Variable Air Volume Systems
What are they?
Variable air volume (VAV) systems supply air at a variable temperature from an air handling unit (AHU). The AHU draws in fresh air and return air from inside the building and either heats or cools it before reaching the occupied space. Within each space, the amount of air is adjusted to control the space temperature using VAV terminals, which may also have some local heating capacity.
VAV systems either use central AHUs located on dedicated plant room floors that serve multiple levels of the building, or may use smaller units in plant rooms on each floor.
Advantages / Disadvantages
- Highly flexible, but ideal for open plan spaces
- Industry standard technology with plenty of industry experience in operation and maintenance
- Potential for high efficiency operation when well controlled and maintained.
- Air-handlers can be (and generally are) configured for the use of outside air for free cooling.
- High component count and heavy dependence on control can make the system prone to energy-intensive failure modes that may also affect occupant comfort.
- Large multi-storey air-handling systems may have difficulty adequately servicing floors with exceptionally high or low loads.
- Large multi-story air-handling systems often less efficient at servicing after-hours loads.
When well controlled and maintained, VAV systems can achieve some of the highest levels of energy efficiency in the market. However, high efficiency is only achieved through superlative control, and relatively minor errors in control can have severe impacts on energy use.
With good management, control, maintenance and commissioning, running costs can be low; without these costs can increase significantly.
A building management system (BMS) is essential to achieve efficient operation.
Retrofit / improvement opportunities
Most VAV systems are capable of being refurbished to achieve a high level of efficiency even if the current level of efficient is poor, provided that basic plant is in reasonable condition. The major areas for improvement are:
- Review and optimisation of control for VAV terminals and air-handlers. The most common failure modes for VAV systems result in fans operating at close to maximum speed all year around and VAV terminals causing extensive heating and cooling conflicts. These issues are largely rectifiable through improved control.
- Optimisation of the use of outside air for free cooling when conditions are appropriate.
- Upgrade of controls for boilers, chillers and associated pumps
- Upgrade of pressure dependent VAVs to pressure independent VAVs, which provide much better feedback on control. This can require floors to be vacated.
- Upgrade of chiller plant. Chillers have 15-20 year lifespan, although they generally have been superseded in terms of efficiency well before the end of their operational life. Replacement of chillers is best undertaken in winter.
Control improvements can be implemented with the tenants in-situ
VAV systems are typically found in large office buildings and some shopping centres from the 1970s onwards. They are less common in smaller buildings.
Floor plate implications
The size of riser ducts can be significant for multi-storey buildings served from a central AHU (located, for example, on the roof or in the basement). The risers may restrict flexibility on floor plates. In buildings with AHUs on each floor, the AHU plant room will take up a significant amount of space.
Temperature control / Occupant comfort
VAV systems can offer a high degree of temperature control when well controlled
The VAV terminals and controls require routine maintenance. Central plant requires standard levels of maintenance. Controls require careful configuration and management; arbitrary adjustment of temperature set-points within VAV systems is a leading cause of energy waste.
How to identify them
VAV systems are commonly found in larger office buildings, but are largely indistinguishable from other AHU based systems to the non-technical observer. Maintenance or operational staff will be able to advise whether a system is VAV.
Questions to ask
- Does the system have an economy cycle to improve energy efficiency?
- Are the VAVs pressure dependent or pressure independent?
- Is there a building management system (BMS)?
- Is the system regularly maintained (at least annually) and seasonally commissioned?
AIRAH Guide DA08 HVAC&R An Introduction
CIBSE Ventilation / Air conditioning / Technical Glossary