VAV Troubleshooting Guide: Variable Air Volume Boxes

How VAV Boxes Work

A Variable Air Volume (VAV) box is a terminal unit in a ducted air conditioning system. It modulates the airflow to a zone based on the zone's temperature. A temperature sensor in the zone signals the VAV controller, which adjusts a damper inside the box. Some VAV boxes also include a reheat coil (hot water or electric) to heat the air when the airflow is reduced to minimum.

Common Problems

• Zone too hot: Possible causes: VAV damper stuck closed, airflow sensor blocked, reheat coil stuck on, controller failure, or insufficient supply air temperature. Diagnose: check the damper position, verify the airflow sensor reading, and check the controller output.
• Zone too cold: Possible causes: VAV damper stuck open (maximum airflow), reheat coil not operating, supply air temperature too low, or thermostat setpoint too low. Diagnose: check the damper position and the reheat valve operation.
• Damper not modulating: The actuator may be failed, the linkage seized, or the controller not sending the correct signal. Diagnose: command the damper to 0%, 50%, and 100% from the BMS and verify the damper moves.
• Airflow sensor reading incorrectly: The airflow sensor (typically a differential pressure type) can be blocked by dirt or the tubing can leak. This causes the controller to modulate based on incorrect data. Diagnose: compare the sensor reading to a handheld anemometer measurement.
• Reheat coil not heating: The hot water valve may be stuck closed, the coil may be air-bound, or the electric heater element may have failed. Diagnose: check the valve position, verify hot water flow, or check heater continuity.
• Noise from the VAV box: Excessive airflow velocity, loose damper linkage, or whistling through the damper at low positions. Diagnose: measure the airflow and compare to the box's design range. Adjust the maximum airflow setting if it's too high.
• Controller communication failure: The VAV controller has lost communication with the BMS. It may operate in a failsafe mode (typically minimum airflow). Diagnose: check the communication wiring and the controller's status indicator.

Diagnostic Procedure

1. Check the thermostat: Verify the setpoint and the actual zone temperature. Is the thermostat calling for heating or cooling? Is the reading accurate (compare to a handheld thermometer)?
2. Check the damper position: Command the damper to fully open and fully closed from the BMS. Verify the damper moves. If it doesn't, check the actuator power and linkage.
3. Check the airflow sensor: Compare the airflow reading to a handheld measurement. If they differ, clean the sensor ports and check the tubing for leaks.
4. Check the reheat coil: Verify the hot water valve or electric heater operates when commanded. Check the valve actuator and the water temperature.
5. Check the supply air: Verify the supply air temperature and static pressure are within the design range. The VAV can only work if the supply air is correct.
6. Check the controller: Look for fault codes or communication errors. Restart the controller if needed. Verify the control parameters (setpoint, PI gains, minimum airflow) are correct.

Prevention

• Clean the airflow sensor ports annually — dust accumulation causes inaccurate readings
• Inspect damper linkages annually — lubricate and tighten as needed
• Verify the actuator stroke annually — command full open and full closed from the BMS
• Calibrate zone temperature sensors annually against a reference thermometer
• Check the reheat valve operation before each heating season
• Verify BMS communication with each VAV controller quarterly
• Review the control parameters — minimum airflow, maximum airflow, and dead band settings