How Does Stairwell Pressurization Keep Smoke Out During Fires?

Stairwell pressurization supplies outdoor air to exit stairs to maintain higher pressure than surrounding spaces, so air flows outward through open doors and keeps smoke out of the escape route. Design balances pressure differentials against door-opening forces, sizes fans for multiple open doors, and uses controls that respond to changing conditions. The physics is simple — air moves from high pressure to low pressure, and smoke can't swim upstream against that flow. The engineering is in the balance: too little pressure and smoke wins; too much pressure and people can't open the doors. Everything in the design serves that balance under real fire conditions.

The design starts with the pressure targets from the smoke control standards, then works out the airflow needed to maintain those pressures given the stairwell's leakage — because no stairwell is airtight, and the supply fan has to overcome leakage through closed doors plus the flow through open doors in the design scenario. Multiple supply injection points (not just one fan at the top or bottom) give more uniform pressure over a tall stair. Controls make or break the system. A constant-volume fan sized for the worst case will over-pressurize the stair when doors are closed; a pressure-controlled variable system with relief adapts. I specify the control strategy as carefully as the fan — sensors, setpoints, and failure modes — because a pressurization system with dumb controls is a liability, not protection.

This system protects the escape route. It gets life-safety rigor.

• Design for the open-door scenario: size fans and controls for multiple doors open during evacuation
• Balance pressure against door force: effective versus smoke and openable by occupants — both, always
• Use multiple injection points: uniform pressure over tall stairs, not a single fan fighting stack effect
• Specify smart controls: pressure-responsive fan control with relief, not constant volume and hope
• Test it like lives depend on it: witnessed acceptance testing and periodic retesting, documented