VAV System Performance
Variable Air Volume system principles, VAV box minimum and maximum airflow settings, duct static pressure control, zone-level testing of VAV terminals, and diagnosing common VAV performance problems.
- Explain how VAV systems modulate airflow to match zone loads and why they are more efficient than constant volume systems
- Verify VAV box minimum and maximum airflow setpoints against design specifications
- Test duct static pressure control and verify the sensor location and setpoint are optimized
- Diagnose common VAV problems including hunting, starving, and simultaneous heating and cooling
Lección 1
VAV System Principles and Components
What Is a Variable Air Volume System?
A Variable Air Volume (VAV) system is a commercial HVAC configuration that varies the volume of supply air delivered to each zone based on the zone's heating or cooling load. Unlike constant volume (CV) systems that run the fan at a fixed speed and vary the supply air temperature to meet loads, VAV systems deliver air at a relatively constant temperature (typically 55 degrees F supply air for cooling) and vary the volume to match the load.
When a zone has a high cooling load, the VAV box opens to deliver more cool air. When the load decreases, the VAV box closes down to deliver less air. At minimum load, the VAV box may be at its minimum airflow position, delivering just enough air for ventilation.
VAV System Components
Air Handling Unit (AHU): The central unit that conditions air and delivers it to the VAV duct system. In a VAV system, the AHU typically has a variable-speed supply fan (VFD-controlled or ECM motor) that adjusts speed to maintain a duct static pressure setpoint.
VAV Terminal Units (VAV Boxes): Individual boxes located in the ductwork above each zone. Each box contains a damper that modulates open or closed based on the zone thermostat signal. Types include:
- Cooling-only VAV box: Simple damper that modulates from maximum to minimum airflow. No heating capability.
- VAV box with reheat: Includes a hot water coil or electric heating element downstream of the damper. When the zone needs heating, the damper goes to minimum position and the reheat coil activates.
- Fan-powered VAV box (parallel): Includes a small fan that draws air from the ceiling plenum and mixes it with primary air. The fan operates during heating mode to supplement airflow when primary air is at minimum.
- Fan-powered VAV box (series): The fan runs continuously and handles all air delivered to the zone. The primary air damper modulates, and the fan provides constant airflow to the zone.
Duct Static Pressure Sensor: A sensor in the supply duct that measures static pressure. The AHU fan speed controller uses this sensor to maintain a constant duct pressure as VAV boxes open and close.
DDC Controller: A digital direct control system that manages the AHU, VAV boxes, and their setpoints. Modern systems use BACnet or similar protocols to communicate between controllers.
Cooling-Only VAV
Damper: Modulates max to min CFM
Heating: None (relies on other zones or baseboard)
Cost: Lowest
Use: Interior zones with consistent cooling loads
VAV with Reheat
Damper: Modulates for cooling, goes to min for heating
Heating: Hot water or electric coil in the box
Cost: Moderate
Use: Perimeter zones with varying loads
Fan-Powered VAV (Parallel)
Damper: Modulates primary air, fan mixes plenum air
Heating: Fan + reheat coil for heating mode
Cost: Higher
Use: Perimeter zones in cold climates
Energy Efficiency of VAV Systems
VAV systems save energy because the fan power cube law applies as the system load decreases. When zones are at partial load (most of the time), VAV boxes close down, duct static pressure would rise, so the AHU fan slows down to maintain the setpoint. A fan running at 60% speed uses only about 22% of full-speed power (0.60^3 = 0.216).
In a constant volume system, the fan runs at full speed all the time regardless of load. The compressor cycles to match the load, but fan energy remains constant. VAV systems save 20-60% of fan energy compared to constant volume systems, depending on the load profile.
VAV systems vary the volume of supply air to match zone loads, using 55 degrees F supply air for cooling. The AHU fan speed modulates to maintain duct static pressure as VAV boxes open and close. The fan power cube law means running at 60% speed uses only 22% of full-speed power, making VAV significantly more efficient than constant volume systems.