What Is Stairwell Pressurization and How Does It Work?

Stairwell pressurization is a smoke-control strategy that supplies air to an exit stair enclosure to maintain positive pressure relative to the fire floor, preventing smoke infiltration. The mechanical design sizes supply fans for the stair volume and leakage paths, limits pressure so doors remain openable, and sequences fan operation with fire alarm activation per the building code's smoke-control provisions. The design analyzes two conditions: doors closed, where the system must overcome leakage to hold pressure, and doors open, where the system must maintain enough airflow velocity through open doorways to resist smoke. Both conditions get calculated. A system that works with all doors closed but collapses when two doors open during evacuation hasn't been engineered.

This is the technical core. With all stair doors closed, the fan pressurizes the shaft; the pressure depends on the fan airflow and the leakage through door gaps, construction joints, and penetrations. Too little airflow and smoke pushes in; too much and the pressure climbs past the door-force limit. The engineer calculates the leakage, selects the fan, and verifies the resulting pressure sits in the acceptable band. Then the open-door condition: when occupants enter the stair, open doors become big leakage paths, and the system must push enough air through them to keep smoke from entering with the people. Multi-point injection — supplying air at several floors rather than one — gives more uniform pressure up a tall stair. Barometric dampers or variable-speed control relieve excess pressure. Every one of these decisions is a calculation, documented for the reviewer who will absolutely check them.

Pressurization lives or dies on controls. The fire alarm system activates the fans on alarm, and the sequence has to be right — wrong fan, wrong direction, or delayed start defeats the system. I document the sequence of operation explicitly: which fans start, which dampers open, what happens on trouble signals. Smoke detectors in the supply air path shut the system down if it's pulling in smoke. Acceptance testing is rigorous and witnessed: pressure measurements with doors in various positions, door-force measurements proving openability, and full sequence verification. I design for testability — measurement ports, accessible dampers, and control points the commissioning agent can actually reach. The drawing set shows the fan and duct layout, injection points, control diagrams, sequences, and the pressure calculations. It's one of the most calculation-heavy small systems in the building, and the permit set reflects that.

• Positive pressure: supply air holds smoke out of the exit stair enclosure
• Two conditions: doors-closed pressure and doors-open airflow, both calculated
• Door forces: pressure limited so occupants can actually open stair doors
• Fire alarm sequence: documented fan start, damper, and shutdown logic
• Acceptance testing: witnessed pressure and door-force measurements before occupancy