VAV Box Minimum Airflow and Reheat at Low Inlet Static
A 24-box VAV system serves a two-story office. Zone 5 is a west-facing perimeter zone with a 1,400-cfm maximum and a 420-cfm minimum plus an 8-kW electric reheat coil. At the box, the design inlet static is 0.50 in. w.c. With the system at design airflow and the zone at 74°F, the box is driven to maximum and the flow hood reads 1,050 cfm at a measured inlet static of 0.35 in. w.c. The factory airflow sensor is calibrated and the damper is verified 100% open. At minimum, with reheat at 100%, the box reads 260 cfm and a 78°F discharge air temperature; the space drifts to 69°F. The manufacturer lists a 25% minimum turndown and requires at least 0.30 in. w.c. inlet static for sensor accuracy. The diffuser is a 4-way ceiling slot with a 700-fpm neck velocity at design.
1. Calculate the flow expected at 0.35 in. w.c. if the box followed a square-law duct/fan relationship, and decide whether the measured 1,050 cfm proves a box fault.
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Expected flow: Q2 = Q1 × sqrt(P2/P1) = 1,400 × sqrt(0.35/0.50) = 1,171 cfm. Measured flow is 1,050 cfm, which is 90% of that estimate and 75% of design. Because the damper is fully open and the sensor is calibrated, the shortfall is mainly an upstream static/duct problem, not proof of a defective box. Check for flexible-duct compression, closed volume dampers, duct leakage, and low fan speed. Also compare the box pressure drop at 1,050 cfm with the manufacturer's catalog. The 0.35 in. w.c. is above the 0.30-in. sensor minimum, but it is below design, so the box cannot deliver design airflow. Do not open the damper further; it is already at 100%.
2. Evaluate the 260-cfm minimum against the manufacturer's turndown and calculate the reheat temperature rise if the 8-kW coil were fully energized at that airflow.
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Turndown = 260/1,400 = 18.6%, below the manufacturer's 25% minimum of 350 cfm. At 260 cfm the airflow sensor is likely outside its accurate range because velocity pressure is very low. Reheat check: 8 kW × 3,412 = 27,300 Btu/h. Temperature rise = 27,300/(1.08 × 260) = 97°F. With 69°F entering air, leaving air could reach about 166°F, well above a typical 120°F high-limit setting. The measured 78°F suggests the coil is staged, the high limit has cycled it, or actual airflow is higher than 260 cfm. Either way, the minimum is not controllable and the space is cold because airflow is too low and reheat is cycling.
3. Recommend the corrective sequence and state what minimum airflow setpoint you would leave in place.
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Sequence: (1) verify inlet static with a second gauge and inspect the duct; (2) verify box airflow with a duct traverse or flow hood at several damper positions; (3) reset minimum to 350 cfm only if the ventilation analysis allows it; if 420 cfm is required, resize or replace the box, or use a fan-powered box; (4) verify reheat staging, airflow interlock, and discharge-air high limit; (5) rebalance and document. Do not raise the minimum damper position without measuring flow. Final minimum should be the greater of the ventilation requirement and the manufacturer's reliable turndown, and it must be achievable at the available inlet static.