What the Standard says, and what is judgement
- Clause 3.8.3 is a sufficiency test, not a percentage. If the outdoor air already supplied to the kitchen or the space next door cannot offset the hoods, dedicated make-up air is required.
- Clause 3.8.4 sizes the transfer openings so the pressure difference between spaces stays at or below 12 Pa. Designing to about 5 Pa leaves margin for filters loading up.
- Clause 3.8.5 interlocks the supply with the exhaust. No supply, no exhaust.
- The 90 % is judgement. Keeping the kitchen slightly negative holds odour in. Take too much and doors get heavy, burners starve and the hood stops capturing.
Where the air enters matters as much as how much
Clause 3.3.2.1 requires supply outlets to be arranged so they do not reduce hood performance or cause excessive cross-draughts over the appliances. In practice that means diffusers on the far side of the kitchen, blowing down at low face velocity. A grille aimed at the front of a canopy pushes the plume straight out of the hood, and it is one of the most common faults we find in an existing fit-out.
Do not forget the gas
Where open flued gas appliances share the room, the products of combustion leave through their flues, and that air has to be counted in the balance too. If the mechanical supply is also the combustion air pathway, AS/NZS 5601.1:2022 requires it at low level and interlocked with the gas. See gas ovens: ventilation air and interlocks.
Tempered or not
Untempered filtered supply is the Perth baseline and it is what most budgets carry. In a hot kitchen, bringing 2000 L/s of summer air in untreated can undo the cooling you paid for, so the options are evaporative cooling, which adds moisture and maintenance, or refrigerated make-up air, which adds cost. That choice belongs in the heat load conversation, not in the exhaust one.
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How much make-up air does a commercial kitchen exhaust need?
AS 1668.2-2012 sets no percentage. Clause 3.8.3 requires direct make-up air when the outdoor air already reaching the kitchen cannot offset the hood exhaust, Clause 3.8.4 caps the pressure drop across transfer openings at 12 Pa, and Clause 3.8.5 interlocks the supply with the exhaust. In open-plan tenancies about 90 % mechanical supply, with the rest transferring, keeps the kitchen slightly negative.
Where should make-up air enter a commercial kitchen?
On the far side from the hoods, blowing down at low face velocity. AS 1668.2-2012 Clause 3.3.2.1 requires supply outlets not to reduce hood performance or cause excessive cross-draughts over the appliances. A supply grille aimed at the front of a canopy pushes the plume out of the hood.
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