Balancing a Branch Without Starving the Fan
A technician is balancing a 4-ton rooftop unit serving ten ceiling diffusers in a small office. The design airflow is 1,600 cfm at 1.0 in. w.c. external static pressure. Room 4 is designed for 200 cfm but the balancing hood reads 300 cfm. The technician closes the branch damper about 50%; room 4 drops to 220 cfm, but total airflow falls from 1,550 to 1,350 cfm, and several other rooms lose about 15%. Room 4 has a 24- by 12-in. return grille and a solid door with no transfer grille or undercut. The fan is a direct-drive ECM set to a fixed speed tap, and the filter is clean. The building owner wants the other rooms left alone because they were 'fine before.'
1. Explain why closing the branch damper reduced total airflow and affected other rooms. Use fan/system curve reasoning.
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Closing the branch damper adds resistance to the system. The system resistance curve becomes steeper, so the fan operating point moves to a lower flow along the fan curve. Even with an ECM on a fixed speed tap, the fan cannot overcome the added resistance without more static pressure capability, so total airflow drops. The reduced total flow lowers the static pressure available at the other branch takeoffs, which is why the other rooms lose about 15%. A single branch damper is therefore not an isolated adjustment; it interacts with the whole system.
2. What is the likely cause of the high airflow in Room 4, and what return-path issue must be checked?
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Room 4 is probably fed by a short, low-resistance branch, an oversized duct or diffuser, or a damper that is missing or wide open. The 300-cfm reading may also have been taken with the door open, which does not represent normal operation. The solid door and small return grille are a problem: when the door is closed, the room can pressurize, causing whistling, reduced supply airflow, and poor return. Check whether a transfer grille, door undercut, or return duct is required, and verify the reading under normal door position.
3. Outline a practical balancing sequence and the final readings you would document.
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Measure total airflow and external static pressure first, then record every diffuser. Adjust branch dampers in small increments, starting with the highest-flow branches, and avoid closing one damper 50% just to fix a single room. Recheck total airflow after each round; if total is high, reduce fan speed rather than throttling every branch. Correct the return path in Room 4 before final readings. Document design vs. actual cfm for each diffuser, total airflow, external static pressure, fan speed tap, filter condition, and door position during the test.