When a fire breaks out in a high-rise or large building, the stairwell is both the primary evacuation route and the most vulnerable path for smoke migration. Staircase pressurisation is the engineered answer: a continuously supplied column of clean air that holds smoke out of the escape route long enough for occupants to reach safety.
Why pressurisation protects the means of egress
Smoke movement in a building is driven by the stack effect, HVAC airflows, wind pressure and the buoyancy of hot combustion gases. Without intervention, smoke can travel from a fire floor to a stairwell within minutes through gaps around doors, in shafts and through leaky construction. Occupants on upper floors then encounter smoke before they reach the exit — the precise scenario that NBC Part 4 and fire-engineering practice seek to prevent.
A pressurisation system introduces a controlled airflow into the protected shaft so that the stairwell pressure is higher than the adjacent corridor at every floor. When a stair door opens, air flows outward into the corridor rather than smoke flowing in. When all doors are closed, the pressure differential itself acts as a barrier. The design objective is a balance point: enough pressure to exclude smoke, not so much that occupants cannot open the door in an emergency.
NBC requirements at a glance
The National Building Code of India (Part 4 — Fire and Life Safety) sets the benchmark for pressurised escape routes. The primary targets are:
- Minimum pressure differential: 50 Pa between the pressurised stairwell and the adjacent corridor or lobby, measured with all stair doors closed.
- Maximum door-opening force: 110 N at the leading edge of the door. This limit exists because occupants in a panic, the elderly and children must be able to open the door without undue effort.
- Air velocity through an open door: Where the design relies on airflow rather than pressure alone at an open door, a minimum face velocity of 0.75 m/s through the doorway is used as the smoke-exclusion criterion at the open-door condition.
The 50 Pa and 110 N limits are not independent — they interact. A well-designed system satisfies both simultaneously through the use of pressure-relief dampers and careful fan selection.
Pressurisation fans, air intake and distribution
Fan selection
Pressurisation fans for stairwells are typically centrifugal or mixed-flow units sized to deliver the design airflow against the leakage resistance of the shaft. The fan must be rated for smoke-duty operation at elevated temperature (usually 200 °C for two hours, in accordance with EN 12101-3 or equivalent), since it must continue operating in a fire condition. Variable-speed drives are increasingly specified to allow field adjustment during commissioning and to handle varying leakage conditions across the range of door-open states.
Air intake location
The intake must draw clean, uncontaminated air. The preferred position is on the exterior facade, as far as practicable from any likely fire-side opening, smoke outlet or car-park exhaust. Rooftop intakes are common for tall buildings but require careful consideration of wind direction variability. The intake must be protected against rain ingress and fitted with motorised dampers that open automatically on system activation.
Distribution within the shaft
Air is typically introduced at multiple levels rather than at a single point. Multi-point injection distributes the pressure more evenly up the shaft and reduces the risk of the uppermost floors being under-pressurised while the lowest floors are over-pressurised. Injection points are usually spaced at every second or third floor, supplied via a vertical duct running inside or alongside the stairwell.
Pressure-relief dampers and barometric relief
Over-pressurisation is the most common field failure mode. When all stair doors are closed simultaneously — as they would be during an alarm when occupants have already evacuated — the fan continues to deliver air into a sealed shaft and the pressure rises rapidly. Without relief, the pressure differential can far exceed 50 Pa and the resulting door-opening force can exceed 110 N, potentially trapping people inside.
The solution is automatic pressure relief. Two approaches are used:
- Barometric-relief dampers (overpressure relief dampers): Gravity- or spring-loaded flaps installed in the shaft wall or at the top of the stairwell. They open passively when the shaft pressure exceeds a set point (typically 60–70 Pa), venting to an external or safe internal space.
- Variable-speed fan with pressure feedback: A pressure sensor in the shaft provides a signal to the VSD to reduce fan speed and maintain the pressure within the design band. This approach is more precise but more complex to commission and maintain.
Both methods can be combined. Relief dampers act as a passive failsafe even where a VSD is fitted.
