VAV Box Explained: How Variable Air Volume Control Works
A VAV (Variable Air Volume) box sits between the central air handling system and individual zones, adjusting how much conditioned air reaches each zone based on that zone’s actual demand — instead of delivering a fixed airflow regardless of need. Jaideo Automation supplies the VAV Box.
How a VAV Box Works
Inside a VAV box, a damper modulates open or closed in response to a zone temperature signal, adjusting the volume of supply air delivered to that zone. An air velocity sensor inside the box confirms actual airflow, closing the control loop between the commanded damper position and the airflow actually being delivered.
Why VAV Instead of Constant Volume
- Energy savings — fan energy drops significantly as total system airflow reduces to match aggregate zone demand, rather than running at full volume constantly
- Zone-level comfort control — each zone gets only the airflow it needs, rather than one setting serving an entire floor
- Better part-load performance — a VAV system naturally throttles back during periods of lower occupancy or milder outdoor conditions
Pressure-Independent vs Pressure-Dependent VAV Boxes
A pressure-independent VAV box uses its internal airflow sensor to maintain a commanded airflow rate regardless of duct pressure fluctuations elsewhere in the system. A pressure-dependent box lacks that feedback, so its actual delivered airflow can drift as system pressure changes — pressure-independent designs are generally preferred in multi-zone systems where duct pressure varies as other VAV boxes open and close.
Reheat VAV Boxes
Some VAV boxes include a reheat coil, allowing the box to add heat to the reduced-volume airstream during periods of low cooling demand — preventing overcooling and excessive humidity in zones that need less airflow but still require some temperature correction.
Airflow Control Isn’t Only About VAV
A VAV box is one tool for controlling airflow, but not the only one. At the AHU, outdoor/return/exhaust dampers control the mix of air entering the system — see our Damper Actuators guide for how those are driven. Fan speed itself is another lever, particularly on variable-speed AHU fans. A complete airflow control strategy typically uses all three together: fan speed for overall system volume, AHU dampers for outdoor air mixing, and VAV boxes for zone-level distribution.
Where VAV Boxes Fit in the System
VAV boxes sit downstream of the AHU, controlled together with it — see our AHU Controllers guide for the upstream control side, and our Air Velocity Sensors guide for the airflow measurement inside the box itself.
Frequently Asked Questions
What is a VAV box?
A VAV box is a device that sits between the central air handling system and individual zones, modulating a damper to adjust the volume of conditioned air delivered to each zone based on demand.
What is the difference between pressure-independent and pressure-dependent VAV boxes?
A pressure-independent VAV box uses an internal airflow sensor to maintain a commanded airflow rate despite duct pressure changes, while a pressure-dependent box lacks that feedback and can drift as system pressure varies.
What is a reheat VAV box used for?
A reheat VAV box includes a heating coil to add heat to the reduced-volume airstream during low cooling demand, preventing overcooling and excessive humidity in zones needing less airflow.