Balancing a Small Commercial Office With a Flow Hood
A technician is balancing a 2,400 ft² dental office with a 5-ton rooftop unit rated 2,000 cfm at 0.60 in. w.c. external static. Eight ceiling diffusers are designed for 250 cfm each. Using a calibrated flow hood, the technician records a total supply airflow of 1,780 cfm. The return plenum static is −0.32 in. w.c.; the supply plenum static is +0.28 in. w.c. One reception-area diffuser reads 170 cfm with its damper wide open, while the adjacent corridor diffuser reads 310 cfm. The filter is new, and the belt-driven blower is on the highest speed tap. The owner complains that reception is warm while the corridor is cold. The technician has not checked fan RPM, belt condition, or duct leakage. The flow hood was zeroed and placed on each diffuser with a factory-supplied square-to-round adapter.
1. Compute the total external static pressure and interpret it against the measured total supply airflow.
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Total external static pressure is the supply plenum static plus the absolute value of the return plenum static: 0.28 + 0.32 = 0.60 in. w.c. That equals the unit’s rated external static pressure. At the rated static pressure, the fan should be near its rated 2,000 cfm, yet the sum of the diffuser readings is only 1,780 cfm, about 11% low. That discrepancy suggests one or more of the following: flow hood error or poor diffuser adapter fit, duct leakage downstream of the measurement point, a slipping or worn belt, incorrect fan RPM, a dirty coil or filter, or a fan curve that does not match the unit. Do not begin by opening dampers; first verify the total airflow with fan RPM, motor current, and a static-pressure fan curve.
2. Explain why the reception diffuser is low and the corridor diffuser is high, and describe how to balance them.
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The pattern points to a branch resistance imbalance, not simply a total airflow problem. The corridor diffuser is probably on a shorter or lower-resistance branch, so it receives more than its share. The reception branch may have a long flex run, extra elbows, a partially closed or disconnected damper, or a crushed duct. Balancing means adding resistance to the corridor branch and verifying the reception branch is fully open and clear. However, because total measured airflow is already low, balancing alone will not fix the warm reception area. First correct total airflow, then adjust the corridor damper while re-reading reception until both are near design. Do not close the corridor damper so far that static pressure rises excessively.
3. What measurement precautions are needed when using a flow hood on ceiling diffusers?
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Zero the hood before use and confirm its calibration. Use the correct adapter that matches the diffuser shape and seals against the ceiling so it does not draw in room air or spill supply air. Avoid pressing the hood too hard and distorting the diffuser or creating a gap. Let the reading stabilize, and take more than one reading per diffuser. Check that ceiling tiles, lights, or nearby walls are not deflecting the pattern. Record the hood’s correction factor if one is required. Because the measured total is significantly below rated airflow, verify the hood against a known reference or a duct traverse before concluding the system is underperforming.