VAV Boxes Starving With the Static Sensor at Setpoint
A 22,000 cfm AHU with a VFD-driven forward-curved fan serves 30 VAV boxes through 180 ft of medium-pressure duct. The duct static pressure sensor is tapped 15 ft downstream of the fan discharge in a straight run; the setpoint is 1.5 in. wg. At 2 pm the VFD is at 52 Hz, the sensor reads 1.5 in. wg, and every terminal on the last third of the duct is 100% open and 15-20% below its airflow setpoint; those zones are 3-4°F warm. Measured AHU airflow is 22,000 cfm, equal to design. A contractor proposes raising the static setpoint to 2.2 in. wg. The fan draws 18 kW now, and each box needs 0.25 in. wg at its inlet.
1. Explain why one-third of the boxes are starved while the sensor reads setpoint, and reconcile that with the AHU measuring design airflow.
Read the worked answer
The sensor is not where the problem is. A tap 15 ft off the fan discharge reads the fan's static, not the pressure at the hydraulically farthest terminal. The last third of a 180 ft medium-pressure run sits behind the full duct friction and fitting losses. Whatever the sensor sees, the far boxes see less:
P_far ≈ P_sensor − ΔP_duct(sensor→far box)
For the far boxes to be short at 100% open, P_far is below the 0.25 in. wg inlet requirement while the sensor holds 1.5 in. wg. That is entirely consistent: a sensor on a low-loss section of the duct controls the near boxes' pressure, not the critical zone.
Why the AHU still reads design airflow
When the far boxes are starved, the near boxes see the higher pressure and take more than their setpoint. The oversupply and undersupply roughly cancel at the fan inlet, so a total airflow measurement looks normal and hides the imbalance. Do not use total airflow as evidence that the terminal side is healthy - sum the individual box readings; the near-end boxes will be well above setpoint.
Rule out the cheap causes first: kinked or water-trapped pneumatic tubing, high/low ports reversed, a transducer reading high, and a damper actuator that is 100% commanded but mechanically decoupled at the far boxes.
2. Evaluate the proposal to raise the setpoint to 2.2 in. wg. Quantify the cost and identify what it does and does not fix.
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What it does: it raises available pressure at the far end by about 0.7 in. wg, which may be enough to satisfy the far boxes.
What it costs: with the boxes already at maximum and the flow bounded by the design airflow, the fan power scales roughly with static: 18 kW × (2.2/1.5) ≈ 26 kW, or about +8 kW at peak. At 2,500 fan hours that is 20,000 kWh ≈ $2,200/yr, before demand charges. If the airflow rises toward 22,000 cfm true (rather than the measured value that includes near-end oversupply), power will go a bit higher still, because fan power follows both pressure and flow.
What it does not fix:
- It permanently over-pressurizes the near-end boxes. Those boxes were already over-delivering; they will now be pinned at minimum damper with high inlet static, which produces noise, damper actuator wear, and elevated duct leakage.
- It masks the sensor-location problem rather than correcting it. The next time the building is rebalanced or a box is added, the same complaint returns.
- It removes the headroom to raise the setpoint later without tripping the drive or the duct pressure class. Check the 2.2 in. wg against the medium-pressure duct rating and the box maximum inlet static before considering it at all.
3. Give the correct fix and the measurement steps that prove it worked, including sensor accuracy considerations.
Read the worked answer
Correct fix
- Relocate the static tap to about two-thirds to three-quarters along the duct, or to the inlet of the hydraulically farthest box, in a straight, fully developed section.
- Set the setpoint so that the critical box has 0.25 in. wg plus a small margin - typically 1.3-1.6 in. wg - and verify by measurement, not by the old number.
- Add a static pressure reset: poll box damper positions and lower the setpoint by 0.05 in. wg every few minutes while every box is below about 90% open; freeze when one box reaches 90%. On a system with this much oversupply at the near end, a reset typically recovers 5-15% of fan energy in the part-load hours without touching the critical zone.
- Check the drive's PID for the symptom the complaint describes: a 52 Hz ceiling with the setpoint satisfied usually means the drive is being limited by the local sensor, not by the motor.
Measurement and uncertainty
- Use a differential transmitter with a 0-2 in. wg span. At 1% of span, the error is 0.02 in. wg; a 0-10 in. wg transmitter would carry 0.1 in. wg of error, which is the same order as the pressure you are trying to protect.
- Confirm the tap with a second, independent gauge at the same port before moving anything. A 0.1 in. wg disagreement between two gauges is usually one bad transducer, not a real duct problem.
- Verify the boxes with a calibrated balometer at each far-end terminal, and the AHU with a pitot traverse (expect about ±5% with a good traverse). A box 15-20% short is outside traverse uncertainty; a box 3% short is not.
- Re-trend for a week after the change: confirm the far boxes drop below 100% open, the near boxes come off their minimum, and the fan kW falls at part load rather than simply moving the problem to the other end of the duct.