What Is a Variable Air Volume (VAV) System?
A VAV system is the standard commercial HVAC architecture: a central air handler supplies air at constant temperature while VAV terminal boxes in each zone vary the airflow to match the zone's load — less air when the zone needs less cooling. It is the baseline system in energy codes (the system others are compared against) and remains the right choice for many office, retail, and school buildings.
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How it works
The air handler pushes 55°F supply air through the ductwork; each zone's VAV box has a damper that modulates airflow based on the zone thermostat, with a reheat coil for zones needing heat while the system cools elsewhere. Variable-speed drives on the supply fan cut fan energy dramatically at part load — the fan laws mean halving airflow uses roughly one-eighth the power. Modern VAV adds demand-controlled ventilation (CO2 sensors) and supply-air-temperature reset for further savings.
VAV vs. the alternatives
Versus constant-volume: far more efficient and comfortable, slightly more complex. Versus VRF: VAV centralizes maintenance at the air handler (one big machine in a mechanical room versus dozens of fan coils), handles ventilation natively, and suits buildings with consistent occupancy patterns; VRF wins on zoning granularity and retrofit flexibility. Versus chilled beams/DOAS: VAV is simpler and cheaper first-cost, less efficient at the margin. The MEP engineer models the options against the building's actual load profiles.
Design and commissioning notes
Good VAV design lives in the details: minimum airflow setpoints low enough to avoid overcooling (a classic comfort complaint), reheat minimized through proper zoning, ductwork sized for the variable flows, and controls sequences that actually implement the energy strategies. Commissioning the airside — verifying every box, sensor, and sequence — is where the designed efficiency becomes real efficiency.
The bottom line
VAV varies zone airflow from a central air handler — the commercial baseline for efficiency and comfort. Design the minimums, reheat, and controls sequences carefully; commission every box.
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