Stairwell pressurization and smoke control systems

In a fire, smoke is usually more dangerous than flames. It spreads quickly through buildings, reduces visibility, and contains toxic gases. In a high-rise building, occupants may need several minutes to descend the stairs, and firefighters use the same stairs to reach the fire floor. Keeping exit stairs clear of smoke is therefore one of the most important life-safety functions in a tall building.

Stairwell pressurization is the most common way to do this in Canada. Fans blow outdoor air into the stair shaft so that the stair is at a higher pressure than the floors around it. When a door to a smoky floor is opened, air flows out of the stair rather than smoke flowing in.

Code basis

Requirements for limiting smoke movement in high buildings come from Part 3 of the National Building Code (NBC) and the provincial codes based on it. The NBC's additional requirements for high buildings generally apply to buildings with a floor level more than 18 m (59 ft) above grade, and to certain other buildings based on occupancy.

The code sets an objective: exit stairs, and certain other areas, must be protected from smoke contamination for a period long enough to allow evacuation and firefighting. Rather than prescribing a single method, the NBC's explanatory notes describe several acceptable measures for high buildings. These include:

• Pressurizing exit stairs with mechanical fans.
• Providing vestibules between floor areas and stairs, sometimes ventilated or pressurized.
• Venting smoke from the fire floor through shafts or exterior openings.
• Using building air handling systems to exhaust the fire floor and pressurize adjacent floors.
• Elevator hoistway protection, particularly for firefighters' elevators.

Provincial codes may modify these measures. Ontario, for example, has its own detailed provisions in the Ontario Building Code. Designers must confirm with the authority having jurisdiction which approach is acceptable for a given building. For broader context, see fire dampers and smoke control under Canadian codes.

How stair pressurization works

A stair pressurization system typically includes:

1. A dedicated supply fan, usually on the roof or at the base of the stair, drawing outdoor air from a location away from smoke exhaust.
2. Ductwork and supply openings into the stair, either at one point or at multiple levels.
3. Controls that start the fan on fire alarm and, in some designs, modulate pressure.
4. Emergency power so the system runs during a power failure.
5. Relief or pressure control to prevent excessive pressure when doors are closed.

Single-injection and multiple-injection

In shorter stairs, air may be injected at one location. In taller stairs, single injection can lead to large pressure differences between the top and bottom, especially if a door near the injection point is open. Multiple injection points, fed by a vertical duct along the stair, distribute air more evenly.

Compensated and non-compensated systems

Doors open and close during evacuation, so the airflow needed varies constantly. Non-compensated systems supply a fixed airflow and accept pressure swings. Compensated systems respond to door operation, using variable speed fans controlled by pressure sensors, barometric relief dampers, or modulating bypass dampers, to keep pressure within an acceptable range.

The pressure window

A stair pressurization system must work within a narrow range:

NFPA values are given here as widely used references; Canadian codes and the authority having jurisdiction govern actual design criteria. The door closer force, the size of the door and the hardware all affect how much pressure difference can be tolerated before the door becomes too hard to open.

Stack effect in Canadian winters

Stack effect is a major design challenge in Canada. In winter, warm air inside a tall building rises and creates pressure differences between the shaft and floors that vary with height: the stair tends to be under negative pressure relative to the floors at the bottom and positive at the top.

In a cold Canadian winter, stack effect pressures in a tall building can be comparable to or larger than the design pressure difference of the stair system. A system that works well in a summer test may fail to keep smoke out of the lower floors in January, or may produce excessive door forces at the top. Good design analyzes the system at winter design conditions, often with network airflow modelling, and multiple injection points or compensation help. See building pressurization and infiltration control.

Cold outdoor air

Stair pressurization fans in Canada supply untempered outdoor air that can be -30 C (-22 F) or colder. During testing or a false alarm, this can freeze sprinkler and standpipe piping in the stair and make the stair uncomfortable. Designs address this with heated supply air, short test durations in winter, or protected piping, depending on the building and the authority's requirements.

Other smoke control approaches

• Zoned smoke control: the fire floor is exhausted, and floors above and below are pressurized using building air handlers.
• Atrium smoke exhaust: large interconnected floor spaces require smoke exhaust sized to keep smoke above occupied levels for a defined period.
• Elevator hoistway pressurization: used to protect elevators intended for firefighter or occupant use during a fire.
• Vestibules: create an intermediate space that can be pressurized or ventilated.

Testing and commissioning

Stair pressurization systems must be tested at installation and periodically after that. Initial acceptance typically includes:

1. Verifying fan rotation, airflow and emergency power operation.
2. Measuring pressure differences across each stair door with all doors closed.
3. Measuring door opening forces at each level.
4. Testing with specified doors open, such as the fire floor door and the exit door at grade.
5. Verifying activation from the fire alarm system and correct operation of dampers, relief and controls.
6. Documenting results for the authority having jurisdiction.

Integrated testing of the fire alarm, smoke control, emergency power and other life-safety systems is required under CAN/ULC-S1001 where the code calls for it. See integrated systems testing and CAN/ULC-S1001.

The National Fire Code and provincial fire codes require smoke control systems to be maintained and periodically tested. Over time, renovations, new doors, changed door closers, tenant alterations and failed dampers erode performance. Stairwell pressurization testing on a regular cycle confirms the system still meets its design criteria.

Common problems

• Door opening forces too high at the top of the stair in winter.
• Insufficient pressure at the bottom of the stair due to stack effect.
• Relief dampers stuck closed or open.
• Fans not starting on fire alarm due to control changes.
• Air intakes located where they could draw in smoke.
• Stair doors propped open or with damaged closers and seals.

Key takeaways

• Stair pressurization keeps smoke out of exit stairs in high buildings so occupants can evacuate and firefighters can operate.
• The NBC sets objectives and lists acceptable measures; provincial codes and the AHJ govern specifics.
• Design must balance minimum pressure against maximum door opening force.
• Winter stack effect in Canadian high-rises can overwhelm poorly designed systems.
• Acceptance testing, integrated testing and periodic retesting are essential.