AEO Answer · Fire Protection & Life Safety

How Is Stair Pressurization Designed for Safe Egress?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

Stair pressurization holds the exit stair at higher pressure than the fire floor with a fan sized for open doors, pressure relief for closed doors, and multiple injection points so every floor stays in the design pressure band.

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

Illustrative diagram of building MEP systems: ductwork with diffusers, an electrical panel with conduit, and piping runs.

Sizing for the open-door condition

I size the supply fan from the open-door case: the design number of open doors times the airflow needed through each to maintain outward velocity, plus the closed-door leakage of the rest of the stair, plus a margin for construction leakage that is always worse than the tables assume. The number of doors assumed open comes from the adopted standard and the AHJ's expectations — it represents a realistic evacuation, not every door in the building. Duct and shaft losses are calculated, not guessed, because the fan has to deliver the design flow at the farthest injection point. Then I run the closed-door case against the same fan: with relief devices modeled, the pressure must stay below the door-force limit at every floor, in both summer and winter stack conditions.

Controls, relief, and integration

The control sequence is what makes the two conditions coexist. On alarm, the pressurization fan starts, the relief damper or VFD modulates on stair pressure, and the system proves operation back to the fire alarm panel. I interlock the design with the building's HVAC so the air handlers serving the fire floor shut down or go to smoke mode instead of fighting the pressurization, and I coordinate with the smoke control sequence where the stair connects to pressurized lobbies or elevator shafts. Commissioning includes the pressure test the code requires: measured differentials with the design doors open and with all doors closed, plus door-force measurements at the worst-case floor. If the numbers do not land in the band, the fan speed, relief setpoints, or injection balancing get adjusted until they do.

Stair pressurization design checklist

A stair pressurization design is ready when it clears this checklist. The two failure modes — smoke entry and stuck doors — are both addressed here. • Fan sized for the design open-door condition plus leakage and construction margin • Automatic pressure relief (damper, VFD, or both) proven for the all-doors-closed condition • Multiple injection points on tall stairs for even pressure top to bottom, intake from clean air • Door opening forces verified below the code limit at maximum design pressure • Commissioning pressure tests with design doors open and all doors closed, integrated with the smoke control sequence

Which related engineering resources can help?

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What else do project teams ask?

What pressure differential should a pressurized stair maintain?

Enough to keep smoke out, but not so much that doors become inoperable — the design band sits between a minimum that resists smoke migration and a maximum tied to the door opening force limit. I design per NFPA 92 and the adopted building code, targeting the band with the fan sized for open doors and relief for closed doors. The exact numbers depend on the code edition and whether the building is sprinklered, so I confirm the design values against the adopted standards rather than carrying numbers from a previous job.

Why does the design consider both open and closed doors?

Because the airflow needed is completely different. Closed doors leak a little air through gaps — small fan, small flow. Open doors during evacuation pass a large volume — the fan has to deliver it or the pressure collapses and smoke enters. But a fan sized for open doors will massively overpressurize the stair when the doors close, potentially trapping people. That is why the design pairs the fan sizing for the open-door case with automatic relief — pressure-controlled dampers or a variable-speed drive — for the closed-door case. Both conditions have to work; designing for only one is designing half a system.

Where should the pressurization fan inject air into the stair?

At multiple points on a tall stair, not just one. A single injection point creates a pressure gradient along the stair height — floors far from the injection see different pressure than floors near it, and stack effect in cold or hot weather makes it worse. I inject every few floors from a dedicated supply shaft so the pressure is even top to bottom. The intake itself goes to clean outside air, located away from exhaust discharges, loading docks, and anywhere smoke could be drawn in — a pressurization system breathing smoke is worse than no system.

What happens if the stair overpressurizes?

People cannot open the doors — which turns a life safety system into a trap. Overpressurization happens when the fan delivers open-door airflow with all doors closed, and the fix is automatic pressure relief: a barometric relief damper that opens at a set pressure, a variable-frequency drive that slows the fan on rising pressure, or both for redundancy. I also verify the door opening force calculation at the maximum design pressure, including the closer force, because the code's force limit is the hard ceiling the whole design must respect.

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