HVAC · Compare
VAV vs CAV air distribution: which system to spec
Variable air volume varies flow at a constant cold supply temp; constant air volume holds flow steady and varies temperature. Zone count decides.
Short answer
Spec VAV for a multi-zone commercial building with varied loads and schedules; spec CAV for a single zone or where airflow itself must stay constant. The single biggest deciding factor is zone diversity: if many spaces peak at different times and want independent comfort, VAV harvests that diversity and saves fan energy at part load. If the load is one space with one setpoint, or a process, pressurization, or exhaust-driven room that needs constant airflow, CAV is simpler, cheaper, and correct.
VAV (variable air volume) vs CAV (constant air volume): side by side
| Factor | VAV (variable air volume) | CAV (constant air volume) |
|---|---|---|
| What varies | Airflow per zone varies; supply air held cold (commonly ~55F) | Airflow held constant; supply temperature varies to meet load |
| Zoning | A box and controller per zone; each holds its own setpoint independently | Single-zone by nature; multi-zone needs heavy dual-duct or multizone hardware |
| Fan energy | VFD rides part-load demand; commonly 30-50% lower fan energy than CAV | Fan moves full design airflow every hour it runs |
| Upfront cost / complexity | Higher first cost; needs DDC network, VFD, static pressure sensor, box controllers | Lower first cost; a thermostat and a few actuators on a single-zone unit |
| Commissioning / balancing | Commission behavior: set min/max at every box, calibrate flow sensors, drive reset sequences | Close to set-and-forget: set flow once, confirm temperature control |
| Performance limits | Low-flow dumping and stratification; reheat waste if box minimum set too high | Uneven comfort across a multi-zone unit; overcools then reheats in dual-duct/multizone |
| Code / standards | 90.1 effectively defaults to VAV with reset above thresholds; 62.1 sets box minimums; Guideline 36 sequences | Fits single-zone applications; single-zone VAV with a VFD often required by code on packaged units |
| Best use | Multi-zone commercial: offices, schools, mixed-use floors with varied loads and schedules | One space/setpoint (warehouse, retail, gym) or constant-airflow needs (labs, exhaust makeup, pressurization) |
| Operator demand | Depends on resets staying enabled and tuned; poor performance is usually uncommissioned controls | Little to commission or tune; steady single operating point |
Which should you pick?
Choose VAV (variable air volume) when
- The building has many zones with different loads, orientations, and schedules that peak at different times
- You want independent room-by-room comfort off a single supply duct
- Part-load fan savings and energy code compliance drive the design on a larger system
- You have an operator and commissioning budget to set box setpoints and keep static and supply-temp resets running
Choose CAV (constant air volume) when
- The load is a single zone with one setpoint (warehouse, single retail box, gymnasium, server closet)
- Airflow must stay constant for a process, constant exhaust makeup, or pressurization (some labs)
- The load runs flat around the clock, so there is no part-load diversity for VAV to harvest
- First cost and simplicity matter more than part-load energy, and a VFD single-zone VAV covers the efficient middle
Bottom line
It depends on zone count and whether constant airflow is a requirement or a compromise. For a multi-zone commercial building with varied loads, VAV is the standard: it zones better and cuts fan energy at part load, but it costs more to build, needs a DDC network with static pressure and supply air temperature reset, and only pays off if the control sequences are actually commissioned and left enabled. For a single zone, a flat process load, or a room that needs constant directional airflow, CAV is cheaper and correct, and adding VAV boxes buys nothing. Single-zone VAV with a VFD is the middle ground that captures fan savings on a packaged unit without any boxes. Match the system to the building's load behavior, not a habit, and let the adopted energy code set the floor.
FAQ
What is the difference between VAV and CAV?
A CAV (constant air volume) system holds the airflow steady and varies the supply temperature to meet the load. A VAV (variable air volume) system holds the supply air at a constant cold temperature, commonly around 55F, and varies the airflow to each zone through VAV boxes. VAV saves fan energy and zones better; CAV is simpler and fits single-zone loads.
Why is VAV more efficient than CAV?
VAV saves fan energy at part load, where fan power can fall toward the cube of the airflow, and it avoids overcooling air only to reheat it the way multi-zone CAV does. A VAV fan slows as zones throttle back; a CAV fan moves full airflow every hour. Published comparisons commonly show 30 to 50 percent lower fan energy, though the real number depends on the building, load profile, and controls.
When should I use CAV instead of VAV?
Use CAV for a single zone with one setpoint, or where airflow must stay constant for a process, pressurization, or constant makeup-air reason, such as some labs and exhaust-driven spaces. On flat loads with little diversity, VAV's boxes and controls buy nothing. Single-zone VAV with a VFD captures fan savings on a single-zone packaged unit without adding boxes.