Code requirements
Smoke exhaust fans may be centrifugal or axial (capable of continuous operation at 280°C for 30 min). A smoke exhaust fire damper that closes automatically must be fitted at the fan inlet and interlocked to stop the fan. The fan should sit at the top of the exhaust system with the outlet pointing up, above the inlet of the pressurisation fan; the vertical gap should be at least 3 m, or the horizontal gap at least 10 m. When any exhaust outlet or damper in the system opens, the fan must start on its own.
1. Place the fan in a dedicated plant room or on the outdoor roof. The room needs fire-rated walls (≥2.0 h) and floor (1.5 h) plus an A-class fire door to separate it from other areas. Where space is tight, use a dedicated enclosure whose surrounding structure has a fire rating of at least 1.0 h and leaves at least 600 mm clearance on both sides of the fan. When sharing a room with other fans, all of the following must hold:
- An automatic sprinkler system is fitted in the room.
- No fan or duct for mechanical pressurisation is allowed in the room.
- No flexible connection is allowed on the exhaust fan or its duct.
Outdoor exhaust fans
2. Exhaust ducts must use non-combustible material. With metal ducts, the internal air speed should not exceed 20 m/s; with smooth concrete or other non-metallic ducts, not exceed 15 m/s. Where combustible material is present above the ceiling, the exhaust duct there must be insulated with non-combustible material and kept at least 150 mm from combustibles.
3. After a fire is confirmed, the exhaust outlets or automatic dampers in the burning smoke compartment must be open; those in other compartments must be closed.
4. Keep the exhaust system separate from ventilation and air-conditioning where possible. If combined, all of the following must hold:
- The outlets, ducts and fans must meet exhaust-system requirements.
- After fire confirmation, the exhaust outlets and fan in the fire area must open and ventilation or AC not related to exhaust must shut automatically within 15 s.
5. The exhaust shaft must be separated from adjacent zones by a wall with a fire rating of at least 1 h. Any access door in that wall must be a C-class fire door. A horizontal exhaust duct passing a fire wall needs a smoke exhaust fire damper. Where it crosses two or more fire compartments, or runs in a corridor ceiling, its fire rating must be at least 1 h. The duct must not cross a lobby or stairwell; if it must, the rating must be at least 2 h.
6. Exhaust outlets must meet the following:
- The outlet must sit within the smoke reservoir.
- On fire, the fire alarm links the outlet or automatic damper open, and a manual opener is provided on site.
- The outlet should direct smoke opposite to evacuation; its distance from a safe exit must be at least 1.5 m (keep it as far as possible).
- The outlet air speed should not exceed 10 m/s.
7. Where the ceiling void is used for indirect exhaust, the neck velocity at the smoke inlet in a sealed ceiling should not exceed 2.7 m/s, and the ceiling must be non-combustible. Where the room outlet is in a grille ceiling, the grille open area must be at least 25% of the net ceiling area.