Above and below 1000 L/s
Kitchen exhaust becomes objectionable above 1000 L/s (Table 3.3), and outlets within a single 6 m radius are counted together for that flow. So a grease fan, a condensate fan and two oven vents on the same patch of roof are one discharge for classification, even though each has its own terminal.
| Flow of the discharge group | What applies |
|---|---|
| Above 1000 L/s | Vertical, 5 m/s or more at the outlet, and 6 m from boundaries, intakes and openings (Cl 3.10.3) |
| Below 1000 L/s | Separation from Table 3.4, growing with flow: 1 m under 200 L/s, 2 m under 400, 3 m under 600, 4 m under 800, 5 m under 1000 |
The velocity is at the outlet, not in the duct
The 5 m/s is measured at the outlet or terminal area. A duct running at 8 m/s tells you nothing if the fan outlet is larger than the duct: the maximum outlet area is the airflow divided by 5. Size terminals for about 6 m/s so a 10 % shortfall in airflow still passes on the day it is commissioned.
Where the 3 m comes from
Council food-business guidelines. Cockburn, Wanneroo, Gosnells, Victoria Park, Melville and Nedlands all write a height into their documents, usually 3 m above a flat roof or 1 m above the ridge, some with a relaxation more than 15 m from any higher structure. Other councils simply refer to AS 1668.2 and set no height at all. Read the guideline of the council that will approve your job, not a neighbouring one.
What this changes on site
- A stack costs sheet metal, stays, a support frame and a wind and roof check. Where no guideline requires it, that money buys nothing.
- Spreading outlets apart to dodge the grouping rarely works: the grease fan alone is usually over 1000 L/s.
- The cheaper fix is almost always moving the intake, not the outlets.
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Does a commercial kitchen exhaust have to discharge 3 m above the roof in WA?
Not under AS 1668.2-2012. Clause 3.10.3(b) applies the 3 m to Type A effluent and kitchen exhaust is Type B. Above 1000 L/s, Type B must discharge vertically at 5 m/s or more and sit at least 6 m from boundaries, air intakes and natural ventilation openings. Cockburn, Wanneroo, Gosnells, Victoria Park, Melville and Nedlands impose a height in their own guidelines, so check the approving council.
How far must a kitchen exhaust outlet be from an air intake?
Six metres once the discharge group exceeds 1000 L/s. Below that, AS 1668.2-2012 Table 3.4 scales the separation with flow: 1 m under 200 L/s, 2 m under 400, 3 m under 600, 4 m under 800 and 5 m under 1000. Outlets within a single 6 m radius are added together to decide which case applies.
More memos
General guidance, not a design. Clause references are to AS 1668.2-2012 with Amendments 1 and 2 (called up by NCC 2022) and AS/NZS 5601.1:2022; NCC 2025 calls up AS 1668.2:2024, where numbering and some values differ, so confirm the edition with your building surveyor. I do the design and calculations as a mechanical engineer (MIEAust); the mechanical contractor installs, commissions and certifies, and a licensed gasfitter certifies gas work. More: kitchen exhaust and ventilation design in Perth · all design memos