A Variable Air Volume (VAV) box is an HVAC terminal unit installed in ductwork to regulate the volume of conditioned air delivered from a central air handling unit (AHU) to a specific building zone. By modulating an internal damper in response to thermostat demand, the VAV box increases or decreases airflow to maintain the desired temperature in its assigned space. This zone-level control strategy distinguishes VAV systems from constant air volume (CAV) systems, which deliver a fixed volume of air regardless of actual thermal load.
Technical Details and Specifications
A VAV box consists of a sheet metal casing, an airflow sensor, a modulating damper, and a damper actuator controlled by a zone thermostat or building automation system (BAS). The airflow sensor, typically a velocity pressure transducer, monitors air volume so the controller can adjust the damper position to meet the setpoint.
- Airflow Range: VAV boxes typically modulate between 20% and 100% of their design maximum CFM (cubic feet per minute). Minimum airflow settings are critical for maintaining adequate ventilation and preventing dumping of unconditioned air.
- Inlet Sizes: Common round inlet duct connections range from 6 inches to 20 inches in diameter, with larger sizes accommodating higher airflow capacities.
- Pressure Drop: Units are generally designed for a pressure drop between 0.2 and 0.5 inches of water gauge (w.g.) at design airflow.
- Control Signals: Damper actuators accept pneumatic signals (3 to 15 psi) or electronic signals (0 to 10 VDC or 4 to 20 mA), depending on the control infrastructure of the building.
VAV boxes are available in two primary configurations: cooling-only and reheat. A reheat VAV box includes a hot water coil or electric heating element downstream of the damper, allowing the unit to warm air when a zone requires heating rather than cooling. Fan-powered VAV boxes incorporate a small integral fan to draw plenum air into the supply stream, improving air circulation at low-load conditions.
Common Applications
VAV boxes are standard equipment in commercial buildings, including office towers, hospitals, universities, government facilities, and large retail environments. They are especially well suited for buildings with multiple zones that experience varying occupancy levels and thermal loads throughout the day. Interior zones with consistent heat gain from lighting and equipment may use cooling-only boxes, while perimeter zones exposed to exterior walls and windows often require reheat boxes to offset heat loss during colder months.
Relevant Standards and Codes
VAV box design, testing, and application are guided by several industry standards. ASHRAE Standard 62.1 establishes minimum ventilation rates that directly influence minimum airflow settings. ASHRAE Standard 90.1 sets energy efficiency requirements that affect reheat limitations and fan power allowances. ARI Standard 880 (now AHRI 880) provides the testing methodology for air terminal unit performance, including sound power levels, which is an important consideration in occupied spaces.
Practical Significance
VAV systems offer significant energy savings compared to constant volume systems because the central supply fan can reduce speed as zone dampers close, lowering fan energy consumption. Individual zone control also reduces simultaneous heating and cooling, cutting overall energy waste. Proper commissioning and regular maintenance of VAV boxes, including actuator calibration and airflow sensor cleaning, are essential to sustaining both occupant comfort and system efficiency over the life of the building.