A stairwell pressurization system keeps a fire escape stair free of smoke by holding it at a higher pressure than the adjacent floors – commonly a design band of 12.5–62 Pa, with the lower figure set by smoke resistance in a sprinklered building and the upper figure set by the force needed to open the door. The door opening force must stay below about 133 N, which is the practical constraint that limits maximum pressure. Airflow follows from total leakage area rather than stair volume, and a 20-storey stair typically needs 10,000–15,000 m³/h to hold pressure with all doors closed.
Calculating the required airflow
Airflow is governed by the orifice relationship Q = 0.827 × A × √ΔP, where A is total effective leakage area. Each closed stair door contributes roughly 0.02 m² of leakage, and stair walls, service penetrations and the roof hatch add more. For a 20-storey stair with 20 doors, total effective leakage area of about 0.5 m² is a reasonable design assumption. Holding 50 Pa then requires 0.827 × 0.5 × √50 = 2.92 m³/s, or about 10,500 m³/h.
The design case that sizes the fan, however, is usually not the all-doors-closed condition but the specified number of open doors. Codes commonly require the system to maintain the minimum differential with one or two doors open, and an open door of 2.0 m² passing air at 0.76 m/s alone accounts for 1.52 m³/s, more than 5,400 m³/h. That is why a two-door-open criterion can roughly double the fan duty and why a pressure-relief or modulating arrangement is essential: a fan sized for the open-door case will over-pressurise the stair and jam the doors when everything is closed.
Design criteria at a glance
| Parameter | Typical requirement | Design target | Consequence if missed |
|---|---|---|---|
| Minimum differential, sprinklered | 12.5 Pa | 25–30 Pa | Smoke migrates into stair |
| Minimum differential, non-sprinklered | 25 Pa or more | 35–45 Pa | Smoke migrates into stair |
| Maximum differential | Set by door force | 50–62 Pa | Doors cannot be opened |
| Door opening force | ≤133 N at the handle | 90–110 N | Occupants trapped, code failure |
| Velocity through one open door | ≥0.76 m/s where specified | 0.8–1.0 m/s | Smoke back-flows into stair |
| Time to reach design pressure | About 60 s from activation | 30–45 s | Stair contaminated during escape |
| Injection points, single-injection limit | Up to roughly 30 m height | 8–10 storeys | Pressure gradient too steep |
| Standby power | Required | Automatic transfer | System lost with utility supply |
Single injection or multiple injection?
- Use a single injection point when the stair is no more than about 30 m tall, roughly 8 to 10 storeys, and stack effect is modest.
- Use multiple injection points when the stair exceeds that height, distributing supply every two to three floors so the pressure gradient along the shaft stays within the 12.5–62 Pa window at every level.
- Use a barometric relief damper when the building is simple and cost governs; the damper opens above a set differential and dumps excess air, though it responds slowly and needs annual proving.
- Use a variable-speed fan with pressure sensors when the stair is tall, doors open frequently, or the design must satisfy both the closed-door and open-door criteria; modulating control typically holds the band far more tightly than mechanical relief.
- Add a vestibule or anteroom when the stair opens directly into a large open-plan floor, since the double-door arrangement dramatically reduces the leakage the fan must overcome.
- Reconsider the concept entirely when the building relies on natural smoke venting and the stair has no dedicated shaft; retrofitting pressurization into a leaky shaft rarely achieves a stable differential.
Stack effect: the load nobody sizes for
In a tall building the temperature difference between inside and outside creates its own pressure gradient, and in winter it opposes the pressurization system at the bottom of the shaft. The magnitude follows ΔP = 3460 × h × (1/Tₒ − 1/Tᵢ) with temperatures in kelvin. For a 50 m stair at an outdoor temperature of −10 °C (263 K) and an indoor temperature of 21 °C (294 K), the calculation gives 3460 × 50 × (0.003802 − 0.003401), or about 69 Pa across the height of the shaft. That is larger than the entire design pressure band, which is exactly why single-point injection fails in tall buildings and why the neutral plane must be considered when choosing injection levels. In summer the gradient reverses, though usually with smaller magnitude.
Commissioning the system
- Measure the differential at every landing with all doors closed, recording values against the 12.5–62 Pa band, and check the top and bottom of the shaft where the gradient is worst.
- Repeat with the specified number of doors open, including the discharge door to outside, which is the most common failure condition.
- Measure door opening force at the handle with a calibrated gauge, confirming it stays below 133 N at every level under the worst pressure case.
- Time the activation sequence from smoke detector or alarm signal to stable design pressure, and verify damper travel and fan ramp are within the required interval.
- Test the system on standby power, and confirm that any interacting systems – lift shaft pressurization, floor exhaust, general HVAC shutdown – sequence correctly rather than fighting each other.
- Test in both heating and cooling seasons where possible, since a system commissioned in mild weather may fail on the first cold day.
Keruite manufactures the axial and centrifugal fans used in stair pressurization and smoke control systems at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every rotor is balanced to G2.5 and each unit is performance-tested against its specified duty before shipment.
Why systems fail in practice
Field failures cluster into four causes. The first is uncontrolled leakage: a stair shaft with unsealed service penetrations, an open roof hatch or a poorly gasketted discharge door can double the assumed leakage area, so the measured differential lands well below the 12.5 Pa minimum even though the fan is delivering design airflow. The second is interaction with other systems. A lift shaft vent, a pressurised lobby, or general HVAC that keeps running in fire mode can each shift the reference pressure the stair is measured against, making a compliant fan look non-compliant. The third is sensor location: a differential transducer referenced to a floor that is itself pressurised reports a number that has no relationship to the smoke barrier. The fourth is simple neglect – relief dampers seize, sensor tubing kinks or fills with dust, and speed drives revert to a default after a power event. Because the system only operates in an emergency, none of these faults announce themselves, which is why periodic proving tests with real pressure and door-force measurement matter more here than in any everyday HVAC system.
FAQ
What pressure should a pressurised stairwell maintain?
Typically a band of 12.5–62 Pa relative to the adjacent floor. Sprinklered buildings commonly accept 12.5 Pa as the minimum, and the upper limit exists because pressure above roughly 62 Pa can push door opening force past the 133 N maximum, trapping occupants.
Why do pressurised stair doors become hard to open?
Because pressure acts across the whole door leaf. A 2.0 m² door at 60 Pa carries 120 N of net force, which adds to closer resistance and latch friction. The fix is pressure relief or a modulating fan, not a stronger closer or a weaker door seal.
How many injection points does a tall stairwell need?
Single injection is generally limited to about 30 m, or 8 to 10 storeys. Above that, inject every two to three floors so stack effect and shaft friction do not push the bottom below the minimum differential while the top exceeds the maximum.
Does stairwell pressurization run continuously?
No. It is a fire-mode system activated by the fire alarm or smoke detection, and it must reach design pressure quickly, commonly within about 60 seconds. It requires standby power, and its dampers and fans should be exercised and proved at scheduled intervals rather than left idle.