How do you size a smoke exhaust fan in Australia?

Smoke exhaust fan sizing in Australia follows NCC 2025 Specification 21. Use Figure S21C2 with two inputs: the height from the lowest floor to the underside of the smoke layer (minimum 2 m above the highest floor), and the design fire load in MW from Table S21C2. The figure gives the required exhaust rate in m³/s.

What temperature rating does a smoke exhaust fan need in Australia?

NCC 2025 S21C3 requires every smoke exhaust fan to run continuously at 200°C for at least 1 hour at full airflow and system resistance. In buildings without sprinklers, the fan must also run at 300°C for at least 30 minutes.

What is the maximum size of a smoke reservoir under the NCC?

NCC 2025 S21C4 limits each smoke reservoir to 2,000 m² in plan area. In enclosed walkways and malls of Class 6 buildings, the reservoir must not exceed 60 m in length. The minimum reservoir depth is 500 mm below the ceiling.

When does the NCC require a smoke exhaust system?

NCC 2025 E2D10 requires smoke exhaust for Class 5-9 buildings not over 25 m effective height that exceed 18,000 m² floor area or 108,000 m³ volume. Class 9b assembly buildings (theatres, nightclubs, exhibition halls) have lower thresholds under E2D16-E2D19.

What are the make-up air requirements for smoke exhaust systems?

NCC 2025 S21C6 requires low-level make-up air for all smoke exhaust systems. Air velocity through doorways must not exceed 2.5 m/s. In multi-storey voids, the average make-up air velocity must not exceed 1 m/s to prevent smoke spread between floors.

Smoke Exhaust Fan Sizing | CCC Engineering

Section 1

What You Need to Know

Smoke exhaust fans pull hot smoke out of a building during a fire so people can escape and firefighters can see. The NCC (National Construction Code) sets out when you need a smoke exhaust system and how to size the fans. AS 1668.1 (the Australian Standard for fire and smoke control) fills in the design detail. Get the fan size wrong and the smoke layer drops below 2 m, blocking exit paths and failing certification.

Section 2

The Rules

Smoke exhaust fans must keep the smoke layer at least 2 m above the highest floor level in the smoke reservoir (NCC 2025, Specification 21, S21C2)
Fan exhaust rates come from Figure S21C2, based on the height to the smoke layer and the fire load in MW from Table S21C2 (NCC 2025, S21C2)
Each fan must run at 200°C for at least 1 hour at full airflow and system resistance (NCC 2025, S21C3)
In buildings without sprinklers, each fan must also run at 300°C for at least 30 minutes (NCC 2025, S21C3)
Smoke reservoirs must not exceed 2,000 m² in plan area, with a minimum depth of 500 mm below the ceiling (NCC 2025, S21C4)
Make-up air velocity through doorways must not exceed 2.5 m/s (NCC 2025, S21C6)
Fan discharge velocity must be at least 5 m/s, and the discharge point must be at least 6 m from any air intake or exit (NCC 2025, S21C5)

Section 3

What This Means in Practice

Take a 1,500 m² Class 6 retail floor with sprinklers and a 4 m ceiling. Table S21C2 gives a design fire load of 5 MW for a sprinklered Class 6 building. You read Figure S21C2 with a height of about 2 m (from the floor to the underside of the smoke layer, keeping the clear layer at 2 m above the floor) and the 5 MW line. That figure gives you the required exhaust rate in m³/s. For this scenario, the exhaust rate is typically in the order of 15-25 m³/s per smoke reservoir, depending on geometry. Confirm the exact rate by reading Figure S21C2 for your specific height and fire load.

The fan itself must be rated to 200°C for 1 hour because the building has sprinklers. Without sprinklers, the fire load jumps to 10 MW for Class 6, and the fan needs a 300°C/30-minute rating on top of the 200°C/1-hour rating. That doubles the exhaust rate and pushes the fan into a larger, heavier, more expensive unit.

Make-up air is just as important as exhaust. If the replacement air cannot get in fast enough, the fan starves and the smoke layer drops. The 2.5 m/s limit on doorway velocity means you need enough open area at low level to feed the fan without creating a wind tunnel that pushes smoke back down. For a 20 m³/s exhaust rate, you need at least 8 m² of free opening area at low level.

Section 4

Key Design Decisions

1

Sprinklered vs. Unsprinklered Fire Load

Design to the sprinklered fire load wherever sprinklers are installed. A sprinklered Class 6 building uses 5 MW instead of 10 MW, which roughly halves the exhaust rate and allows a smaller, cheaper fan.

Trade-off: Sprinklers add capital cost (typically $30-60/m²), but the smaller smoke exhaust system and lower fire load save on fan size, ductwork, and structural support. Most commercial buildings install sprinklers anyway for other NCC compliance reasons.

2

Number of Smoke Reservoirs

Split large floor plates into multiple smoke reservoirs using smoke curtains or bulkheads. Each reservoir is limited to 2,000 m² and gets its own fan or fan zone.

Trade-off: More reservoirs mean more fans and more ductwork, but each fan is smaller and easier to install. Fewer, larger reservoirs need bigger fans that are harder to fit in the ceiling void or on the roof.

3

Fan Location: Roof-Mounted vs. In-Line

Roof-mounted fans are the most common choice. They sit outside the fire compartment, making maintenance easier and keeping the hot motor away from occupied space. In-line fans inside ductwork can work but need fire-rated enclosures.

Trade-off: Roof-mounted fans need structural support, weatherproofing, and acoustic treatment. In-line fans save roof space but must be accessible for AS 1851 inspections inside the ceiling void or riser.

4

Make-Up Air Strategy

Provide low-level openings (automatic doors, louvres, or breakout panels) sized so air velocity stays below 2.5 m/s through any doorway. In multi-storey voids, keep average make-up air velocity below 1 m/s.

Trade-off: Automatic make-up air louvres (typically $3,000-8,000 per opening) give reliable performance. Relying on manually opened doors or breakout panels is cheaper but less predictable during an actual fire.

Section 5

Who Needs to Know What

Section 6

References

National Construction Code 2022, Volume One, Part E2 — Smoke hazard management
National Construction Code 2022, Volume One, Specification 21 — Smoke exhaust systems
AS 1668.1:2015 (Amdt 1:2018), The use of ventilation and airconditioning in buildings — Part 1: Fire and smoke control in buildings
AS 1851-2012, Routine service of fire protection systems and equipment
AS 4254, Ductwork for air-handling systems in buildings
AIRAH DA05, Guide to Fire and Smoke Control in Buildings Using AS 1668 Part 1
ASHRAE Handbook - HVAC Applications, Chapter 53: Smoke Management (international reference)

Smoke Exhaust Fan Sizing and Specification

What You Need to Know

The Rules

What This Means in Practice

Key Design Decisions

Sprinklered vs. Unsprinklered Fire Load

Number of Smoke Reservoirs

Fan Location: Roof-Mounted vs. In-Line

Make-Up Air Strategy

Who Needs to Know What

Need this engineered for your project?

References

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