Stairwell Pressurisation Systems
What You Need to Know
Fire stairs must stay smoke-free during a fire. Stairwell pressurisation systems pump air into the stair shaft to keep smoke out. The NCC (National Construction Code) requires these systems in most buildings over 25 m in effective height. AS 1668.1 (the Australian Standard for fire and smoke control) sets the design rules: 50 Pa minimum pressure, 1 m/s airflow through open doors, and 110 N maximum door opening force.
The Rules
- Fire-isolated exits in buildings over 25 m effective height need automatic air pressurisation (NCC 2025, E2D4)
- The system must serve the entire exit, not just part of it (NCC 2025, E2D4)
- Stairwell pressure must be at least 50 Pa above the adjacent floor when all doors are closed (AS 1668.1)
- Airflow through open doors must be at least 1 m/s to stop smoke entering the stair (AS 1668.1)
- The force to open any stair door must not exceed 110 N at the handle with the system running (AS 1668.1)
- The system must be controlled by a smoke detection and alarm system installed to Specification 20 (NCC 2025, E2D3)
- Design for 3 doors open at the same time: 1 exit door plus 2 consecutive floor doors (AS 1668.1)
What This Means in Practice
Take a 30-storey residential tower with a single fire stair. The pressurisation fan must push enough air to hold 50 Pa across every closed door and still deliver 1 m/s through 3 open doors at the same time. A standard fire door is about 2 m². Three open doors need roughly 6 m³/s of air just for velocity. Add leakage through all 30 closed doors and you are looking at a fan capacity in the order of 7-8 m³/s.
The fan needs a dedicated shaft for supply air and a second shaft (or relief path) for pressure relief. When all doors close, the stair would over-pressurise without relief. Most Australian systems now use variable speed drives (VSDs) with a pressure sensor in the stair. The fan speeds up when doors open and slows down when they close. Barometric relief dampers or motorised relief dampers handle the rest.
Stack effect in tall buildings pushes air up in winter and down in summer. This works with pressurisation on some floors and against it on others. Air injection points every 3-5 floors help keep pressure even from bottom to top. Without multiple injection points, the top floors may be over-pressurised while the bottom floors are under-pressurised, or the reverse.
Key Design Decisions
Single Injection vs. Multiple Injection Points
Use multiple injection points (every 3-5 floors) for buildings over 10 storeys. A single injection point at the base works for shorter buildings but creates uneven pressure in tall ones.
Relief Air Strategy
Install either barometric relief dampers (passive, spring-loaded) or motorised dampers controlled by a differential pressure sensor. Pair either option with a VSD on the supply fan for responsive control.
Dedicated Shafts vs. Shared Risers
AS 1668.1 requires dedicated shafts for pressurisation supply and relief air. No other building services can share these shafts. Plan them into the core layout from day one.
Alternatives for Buildings Under 25 m
For certain building classes under 25 m with a rise of more than 2 storeys, the NCC offers options including stairway pressurisation, zone pressurisation, smoke detection, and sprinklers (NCC 2025, E2D9). Pressurisation is not always the simplest or cheapest path.
Who Needs to Know What
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References
- AS 1668.1:2015 (Amdt 1:2018), The use of ventilation and airconditioning in buildings — Part 1: Fire and smoke control in buildings
- National Construction Code 2022, Volume One, Part E2 — Smoke hazard management
- National Construction Code 2022, Volume One, Part D3 — Construction of exits
- AS 1851-2012, Routine service of fire protection systems and equipment
- AS 1670.1-2018, Fire detection, warning, control and intercom systems — System design, installation and commissioning — Fire
- ASHRAE Handbook - HVAC Applications, Chapter 53: Smoke Management (international reference)