 1938, air-conditioning was defined as the simultaneous control of temperature, humidity, air movement and the quality of the air in the space. The definition still holds today, though many people would add noise control to the list.

For optimum comfort, one should have warm feet and a cool head without noticeable draught. In the workplace, good air movement should be maintained to avoid stuffiness and dilute pollutants such as CO2, ozone and formaldehyde. However, according to the ISIAQ-CIB Task Group TG 42, air velocity should not exceed 0.18-0.2m/s.

Since raised access floors have become standard in offices, underfloor air conditioning has become more popular. There are high costs associated with the maintenance and reconfiguration of systems above workstations and, on occasion, leaks of water can cause serious and expensive damage to computer networks. It has also been found that air introduced at a low level offers improved conditions in the workspace as well as additional benefits in time required for maintenance.

Many aspects of office design are now affected by legislation and operators must adapt air-conditioning systems to meet changing requirements and office layouts. Air-conditioning systems have tended to be either central station systems in a variety of configurations, or smaller diversified plants such as fan-coil and VRV systems. In the drive for economy, systems have tended towards higher volume/higher temperature differential solutions with fewer units, but this has led to increased complaints of draughts and noise. Underfloor systems can permit a greater degree of user control, even individual user control, which is becoming an increasingly significant consideration.

This CPD considers the different types of underfloor air-conditioning system available and some of the associated design considerations.


Displacement ventilation
This is an early form of ventilation, developed in Sweden in 1978. In a displacement ventilation system, air is supplied at low level and extracted at high level. As it is essentially a ventilation system with limited cooling potential (40-60W/m²) with limited control options, it does not fully meet the definition of air-conditioning. Displacement systems often serve large areas and therefore must try to fulfil simultaneous demand for both heating and cooling. 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, this warm air is naturally buoyant, so it rises to a high level and is exhausted before completely heating the space, and the heating efficiency is therefore impaired. Another disadvantage of such large zones is that systems can provide only an average humidity, often leading to complaints from users.

Swirl grilles at floor level
With these systems, air is introduced at a temperature approximately 5°C lower than room temperature 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. The small temperature difference between the supplied air and air already in the room is intended to avoid complaints of draught, but it means that many diffusers are required to meet the cooling load. Positioning these in suitable locations is often difficult to achieve.

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