What is variable air volume?

Variable air volume (VAV) means the zone gets more or less supply air as load changes, instead of a constant-volume blast with reheat doing all the work. In a commercial VAV system, a central air handling unit feeds duct static; each zone's VAV box meters that air to the space. The contrast with constant air volume (CAV) is simple: CAV holds airflow fixed and varies temperature; VAV varies airflow (and may still reheat at minimum flow).

The zone control loop

Space temperature (sometimes CO₂ or occupancy) resets an airflow setpoint between the box minimum and maximum. The VAV controller then drives the damper so delivered airflow tracks that setpoint. Occupied, unoccupied and standby modes usually change the minimum, the temperature setpoints, or both. When the box is at minimum and the zone is still cold, a reheat coil (hot water or electric) picks up the rest of the load.

Pressure-independent VAV boxes

Most commercial boxes are pressure-independent: two cascaded loops. The temperature (or IAQ) loop writes an airflow setpoint in CFM or l/s. A faster flow loop reads inlet differential pressure or a flow sensor, applies the box K-factor, and moves the damper so measured airflow holds even when upstream duct static swings. Pressure-dependent boxes skip the flow loop and map damper position more directly to load; they drift when static changes and are rarer on modern BMS jobs.

What the BMS sees

A typical VAV point list includes zone temperature, airflow setpoint, measured airflow, damper command and/or position, reheat command (valve or SCR), and occupancy or minimum-flow mode. Many controllers also expose box discharge temperature and a local override or occupancy sensor. Those points live on the BMS already; analytics reads them rather than inventing a parallel sensor set.

Fault signatures analytics catch

Damper hunting at or near minimum often means a noisy flow signal, aggressive PID, a minimum below the controllable range, or duct static that is too high for stable low-flow control. Verify pickup tubing and calibration before retuning. A stuck or failed damper shows as command versus position mismatch, or airflow stuck high or low while the loop keeps writing. Airflow sensor drift, a bad K-factor, or a clogged pickup makes the controller look happy while a hood or traverse disagrees. Reheat fighting cooling shows as simultaneous heat and cool, a wrong mode or deadband, or a sensor lying about minimum flow so the box reheats while the AHU still cools hard.

Trend points that matter

When diagnosing a VAV box, co-trend airflow setpoint, measured flow, damper command (and position if separate), zone temperature, reheat command, and upstream duct static when the AHU exposes it. That set separates control hunting from sensor lies from plant-side static problems. FrostLogic Explore is Sensor Intelligence: it reads those BMS points and returns a prioritised queue. It is not a CMMS and not a dashboard replacement. Writes start off; predictive maintenance in Explore is the commercial path for VAV degradation ranked by cost and urgency.