NCC 2022 Volume One - Building Code of Australia Class 2 to 9 buildings


Specification 21 Smoke exhaust systems

Specification 21 Smoke exhaust systems

This Specification describes the requirements for mechanical smoke exhaust systems.

(1) Smoke exhaust fans must have a sufficient capacity to contain the smoke layer—

  1. within a smoke reservoir formed in accordance with S21C4 and not less than 2 m above the highest floor level; and
  2. above the top of any openings interconnecting different smoke reservoirs.

(2) Exhaust rates must be determined in accordance with Figure S21C2, with the height measurement taken from the lowest floor level to the underside of the smoke layer and the fire load determined in accordance with Table S21C2.

Table S21C2 Fire load (MW)
Classification Fire load (MW) for unsprinklered buildings Fire load (MW) for sprinklered buildings
Class 2, 3 or 5 5 1.5
Class 6 10 5
Class 7 or 8 15 5
Class 9 — generally 5 1.5
Class 9b buildings covered by Part I1 (see Note) or exhibition halls 10 5
Table Notes

If the smoke reservoir above the stage is smoke separated from the audience area, the fire load specified applies to the stage area only and the fire load for the audience area is as per Class 9 generally.

Figure S21C2 Smoke exhaust rate

Each smoke exhaust fan, complete with its drive, flexible connections, control gear and wiring must—

  1. be constructed and installed so that it is capable of continuous operation (exhausting the required volumetric flow rate at the installed system resistance) at a temperature of 200°C for a period of not less than 1 hour; and
  2. in a building not fitted with a sprinkler system, be capable of continuous operation at a temperature of 300°C for a period of not less than 30 minutes; and
  3. be rated to handle the required volumetric flow rate at ambient temperature to be capable of exhausting cool smoke during the early stages of a fire and to allow routine testing; and
  4. have any high temperature overload devices installed, automatically overridden during the smoke exhaust operation.

(1) A fire compartment must be divided at ceiling level into smoke reservoirs formed by smoke baffles/curtains of non-combustible and non-shatterable construction.

(2) The horizontal area of a smoke reservoir must not exceed 2000 m2 and in enclosed walkways and malls of a Class 6 building must not exceed 60 m in length.

(3) Smoke reservoirs must be of sufficient depth to contain the smoke layer and must not be less than 500 mm below an imperforate ceiling or roof.

(4) Within a multi-storey fire compartment

  1. a non-combustible bulkhead or smoke baffle/curtain must be provided around the underside of each opening into a building void to minimise the spread of smoke to other storeys; and
  2. the depth of the bulkhead or smoke baffle must be not less than the depth of the smoke reservoir provided under (3) plus an additional 400 mm.

Smoke exhaust fans and vents must be located—

  1. such that each smoke reservoir is served by one or more fans with the maximum exhaust rate at any one point limited to avoid extracting air from below the smoke layer; and
  2. to prevent the formation of stagnant regions resulting in excessive cooling and downward mixing of smoke; and
  3. at natural collection points for the hot smoky gases within each smoke reservoir having due regard to the ceiling geometry and its effect on the migratory path of the smoke; and
  4. away from the intersection of walkways or malls; and
  5. to ensure that any voids containing escalators and/or stairs commonly used by the public are not used as a smoke exhaust path; and
  6. to discharge directly to outdoor with a velocity of not less than 5 m/s, at a suitable point not less than 6 m from any air intake point or exit.

(1) Low level make-up air must be provided either automatically or via permanent ventilation openings to replace the air exhausted so as to minimise—

  1. any disturbance of the smoke layer due to turbulence created by the incoming air; and
  2. the risk of smoke migration to areas remote from the fire due to the effect of make-up air on the air balance of the total system.

(2) The velocity of make-up air through doorways must not exceed 2.5 m/s.

(3) Within a multi-storey fire compartment, make-up air must be provided across each vertical opening from a building void to the fire-affected storey at an average velocity of 1 m/s so as to minimise the spread of smoke from the fire-affected storey to other storeys.

(1) Each smoke exhaust fan must be activated sequentially by smoke detectors complying with Specification 20 and arranged in zones to match the smoke reservoir served by the fan(s).

(2) Subject to (3) and (4), an air handling system (other than individual room units less than 1000 L/s and miscellaneous exhaust air systems installed in accordance with Sections 5 and 6 of AS 1668.1) which does not form part of the smoke hazard management system must be automatically shut down on the activation of the smoke exhaust system.

(3) In a single storey fire compartment, air handling systems in all non fire-affected zones may operate on 100% outdoor air to provide make-up air to the fire-affected zone.

(4) Within a multi-storey fire compartment, air handling systems in all non fire-affected zones and storeys must operate at 100% outdoor air to provide make-up air to the fire-affected storey via building voids connecting storeys.

(5) Manual override control and indication together with operating instructions for use by emergency personnel must be provided adjacent to the fire indicator panel in accordance with the requirements of clauses 4.11 and 4.13 of AS 1668.1.

(6) Manual control for the smoke exhaust system must also be provided at a location normally used by the stage manager in a theatre.

(7) Power supply wiring to exhaust fans together with detection, control, and indication circuits (and where necessary to automatic make-up air supply arrangements) must comply with AS 1668.1.

A smoke detection system must be installed in accordance with Specification 20 to activate the smoke exhaust system.