Lift-well and lobby pressurisation
Lift shafts present a separate but related hazard. The stack effect in a tall building can draw smoke up through the lift shaft to upper floors even when the fire is contained on a lower level. NBC Part 4 requires pressurisation of lift wells for buildings above a specified height. The approach mirrors stairwell pressurisation: a dedicated fan, clean-air intake, multi-point injection and relief provisions. Lift lobbies — the protected lobbies that separate the lift doors from the general floor area — are pressurised as a second line of defence, maintaining a pressure buffer between the lift shaft and the occupied floor.
Lobby pressurisation systems are commonly integrated with the stairwell system at the controls level but must operate as mechanically independent circuits so that a fan failure in one does not compromise the other.
Basement and car-park smoke extraction
Basements and below-grade car parks cannot be pressurised in the conventional sense — there is no vertical stack to work with. Instead, these areas rely on mechanical smoke extraction: high-temperature exhaust fans draw smoke-laden air out of the space and discharge it externally, while make-up air is admitted through separate low-level openings or a dedicated supply fan. The system is designed to maintain a tenable environment in the escape routes and to assist fire-brigade operations.
Jet-fan systems are widely used in large open-plan car parks where conventional ductwork would be impractical. Impulse fans mounted at soffit level direct airflow towards extract points and prevent smoke stratification. CO monitoring is integrated with the jet-fan controls so that the ventilation system serves both smoke-control and normal air-quality functions. See our fire fighting & protection service page for a broader view of fire system integration.
Interface with the fire alarm system
Pressurisation and smoke-extraction systems must be under the control of the fire alarm panel. Auto-start on detection is not optional — it is a life-safety requirement. The standard interface involves:
- Activation of all pressurisation fans on a general alarm signal from the fire detection and alarm system (FDAS).
- Activation of smoke-extract fans on the relevant zone or floor signal, with selective override capability for the fire brigade.
- Automatic closure of any normally open ventilation or make-up air dampers on non-pressurised floors to prevent smoke spread.
- Feedback of fan run status, damper position and fault signals to the FDAS or building management system (BMS) for monitoring.
The cause-and-effect matrix for the smoke-control system should be reviewed and agreed with the local fire authority and approved fire consultant at design stage, not at commissioning.
Testing and commissioning
A pressurisation system that has not been properly commissioned is worse than useless — it creates a false sense of security. The commissioning process for a staircase pressurisation system includes:
- Differential pressure measurement: Readings taken at each floor level with all stair doors closed, to confirm the 50 Pa target is met throughout the shaft height.
- Door-opening force measurement: A push-pull gauge applied at the leading edge of each stair door to verify the 110 N maximum is not exceeded.
- Airflow measurement: Pitot-tube traverse or anemometer readings at the fan outlet and at each injection point to confirm design flow rates.
- Pressure-relief performance: Verification that relief dampers open at the correct pressure and re-seat cleanly on pressure reduction.
- Fire-alarm interface test: Witness test of auto-start, cause-and-effect logic, BMS feedback and manual override functions.
All readings should be recorded in a commissioning report that is included in the building’s O&M documentation. For complex projects or high-rise buildings, a third-party witness test by the fire authority or an independent commissioning specialist is advisable.
Common installation errors
The gap between design intent and installed performance is wide in smoke-control systems. The most frequently observed problems include:
- Leaky stairwell shafts: Gaps around cable penetrations, unsealed door frames and incomplete masonry allow air to escape at low pressure, preventing the shaft from reaching the 50 Pa target. Air-tightness must be addressed at construction stage, not remediated post-installation.
- No pressure-relief provision: Systems installed without barometric-relief dampers or VSD control routinely exceed 200 Pa under all-doors-closed conditions. The door becomes impossible to open from the fire side.
- Wrong fan selection: A fan selected at design-point duty only may stall or over-deliver when the system resistance changes as doors open and close. Fan selection must cover the full operating range with a stable curve.
- Dampers not interlocked: Motorised dampers on normal ventilation systems that share shafts or plantrooms with pressurisation equipment must close on alarm. Where the interlock wiring is incomplete or the BMS logic has not been programmed, the two systems fight each other.
- Single-point air injection: Injecting all the air at the base of the shaft creates a large pressure gradient — the bottom floors are over-pressurised while the top floors are under-pressurised. Multi-point distribution must be verified at commissioning.