Variable Air Volume System

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.


  • 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.


  • 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.


How does a VAV system differ from a constant volume system?
A VAV system differs from a constant volume system in that it supplies air at a variable flow rate to meet the changing cooling demands of individual zones, whereas a constant volume system supplies air at a fixed flow rate regardless of the zone’s cooling requirements. This flexibility allows VAV systems to optimize energy consumption and provide more precise temperature control.
What are the key components of a VAV terminal unit?

A VAV terminal unit typically consists of a thermostatically controlled damper, an airflow sensor, and a controller. The damper regulates the amount of air entering the zone, while the airflow sensor monitors the airflow rate and provides feedback to the controller. The controller adjusts the damper position to maintain the desired airflow rate based on the zone’s cooling requirements.

How do VAV systems achieve energy efficiency?

VAV systems achieve energy efficiency by operating the main supply and extract fans at reduced speeds for much of the year, when the overall volume of air required by the various zones is low. This reduces the energy consumption of the fans, which are typically the most significant user of energy in a centralized air system. Additionally, VAV systems can optimize energy consumption by only providing the required amount of cooling to each zone, reducing waste and minimizing energy losses.

Can VAV systems be used in applications other than office buildings?

While VAV systems are commonly used in office buildings, they can also be applied to other types of buildings that require zoned cooling, such as hospitals, universities, and shopping centers. However, the suitability of a VAV system depends on the specific cooling requirements and layout of the building, and may require modifications to accommodate unique demands or constraints.

How do VAV systems handle zone pressurization?

VAV systems typically use a combination of pressure sensors and controllers to maintain a neutral pressure in each zone. The pressure sensors monitor the zone pressure and provide feedback to the controller, which adjusts the damper position to maintain the desired pressure. This ensures that the air is distributed evenly throughout the zone and prevents over- or under-pressurization.

What are some common design considerations for VAV systems?

Common design considerations for VAV systems include determining the optimal zone layout and sizing, selecting the appropriate type and number of VAV terminal units, and designing the ductwork and piping systems to accommodate the variable airflow rates. Additionally, designers must consider factors such as acoustic performance, air quality, and system redundancy to ensure that the VAV system meets the building’s specific requirements and occupant needs.