VAV System Starved Airflow and Failed Duct Leakage Test
A new VAV system has 12 terminal boxes, each designed for 300 L/s, for a total of 3,600 L/s. After start-up, the fan delivers 2,800 L/s at 750 Pa static, while the design static is 600 Pa. Several VAV boxes read only 180–220 L/s at design static, and one box is at minimum. A duct leakage test at 500 Pa shows 8% leakage; the specification allows 4%. The balancer wants to open all VAV dampers, increase fan speed to design static, and then balance. The mechanical contractor says the duct is 'tight enough' because the smoke test showed no major leaks. The fan is a 15 kW plug fan with a VFD, and the return-air plenum is not yet sealed.
1. What is the correct diagnostic sequence before changing fan speed or balancing?
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Do not increase fan speed first. Confirm the design airflow, static pressure setpoint, and the location of the static pressure sensor. Verify fan rotation, VFD output, filter condition, coil pressure drop, and that all access doors and plenum panels are closed. Check that the VAV boxes are calibrated and that their dampers are in the correct control mode, not manually closed or stuck at minimum. Measure static pressure at the fan outlet and at the sensor, and compare with the fan curve.
Then inspect the duct system for restrictions and leakage. The 8% leakage at 500 Pa is a real defect, not a minor smoke-test finding, and the unsealed return plenum can short-circuit air or change the system resistance. Look for closed fire dampers, crushed flexible ducts, missing turning vanes, and duct liner blocking the airway. Do not open all VAV dampers as a diagnostic step because that destroys the control signal and can overload the fan. Fix the known leakage and obvious restrictions first, then re-test the fan and terminals before balancing.
2. Interpret the leakage test and static pressure readings; identify likely causes and consequences.
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The leakage test at 500 Pa shows 8% leakage against a 4% allowance, so the duct system fails the specification. At 500 Pa, 8% of 3,600 L/s is about 288 L/s of leakage, but leakage varies with pressure; the point is that the system is losing air before it reaches the terminals. The fan is delivering 2,800 L/s at 750 Pa, which is 800 L/s below design and 150 Pa above design static. That combination suggests the fan is operating on a steeper system curve than design, not simply that the fan is too small.
Possible causes are a restriction such as a closed damper, a collapsed flex duct, a fire damper out of position, or a static sensor located where it reads artificially high. Excessive leakage can also change the system curve, but leakage usually reduces terminal flow rather than raising fan static. The VAV boxes reading 180–220 L/s are consistent with a starved system. If the balancer increases fan speed without correcting the restriction, the fan may hit the end of its curve, draw more power, and still not deliver design airflow. The failed leakage test must be corrected before balancing because any balance achieved with leakage will change once the leaks are sealed.
3. Specify corrective actions, re-testing, and balancing sequence.
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First, seal the duct leaks to achieve 4% or less. Pay particular attention to transverse joints, access doors, plenum connections, and the unsealed return-air plenum. Repair or replace damaged gaskets and apply the specified sealant. Re-test the duct at the specified pressure, typically 1.25 times the design static, so about 625 Pa for this 500 Pa system unless the specification states otherwise. Record the leakage and compare it with the allowable class.
Next, find and correct the system restriction. Check fire dampers, volume dampers, flex duct, and duct transitions. Verify the static sensor location and calibration. Then check the fan curve at the corrected system resistance and adjust the VFD to the design static, not above it. Calibrate the VAV boxes and confirm minimum and maximum airflow settings. Balance the terminals to 300 L/s each within the specified tolerance, usually ±10%, beginning with the farthest terminal and using the proportional balance method. Recheck fan airflow, static, and power after sealing. Document the leakage test, fan curve, terminal readings, and final balance report.