Size the stairwell pressurization fan from closed-door leakage, wall leakage and the open-door velocity criterion, then check the door opening force — using the leakage method of NFPA 92 and EN 12101-6.
Preliminary sizing only. The final design must be verified against the adopted code, the authority having jurisdiction and commissioning tests; a pressure relief path or variable-speed control is needed so the pressure does not exceed the door-force limit when all doors close.
A pressurized stair has to meet two conditions at once. With the doors closed, the stair must be held above the floors by a minimum pressure difference so smoke cannot leak in. With one or more doors open, enough air must flow out through the open doorway to push smoke back. The fan is sized for the sum of both, plus allowances for leakage nobody can measure in advance.
NFPA 92 sets the minimum pressure difference across a smoke barrier at 12.4 Pa (0.05 in. w.g.) in a sprinklered building. In a non-sprinklered building the minimum rises with ceiling height because hotter, more buoyant smoke pushes harder: about 25 Pa for a 2.7 m ceiling. EN 12101-6 Class A systems commonly design to 50 Pa with doors closed and 0.75 m/s through the open door. Use the class or value your code and authority require — the presets are starting points.
More pressure is not always safer. Every pascal across a door pushes against the person trying to open it into the stair. NFPA 101 caps the opening force at 133 N (30 lbf); many EN-based specifications use 100 N. The tool reports the maximum pressure difference the door allows with the closer force you entered — if your design ΔP is above it, a person may not be able to open the door to escape.
This is also why a pressurized stair needs a relief path or a variable-speed fan: the airflow sized for an open door has nowhere to go when that door closes, and the pressure climbs above the door-force limit.
Air injected only at the bottom of a tall stair leaves through the first open door it meets, and the upper floors lose pressure. Tall stairs are therefore fed at several levels; one supply point every three floors is a common rule. The tool suggests a number of injection points on that basis and splits the airflow evenly.
10-floor stair, sprinklered (12.5 Pa), single-leaf doors opening out of the stair (0.02 m²), one exit door, average walls, 22 m perimeter × 3.5 m floors, one 0.9 × 2.1 m door open at 1.0 m/s.
With 25% unidentified and 15% duct leakage the fan needs about 3.5 m³/s (≈ 7,430 CFM), split over four injection points.
NFPA 92 requires at least 12.4 Pa in sprinklered buildings and higher values in non-sprinklered buildings (about 25 Pa for a 2.7 m ceiling). EN 12101-6 Class A systems typically use 50 Pa with doors closed.
NFPA 101 limits it to 133 N (30 lbf) at the latch. Many EN-based specifications use 100 N. The force includes the door closer, so a strong closer leaves less room for pressure.
The fan is sized for an open door. When every door closes, that air has no outlet and the pressure rises until doors become too hard to open. A barometric relief damper, a vent to outside or a variable-speed fan controlled by a pressure sensor prevents it.
Short stairs can work with a single injection point. Tall stairs are usually fed at several levels, commonly one point every three floors, so an open door near the fan does not starve the upper floors.
No. It gives a preliminary fan size. Stack effect, wind, building leakage, lobby and lift shaft interaction and commissioning test results all affect the final design and must be checked by the engineer of record.
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