Introduction to VAV Boxes

Lesson 2.1

What Is a VAV Box

Most commercial buildings use a central air handling unit to cool air down to about 55 degrees F, then push it through ductwork to every zone in the building. A VAV box (variable air volume) sits between that main duct and the space it serves. Its job is simple: control how much of that cold air gets into the room.

The "variable" part is what matters. Older constant-volume systems blew the same amount of air all the time, whether the room needed it or not. A VAV box has a motorized damper inside it that opens and closes to match the actual cooling load.

Here's something that trips up a lot of new techs: the controller doesn't command a damper position directly. It works to a flow setpoint, a CFM target. Every VAV box is commissioned with a minimum and a maximum CFM. The controller reads the flow sensor, compares the actual CFM to the active setpoint, and drives the damper to match. The damper position is the output of that calculation, not something you set directly.

Most VAV boxes also have a reheat coil: a small hot water coil (or sometimes electric strips) downstream of the damper. When the zone needs heat, the damper holds at its minimum position to keep fresh air moving, and the reheat coil warms that air before it reaches the space.

Meet Your Points

FLOW: Airflow (CFM) (Input): A differential pressure sensor across the inlet that tells the controller how much air is actually flowing through the box in CFM. The controller doesn't command a damper position directly; it chases a CFM setpoint.

DMP-CMD: Damper Command (Output): Analog output signal from the controller to the damper actuator. This is the result of the flow control loop.

HTG-VLV: Heating Valve (Output): Analog output signal controlling the hot water reheat valve position. Opens to add heat to the discharge air when the zone is below heating setpoint.

DAT: Discharge Air Temperature (Input): Measures the temperature of air leaving the VAV box into the space.

ZN-T: Zone Temperature Sensor (Input): Measures the air temperature in the space being conditioned. This is the primary feedback signal.

OCC: Occupancy Sensor (Input): Binary signal from a sensor or schedule telling the controller whether the space is occupied.

Four inputs feeding information to the controller. Two outputs controlling the hardware. That's the full I/O for a typical VAV box.

How It Thinks

The VAV controller is always in one of four modes based on occupancy and zone temperature relative to setpoint:

Cooling: Zone temperature is above the cooling setpoint. The CFM setpoint ramps toward maximum. The heating valve stays closed. Damper: Opening, HTG Valve: Closed.

Heating: Zone temperature is below the heating setpoint. The CFM setpoint holds at minimum. The heating valve opens to warm that air. Damper: Minimum, HTG Valve: Opening.

Deadband: Zone temperature is between the heating and cooling setpoints. CFM setpoint holds at minimum. Heating valve closed. The system is at rest. Damper: Minimum, HTG Valve: Closed.

Unoccupied: The occupancy signal is off. The CFM setpoint drops to its unoccupied minimum or the damper shuts completely. Setpoints widen to save energy. Damper: Closed, HTG Valve: Standby.