VAV vs CAV Air Distribution
A side-by-side comparison of the two main all-air distribution strategies — constant air volume and variable air volume — explaining how each maintains comfort, their energy and zoning differences, and where each fits.
Once an air handling unit has conditioned the air, it must be distributed to many zones whose cooling needs differ and change through the day. The two classic strategies are Constant Air Volume (CAV), which delivers a fixed amount of air and varies its temperature, and Variable Air Volume (VAV), which keeps the supply temperature steady and varies the amount of air to each zone.
The choice affects comfort, energy use and cost for the life of the building. VAV has become the default for larger multi-zone commercial buildings because it saves fan energy and controls many zones independently, while CAV remains simpler and appropriate for single-zone or specialised spaces.
How it works
In a CAV system the supply airflow is essentially fixed. To match a zone whose load has fallen, the system raises the supply temperature — often by reheating the air. Comfort is maintained, but the fan keeps moving the full air quantity at all times, and any reheat adds energy, so part-load operation is inefficient.
In a VAV system the supply air is kept cold and constant in temperature, and a VAV terminal box in each zone throttles the airflow up or down to meet that zone’s current load. As loads fall, the boxes close in, total airflow drops, and a variable-speed (VFD) supply fan slows to maintain duct pressure — sharply reducing fan energy because fan power falls steeply with airflow.
Zoning is where VAV shines. Each VAV box is controlled by its own thermostat, so a sunny west office and a shaded interior room are served independently from one AHU. CAV typically serves a single zone or a group with the same profile, so independent control needs separate units or reheat.
Ventilation needs care in VAV. Because airflow falls at low load, the design must ensure each zone still receives its minimum fresh-air requirement — handled with minimum airflow setpoints, dedicated outdoor-air systems, or demand-controlled ventilation. CAV inherently delivers constant air, so minimum ventilation is straightforward.
Controls and complexity differ. VAV needs box controllers, a pressure-controlled fan and BMS integration, so it costs more to install and commission but pays back through energy savings and flexible comfort. CAV is mechanically simpler, cheaper upfront, and easy to maintain, which keeps it relevant for the right applications.
Main types
In the UAE
- VAV is widely used in UAE multi-zone offices and commercial towers, where varied solar exposure and occupancy make independent zone control and fan-energy savings valuable.
- VAV designs must protect minimum fresh-air rates at low load to meet indoor-air-quality requirements, often using minimum setpoints or dedicated outdoor-air systems.
- Reducing fan energy through VAV and VFDs supports Estidama and UAE energy efficiency goals, given how large the cooling and air-movement share of building energy is.
How GPR applies this
GPR designs and installs both CAV and VAV air distribution across Abu Dhabi, selecting the strategy that suits each building. For multi-zone offices and towers we deliver VAV systems with VFD fans, zone boxes and BMS control to save energy and give independent comfort, while protecting minimum ventilation. For single-zone and specialised spaces we apply efficient CAV. Every system is balanced and commissioned to deliver the design airflow and indoor air quality.
Frequently asked questions
What is the main difference between VAV and CAV?
CAV keeps airflow constant and varies the supply temperature (often with reheat); VAV keeps supply temperature constant and varies the airflow to each zone to match its load.
Why is VAV more energy efficient?
When loads fall, VAV boxes reduce airflow and the variable-speed fan slows down. Because fan power drops steeply with airflow, this saves large amounts of energy at part load.
When is CAV the better choice?
For single-zone or specialised spaces where loads are steady or independent zoning is not needed — CAV is simpler, cheaper to install and easy to maintain.
How does VAV handle fresh-air ventilation at low load?
With minimum airflow setpoints at each box, a dedicated outdoor-air system, or demand-controlled ventilation, so each zone still receives its required fresh air.
Is VAV more expensive than CAV?
It usually costs more to install and commission due to box controllers, a pressure-controlled fan and BMS integration, but it typically pays back through energy savings and better zone comfort.