Building Pressurization Conflict Between Lab Exhaust and Stair Pressurization
A research building has 20 fume hoods on floors 2–5, a dedicated lab exhaust fan, and a stairwell pressurization system for the egress stairs. During a fire-alarm test, the stair pressurization fan ramps up and the building's main entrance doors become very hard to open - a technician measures roughly 45 lbf to crack the door. At the same time, lab spaces on floor 3 show a slight negative pressure relative to the corridor, and the corridor shows a slight positive relative to the lab. The building automation shows the lab exhaust fan at 90% speed and the stair pressurization fan at 100%. The fire marshal requires the stair pressurization to hold 0.05–0.10 in. w.c. in the stairwell. The lab safety officer requires labs to be negative to the corridor. The TAB firm is asked to "tune the stair fan down" to fix the door force.
1. Explain the conflict and why simply turning down the stair fan is the wrong first move.
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The conflict is between two life-safety requirements that share the building envelope:
- Stair pressurization must hold 0.05–0.10 in. w.c. in the stairwell to keep smoke out - a code/fire-marshal requirement.
- Lab negative pressure must keep labs negative to the corridor to contain hazardous fumes - an occupational-safety requirement.
Turning down the stair fan to fix door force:
- May drop stairwell pressure below the 0.05 in. w.c. minimum, violating the fire marshal's requirement and defeating smoke control.
- Does not address the lab being positive to the corridor - that is a separate exhaust/supply balance problem (lab exhaust at 90%, supply likely over-delivering, or transfer air path inadequate).
- The 45 lbf door force is a door-closer/egress issue governed by accessible-door force limits (typically ≤ 30 lbf to open, with some codes allowing higher only under specific conditions); the correct fix is often a pressure-relief damper or barometric relief in the stair, not reducing the fan.
So the stair fan is the wrong lever; the real fixes are relief/relief-damper design and rebalancing lab exhaust vs. supply.
2. Describe the diagnostic measurements and the corrective actions you would recommend, keeping both requirements satisfied.
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Diagnostics:
- Measure stairwell pressure at multiple heights with a calibrated differential manometer referenced to the adjacent occupied space - confirm it is within 0.05–0.10 in. w.c.
- Measure door force with a calibrated force gauge at the handle, per the applicable egress standard, on the worst-case door.
- Measure lab-to-corridor pressure differential with a sensitive manometer (target typically −0.02 to −0.05 in. w.c. for labs).
- Measure lab exhaust flow (hood face velocity and exhaust duct flow) and lab supply flow; compute the net exhaust/supply balance and the transfer air path from corridor to lab.
- Check the relief path - is there a barometric relief damper or relief louver in the stair, and is it functioning?
Corrective actions:
- Add or repair stair relief (barometric relief damper sized to relieve excess pressure during fan operation) so stair pressure stays in band while door force drops below the egress limit.
- Rebalance lab exhaust vs. supply: increase lab exhaust or reduce lab supply so the lab is negative to the corridor, and ensure adequate transfer air (undercut doors or transfer grilles) from corridor to lab.
- Coordinate the stair pressurization and lab exhaust controls so they do not fight (e.g., sequence, interlocks), and re-test both requirements after adjustment.
- Document all measurements and the final state, with sign-off from both the fire marshal's criteria and the lab safety officer's criteria.