How to size smoke control pressurization airflow
A pressurized stairwell or smoke control zone works by holding the protected space at a higher pressure than the fire floor so smoke cannot migrate in. The supply fan has to deliver two things at once: the leakage airflow that escapes through door gaps, wall penetrations and construction cracks while all doors are closed, and the open door airflow that keeps a velocity through a door while occupants evacuate. This calculator sizes both using the relationships in NFPA 92.
Leakage is calculated with the standard orifice form Q = 2,610 × A × √ΔP, where A is total leakage area in square feet, ΔP is the pressure difference in inches of water gauge, and the constant already includes a flow coefficient of 0.65. A design safety factor of about 25% on the leakage result is common practice because as-built leakage almost always exceeds the calculated value.
NFPA 92 sets the minimum design pressure difference at 0.05 in. w.g. for sprinklered buildings, with higher minimums where ceilings are tall and the building is not sprinklered, and caps the practical maximum near 0.35 in. w.g. The ceiling exists because the maximum force required to open an egress door is limited to 30 lbf, so over-pressurizing a stair can trap the very people it was built to protect. Use these results for preliminary sizing and have the final design, including the door force check and acceptance testing plan, reviewed by a licensed fire protection engineer.
Frequently asked questions
No. Too much pressure makes egress doors impossible to open. Code limits door opening force to 30 lbf, so the upper bound has to be checked alongside the minimum.
Add up the gaps around doors and windows, elevator door clearances and wall penetrations. Where measurement is impractical, use the published leakage areas for each construction type and verify with acceptance testing after installation.