What Is a VAV System?

A Variable Air Volume (VAV) HVAC system modulates the volume of conditioned air delivered to each zone in response to the zone's load. Unlike a Constant Air Volume (CAV) system — which always delivers the same airflow and varies temperature — a VAV system varies airflow while maintaining a relatively constant supply air temperature (typically 55°F for cooling).

VAV is the dominant HVAC strategy for commercial office buildings in the United States because it saves significant fan energy (fan power varies with the cube of airflow — halving airflow reduces fan power by 87.5%) and provides individual zone temperature control without separate cooling systems.

VAV System Components

Air Handling Unit (AHU): The central unit contains the cooling coil, heating coil (or preheat coil), filters, and supply fan. In a VAV system, the fan is typically driven by a Variable Frequency Drive (VFD) that reduces fan speed as system airflow demand decreases.

VAV Terminal Boxes: Each zone has a VAV box — a factory-built damper assembly with controls. As the zone's load changes, the zone thermostat sends a signal to the VAV box controller to open or close its damper, modulating airflow between a minimum and maximum CFM setpoint.

Ductwork: High-pressure supply ductwork runs from the AHU to each VAV box. Downstream of each box, low-pressure flexible ductwork connects to the supply diffusers. The high-pressure section must be designed and sealed to handle the static pressure produced by a VFD-controlled fan.

Building Automation System (BAS): VAV systems are controlled by DDC (Direct Digital Control) systems. The BAS maintains the supply air temperature setpoint, resets the duct static pressure setpoint based on terminal box demand, and coordinates economizer operation.

Single-Duct VAV vs. Dual-Duct VAV

Single-duct VAV is the standard: one duct carries cold supply air to all VAV boxes. Each box has a reheat coil (electric, hot water, or steam) for zones with heating loads. When a zone needs heat, the VAV box goes to its minimum position and the reheat coil activates.

Dual-duct VAV uses two parallel duct systems — one cold, one warm. A mixing box blends the two airstreams to achieve the desired zone temperature. Dual-duct systems provide better simultaneous heating and cooling capability but are more expensive and use more duct space.

Minimum Airflow and Ventilation

One challenge with VAV systems is maintaining adequate outdoor air ventilation when airflow is reduced. ASHRAE Standard 62.1 requires a minimum outdoor air fraction for each zone. As VAV boxes throttle down, the percentage of outdoor air in the supply air can drop below the required minimum. Modern VAV systems use ASHRAE 62.1 Appendix A (Multiple Zone HVAC System) calculations and CO₂-based demand control ventilation to manage this.

Duct Static Pressure Control

The VFD on the supply fan is controlled to maintain a static pressure setpoint in the supply duct — typically measured at a point 2/3 of the way along the longest duct run. ASHRAE Guideline 36 recommends resetting the duct static pressure setpoint based on terminal box demand (trim-and-respond logic), which saves additional fan energy compared to a fixed static pressure setpoint.

When to Use VAV

VAV is ideal for multi-zone commercial spaces where loads vary significantly between zones and throughout the day. It is less appropriate for spaces with high latent loads (kitchens, natatoriums), very small zones (where minimum airflow would be uncomfortably cold), or buildings where the complexity and cost of controls outweighs the energy savings — such as small commercial or retail spaces where a split system or fan coil unit system may be more practical.