Introduction to VAV Boxes

Most commercial buildings use a central air handling unit to cool air down to about 55°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. That wastes energy. A VAV box has a motorized damper inside it that opens and closes to match the actual cooling load. More people in a conference room? Damper opens. Empty office on a cool morning? Damper closes down to its minimum.

Here is something that trips up a lot of new techs: the controller does not 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 minimum is what ventilation code requires even when there is no call for cooling, and the maximum is how much the box is allowed to deliver at full demand. 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. When a box is "at minimum," that means airflow is at its minimum CFM setpoint, not that the damper is at a fixed percentage.

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. This is how a single system can cool some rooms and heat others at the same time.

As a controls technician, the VAV box is one of the first things you will commission on a job. You will wire the sensors, map the points in the controller, verify the damper strokes correctly, and make sure the sequence of operations matches the mechanical engineer's design. Understanding what each part does, and what the controller is trying to accomplish, is the foundation for everything else.