Stair Pressurization: Nonuniform Pressure and Door Force
During acceptance testing of a 10-story building's stair pressurization system, the project design calls for 0.10 in. w.c. minimum and 0.25 in. w.c. maximum differential across closed stair doors, with a maximum door-opening force of 30 lbf. With both stair fans at full speed and all doors closed, the top-floor stair-to-corridor differential is 0.04 in. w.c. At the third floor it is 0.11 in. w.c. When the egress door on the ground floor and two doors at the third floor are opened, the top-floor differential goes to negative 0.02 in. w.c. Fan current is 78% of full-load amps, and fan discharge pressure is within design. The contractor proposes increasing fan speed to 60 Hz and raising the setpoint. The fire marshal is scheduled for a retest.
1. Interpret the results and explain why increasing fan speed may not solve the problem.
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The pressure profile is not simply a fan-capacity problem. The third floor is within range at 0.11 in. w.c., but the top floor is only 0.04 in. w.c. That points to a leakage or flow-path problem between the stair and the building at lower floors, or a stair shaft that is not sealed, such as a roof hatch, shaft penetrations, or door undercuts. Increasing fan speed will raise the lower floors above 0.25 in. w.c. and may exceed 30 lbf door force before the top floor reaches 0.10. The negative top-floor reading with doors open shows the fan cannot overcome the loss path; more speed may simply increase leakage and make door operation harder.
2. Describe the test procedure and instruments needed to diagnose the cause.
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Use calibrated manometers at every floor, a door-force gauge, and an anemometer or flow hood at the fan. With all doors closed, record stair-to-corridor differential at every floor and record fan flow and current. Then open the required doors one at a time and record the same points plus door-opening force. Verify fan start on signal, damper position, and automatic transfer. Perform an air-leakage survey of the stair enclosure, including door frames, weatherstrip, roof hatch, and wall and floor penetrations. Compare the measured pressure profile with the design leakage assumptions. The goal is to locate the dominant loss path before changing fan speed.
3. Recommend corrective actions and acceptance criteria for the retest.
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Seal door frames, weatherstrip, and penetrations; verify that the roof hatch and stair doors close and latch; and balance the pressure profile with relief dampers or a variable-speed fan controlled by the top-floor differential pressure. Do not simply raise the setpoint. Acceptance: 0.10 to 0.25 in. w.c. at every floor with all doors closed, positive pressure with the required doors open, door force no more than 30 lbf, and fan current within nameplate. Retest every floor with the fire marshal present and document all readings, including the open-door condition.