VAV Minimum Airflow, Reheat, and Ventilation in an Interior Zone
A 20,000-cfm VAV air handler serves an interior office zone with a design cooling load of 4.5 tons and design airflow of 2,000 cfm. Supply air is 55°F. At a low-occupancy winter condition the zone load falls to 0.5 ton, but the VAV box minimum is set at 40% (800 cfm). The zone temperature drifts to 70°F and the reheat coil energizes. The zone has 2,000 ft² and 12 occupants. The design basis is 5 cfm/person plus 0.06 cfm/ft² outdoor air. The owner wants to reduce reheat energy without causing IAQ complaints.
1. Estimate the reheat energy at the low-load condition and explain why it occurs.
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At 800 cfm, 55°F supply, and a 70°F zone, the sensible cooling delivered is:
Q_s = 1.08 × cfm × ΔT = 1.08 × 800 × (70 - 55) = 12,960 Btu/h ≈ 1.08 tons
The zone load is only 0.5 ton, or 6,000 Btu/h. The excess cooling is:
12,960 - 6,000 = 6,960 Btu/h
The reheat coil must add about 7,000 Btu/h to keep the zone from falling below 70°F. If this condition persists 2,500 h/year, reheat energy is about 17.4 MMBtu/year. The root cause is the 40% minimum airflow, not a faulty reheat valve. The box is delivering far more air than the zone needs at low load.
2. Calculate the ventilation minimum and recommend a new VAV minimum airflow.
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Ventilation requirement:
V_ot = 12 occupants × 5 cfm/person + 2,000 ft² × 0.06 cfm/ft²
V_ot = 60 + 120 = 180 cfm
As a fraction of design airflow, 180/2,000 = 9%. A practical minimum must also satisfy diffuser throw, ventilation effectiveness, and box controllability, so 9% may be too low. A minimum of about 300 cfm (15%) is a reasonable target:
Q_s = 1.08 × 300 × 15 = 4,860 Btu/h ≈ 0.41 ton
This is below the 0.5-ton zone load, so reheat is essentially eliminated at this condition. A 400-cfm minimum gives about 0.54 ton and would still cause slight reheat. Recommend reducing the minimum toward 300 cfm, with a CO2 sensor and a documented check that the AHU outdoor-air intake still delivers the required outdoor air.
3. What other checks are needed before lowering the minimum?
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Check diffuser throw and velocity at the reduced minimum; low flow can cause dumping, stratification, or poor air distribution. Verify the AHU outdoor-air fraction at the new minimum-reducing VAV box flow can reduce outdoor air to the zone unless the intake or CO2 control is reset. Check ventilation effectiveness and CO2 response, humidity at part load, reheat-valve leakage, fan curve and static-pressure reset, freeze protection, and whether the zone has exhaust or pressurization requirements. Also confirm that the minimum does not conflict with the design basis for the other zones on the same AHU. Lowering the minimum is appropriate only if these checks pass.