Comparison

VAV vs CAV: Variable or Constant Air Volume Systems

Constant-volume systems vary the supply temperature. Variable-volume systems vary the airflow. Compare energy use, comfort control, cost and applications.

Mechanical, HVAC & Fluids · Last reviewed October 15, 2026

Short answer: CAV is simple and suits spaces with steady loads and tight ventilation needs. VAV saves fan energy where loads vary by zone and time, which is most commercial buildings.

	CAV (constant air volume)	VAV (variable air volume)
How it meets load	Fixed airflow. Supply temperature changes (reheat or cycling)	Supply temperature stays fixed. Airflow to each zone changes with the load
Fan energy	Fan runs at full flow	Fan slows with demand (VFD). Power falls steeply as airflow falls
Zone control	One zone per unit, or reheat for several	Many zones on one air handler through VAV boxes
Ventilation	Constant airflow keeps outdoor air steady	Minimum airflow and outdoor-air control are needed so ventilation holds at low load
Complexity and cost	Lower first cost, simple controls	Higher first cost: boxes, sensors, static-pressure control
Comfort risks	Overcooling or reheat energy at part load	Dumping or poor mixing at low airflow, noise at high airflow
Typical use	Labs, operating rooms, small buildings, constant-load process spaces	Offices, schools, hospitals general areas, multi-zone commercial buildings

How to choose

Do loads vary by zone and over the day? If yes, VAV usually pays back in fan and reheat energy.
Does the space need a constant air change rate, such as a lab or an isolation room? That favors CAV or a VAV with a controlled minimum.
How many zones? One zone per air handler favors CAV. Many zones favor VAV.
Check the standard. Energy codes set limits on reheat and fan power, which often steer large systems to VAV.

The fan-law reason VAV saves energy

For a fan where the affinity laws apply, shaft power varies about with the cube of speed. At 70% of design airflow, power is about 0.7³ = 0.343, roughly a third. A duct static-pressure setpoint that resets with demand saves more than a fixed setpoint.

Common mistakes

Setting the VAV minimum airflow too low and losing ventilation or mixing.
Leaving the duct static-pressure setpoint fixed at design.
Using reheat at high airflow when the minimum could be lower.

Two scenarios

A ten-story office building with perimeter and interior zones that need different cooling through the day. A variable air volume system sends each zone only the air it needs, and the fan slows down at part load. VAV is the usual choice.

A laboratory that needs a fixed number of air changes every hour for safety. A constant volume system, or a VAV system with a controlled minimum, holds the airflow steady while the supply temperature changes to meet the load.

Cost and life cycle

First cost: VAV costs more because of the terminal boxes, sensors and controls.
Energy: VAV saves fan energy and, with good controls, reheat energy. A poorly tuned VAV system can lose most of the saving.
Commissioning: VAV systems need setpoint tuning and checking of the minimum airflows. Plan for it.
Flexibility: VAV zones can be re-zoned by adjusting boxes and controls, which suits tenant changes.

Related tools and labs

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Questions

Does VAV always use less energy?

Not always. A poorly controlled VAV system with high minimums and fixed static pressure can waste more than a simple CAV system.

Can a lab use VAV?

Yes, with constant-volume or tracked exhaust and controlled minimum flow so room pressure and air change rates hold.

What is a VAV box with reheat?

A terminal unit that varies airflow down to a minimum, and then heats the air when the zone still needs heating. Reheat uses energy, so the minimum airflow should be as low as ventilation rules allow.

What is duct static pressure reset?

A control strategy that lowers the fan's duct static pressure setpoint when the boxes are mostly closed, which saves fan energy.

Do energy codes favor VAV?

Many energy codes limit reheat and fan power in ways that lead large multi-zone systems toward variable air volume. Check the code edition that applies.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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