Stairwell Pressurisation Calculator

Pressurising a fire escape stair keeps smoke out so occupants can escape and firefighters can work. The supply fan must hold a design pressure across closed doors while also delivering enough air through the doors that are open during evacuation. This calculator estimates the required airflow and fan duty from the design pressure, the door area, the number of open doors and building leakage, to NFPA 92 / ASHRAE smoke-control practice.

The design must satisfy two competing cases: enough pressure with all doors closed (without exceeding the maximum door-opening force), and enough airflow velocity through the doors that are open during egress. A relief or modulating arrangement usually reconciles them. Leakage areas, the number of simultaneously open doors and stack effect all matter. Always design to NFPA 92 or the local smoke-control code and coordinate with the fire strategy.

For a stair with a 2.0 m² door and a 50 Pa design pressure, the open-door velocity is about √(2 × 50 / 1.2) ≈ 9.1 m/s, giving roughly 18 m³/s per open door before leakage. With two doors assumed open plus construction leakage, the fan is selected for the combined airflow and system resistance, while relief keeps the closed-door force within limits.

Two design cases

The closed-door case sets the pressure and the door-opening force limit; the open-door case sets the airflow to hold a smoke-blocking velocity. A single fixed fan rarely satisfies both, so a barometric relief damper or a modulating/variable-speed fan is used.

Leakage and stack effect

Real buildings leak through door gaps, construction joints and shafts, and tall buildings have stack effect that helps or hinders depending on season. Realistic leakage areas are essential to a credible airflow.

Coordinate the strategy

Pressurisation is one part of the smoke-control and fire strategy. Coordinate it with the fire alarm interface, door hardware, relief paths and any zoned smoke management, and commission it to the code test criteria.