Interior VAV Zone Held at Minimum While Reheat Saturates
A ten-storey office building has an interior open-plan zone of 900 ft² with 12 occupants, served by one VAV box with a hot-water reheat coil. Design airflow is 1,200 cfm and the commissioning report set the minimum at 30% of design, 360 cfm. The AHU discharges 58 °F, and the box drops to minimum whenever the zone is satisfied. Since the tenant added a break area with two large printers, occupants complain of a steady cold draught at their desks between 13:00 and 17:00. The BAS trend shows: zone temperature 70.5 °F against a 72 °F setpoint; damper steady at 360 cfm all afternoon; reheat valve at 100%; reheat leaving-air temperature 64 °F. The coil is rated 2.5 MBH at 400 cfm with 180 °F water and 55 °F entering air. The zone sensible load at these conditions is about 2.6 MBH. Diffusers are ceiling supply with a zone air-distribution effectiveness of 0.8, and the project applied ASHRAE 62.1 office rates.
1. Using the trend data, decide whether the minimum airflow, the discharge air temperature, or the reheat coil capacity is the primary cause of the zone falling short. Show the arithmetic.
Read the worked answer
Check the coil duty before blaming the valve. At 360 cfm and 58 °F entering air, the measured 64 °F leaving temperature means the coil is delivering 1.08 × 360 × (64 − 58) = 2,330 Btu/h. To hold 72 °F with a 2,600 Btu/h zone load, the box would have to deliver air at 72 − 2,600/(1.08 × 360) = 72 − 6.7 = 65.3 °F. With the coil capped at 64 °F, the zone settles at 64 + 6.7 = 70.7 °F - which is exactly what the trend shows (70.5 °F, within sensor tolerance).
The coil is not failed; it is saturated. It was rated for a 5.8 °F rise at design (2.5 MBH at 400 cfm, 55 °F entering), so asking it to reheat 360 cfm of 58 °F air by 7.3 °F was never within its capability. Discharge air temperature is not the problem at 58 °F, and the coil is not undersized against its own schedule. The primary cause is the 30% rule-of-thumb minimum, which was never checked against either the ventilation requirement or the reheat capacity downstream of it.
2. Calculate a ventilation-based minimum for the zone and test whether the reheat coil can hold 72 °F at that airflow. Identify any turndown concern.
Read the worked answer
ASHRAE 62.1 breathing-zone requirement: Vbz = (Rp × Pz + Ra × Az)/Ez = (5 × 12 + 0.06 × 900)/0.8 = (60 + 54)/0.8 = 143 cfm. Add margin for flow-sensor tolerance and diffuser performance and set the minimum at 160–180 cfm.
At 180 cfm and 58 °F supply the box removes 1.08 × 180 × (72 − 58) = 2,722 Btu/h, slightly more than the 2,600 Btu/h load, so the zone floats at roughly 71.7 °F with the reheat valve nearly closed. Reheat capacity is no longer the limiting factor because almost no reheat is called for. At 143 cfm the box would remove only 2,162 Btu/h and the zone would drift to about 72.8 °F, which is why 160–180 cfm is the right band rather than the calculated bare minimum.
Turndown is the real engineering check: 180 cfm through a 1,200 cfm box is 15% of maximum, at the edge of reliable inlet-probe measurement. Either select a 600–800 cfm box with 10:1 turndown, or hold 250 cfm and accept a small reheat load - at 250 cfm the coil only needs to reach 72 − 2,600/(1.08 × 250) = 62.4 °F, comfortably inside its range. Both are defensible; the point is that the minimum must be calculated and then checked against the box hardware.
3. List the sequence, programming and commissioning changes you would make, and state the acceptance criteria.
Read the worked answer
- Recalculate the minimum for every box in the building from 62.1, with a documented floor for air distribution; do not carry the 30% rule forward.
- Put the reheat valve under discharge-air-temperature control with a 10–15 °F deadband above box inlet temperature, and interlock it so it cannot open until the damper is at minimum.
- Verify the factory airflow calibration at the new minimum with a flow hood, not with the BAS value.
- Calibrate the leaving-air temperature sensor and the zone sensor; a 1.5 °F sensor error changes the diagnosis.
- Confirm the AHU static and supply-air-temperature resets are active, so boxes near the fan are not pushed to minimum by excess duct pressure.
Acceptance: zone holds 72 ± 1 °F across the afternoon, reheat valve duty below 20% of the occupied period, no hunting at the damper, and no new hot spots elsewhere on the floor. Before locking the new minimum, check the diffuser at 180 cfm in a 9 ft ceiling - a 4-way diffuser at very low flow can dump cold air, so confirm the throw and the ADPI at minimum, not just at design.