Variable air volume (commonly known as VAV) cooling is an all-air system which can satisfy the individual cooling requirements of multiple zones, typically within office buildings. This is achieved by supplying air at a constant temperature from a central plant to one or more VAV terminal units in each zone.
The terminal units contain thermostatically-controlled dampers which regulate the amount of air entering the zone in response to the requirement for cooling. For example, the volume of air (and hence the cooling) supplied to a south facing zone on a sunny day will be higher to offset the heat gain.
The primary benefit of VAV over constant volume is its ability to simultaneously provide the required level of cooling to any number of zones within a building. VAV systems can be particularly energy efficient as a result of their ability to operate the main supply/extract fan(s) at reduced speeds for much of the year, when the overall volume of air required by the various zones is low (fans are generally the most significant user of energy in a centralised air system).
When the cooling load is low, the VAV terminal unit will throttle the supply air down to a minimum level of around 40 per cent of the maximum volume. There are two reasons why the volume cannot be allowed to go lower. First, the minimum requirement for fresh air must be maintained, and second, the velocity of air leaving the diffuser must not drop too low if an acceptable level of air circulation is to be ensured within the space.
At low velocities the Coanda effect (that which causes the air to adhere to and move along the underside of the ceiling) will be lost and the air will dump from the diffusers, leading to cold draughts. It is particularly important that VAV systems are designed and commissioned to a high standard to ensure that a satisfactory balance between cooling needs and ventilation rate is achieved.
Most types of VAV terminal unit can incorporate a heating device, which can boost the temperature of the supply air if conditions within the zone require it. Alternatively, perimeter zones can be heated by radiators or convectors. In many buildings, perimeter heating is the preferred option as it helps counter the effect of cold down-draughts from windows.
Fan-assisted VAV
The use of fan-assisted terminal units can ensure that dumping of the supply air will not occur. These units contain a small fan which mixes the supply air with re-circulated room air, and provides a virtually constant volume supply to the occupied space. The mixing of the two streams is controlled to achieve an air temperature that satisfies the cooling load for the zone being served. The disadvantages of fan-assisted terminal units are generally higher capital and maintenance costs, increased total fan power, and the potential for high noise levels in the occupied space.
Benefits
- Used in buildings with multiple zones to match the particular cooling/heating demands of each zone
- Can be relatively energy efficient due to the ability to reduce the speed of the supply/extract fan(s) during periods of low to moderate loads
- A correctly designed and commissioned system will give good temperature control
- VAV systems are particularly suited to buildings with a year-round cooling load.
Limitations
- Space requirements are high in both the plant room and ceiling voids
- Design and commissioning is particularly important if good system performance is to be achieved in terms of comfort and energy efficiency
- The design of some VAV systems is simplified by allowing the terminal units to bypass air that is not required. However this approach can result in oversized plant, wasteful fan power, and increased capital cost
- Fan-assisted terminal units generally have higher capital and maintenance costs and the potential for increased noise levels in the occupied space.
