- Project plans
- Project activities
- Legislation and standards
- Industry context
- Specialist wikis
Last edited 16 Oct 2020
Variable air volume VAV
Ventilation is necessary in buildings to remove ‘stale’ air and replace it with ‘fresh’ air:
- Helping to moderate internal temperatures.
- Replenishing oxygen.
- Reducing the accumulation of moisture, odours, bacteria, dust, carbon dioxide, smoke and other contaminants that can build up during occupied periods.
- Creating air movement, which improves the comfort of occupants.
Very broadly, ventilation in buildings can be classified as ‘natural’ or ‘mechanical’. Mechanical ventilation systems can also include heating, cooling, humidity control and air filtration. These functions are often described collectively as HVAC (Heating Ventilation and Air Conditioning).
- Variable air volume (VAV), in which the temperature of the supply air remains constant, but the volume varies (also known as variable volume, constant temperature VV-CT).
- Constant air volume (CAV) in which the volume of air supply remains constant, but the temperature varies (also known as constant volume, variable temperature CV-VT).
- Variable volume, variable temperature (VV-VT sometimes referred to as variable volume and temperature - VVT).
In more complex systems, where spaces have different heating or cooling demands, there may be additional local control of the amount of air that enters each space. Typically, cool air is supplied by an air handling unit, and thermostatically controlled dampers regulate the amount of air that enters each space. The damper must always remain partially open to allow some ‘fresh’ air into the space.
The fans in the air handling unit are adjusted (variable frequency drive VFD) to control the air pressure in the ductwork. Refrigerant flow is also adjusted to ensure that the air temperature remains constant. VAV terminal units may include fans that re-circulate a proportion of internal air along with the ‘fresh’ supply air to reduce the cooling load.
Where variations between spaces mean that some local heating is required in to maintain constant temperatures throughout a building, VAV terminal units may re-heat the supply air. Despite the apparent waste of re-heating previously cooled air, this can be more economic that providing a warm air supply from the air handling unit when there is only limited heating demand. Heat may be provided in VAV terminal units by electrical elements or by hot water coils.
 Find out more
 Related articles on Designing Buildings Wiki
- Air conditioning.
- Building services.
- Chiller unit.
- Computational fluid dynamics.
- Constant air volume.
- Cross ventilation.
- Dew point.
- Interstitial condensation.
- Mechanical ventilation.
- Natural ventilation.
- Passive building design.
- Solar chimney.
- Stack effect.
- Thermal comfort.
Featured articles and news
So why not write something?
LETI publishes guidance for energy efficient home retrofits.
Predictions about adequate post-pandemic IAQ in non-domestic buildings.
Government publishes plans to 'build back greener'.
The contentious nature of claims associated with cladding, fire safety and EWS1 forms.
ECA comments on low-carbon heating systems initiative and Heat and Buildings Strategy.
Cinders and other forms of domestic rubbish created filth but also generated great wealth.
CIC 2050 Group requests input to find out priorities for future industry leaders.
IHBC publishes response to consultation.