Toilet and Kitchen Exhaust Requirements

What You Need to Know

Toilet and kitchen exhaust air is dirty air. It contains odours, moisture, grease and contaminants that cannot go back into the building. AS 1668.2 bans recirculation of this air. It must go straight outside through a dedicated exhaust system. Kitchen exhaust also has strict discharge rules under AS 1668.2, including vertical discharge at roof level. Get either system wrong, and you risk failed compliance, odour complaints, and fire safety issues.

• Toilet exhaust must not be recirculated. It must discharge directly outside (AS 1668.2, Section 3)
• Kitchen exhaust must not be recirculated. It must discharge directly outside (AS 1668.2, Section 3)
• A single toilet, shower or bathroom needs at least 25 L/s of exhaust air (AS 1668.2, Table A1)
• A domestic kitchen or laundry needs at least 40 L/s of exhaust air (AS 1668.2, Table A1)
• Commercial kitchen exhaust rates are set by hood type and cooking process (AS 1668.2, Section 3)
• Kitchen exhaust must discharge vertically at a minimum 5 m/s velocity from above the highest roof level (AS 1668.2, Cl 3.10.3)
• Kitchen exhaust discharge points must be at least 6 m from property boundaries and sensitive receptors (AS 1668.2, Cl 3.10.3)
• Kitchen exhaust systems exceeding 1,000 L/s must discharge vertically (AS 1668.2)
• Make-up air must replace at least 80% of exhausted kitchen air (AS 1668.2)
• Kitchen exhaust ductwork longer than 10 m with a flame source needs a UL1046-rated fire safety device (AS 1668.1)
• Fire dampers must not be installed on cooking hood exhaust ducts (AS 1668.1)

Take a ground-floor restaurant with a commercial kitchen in a multi-storey building. The kitchen exhaust duct must run from the hood, up through a dedicated riser, and discharge vertically at roof level. That riser needs space on every floor it passes through. The discharge point must sit at least 6 m from the property boundary and any outdoor air intakes.

If the kitchen exhaust is 1,200 L/s, you need a duct riser sized for that flow at 5 m/s discharge velocity. At 5 m/s, a round duct needs roughly 550 mm diameter. That riser must be fire-rated, have cleaning access every 3 m on horizontal runs, and have no fire dampers.

The kitchen also needs make-up air. If the exhaust is 1,200 L/s, the make-up air system typically supplies around 960 L/s (approximately 80% of exhaust). The rest comes as transfer air from the dining area, which keeps the kitchen at slight negative pressure. This stops cooking odours from pushing into other spaces. The make-up air unit and exhaust fan must be interlocked so they start and stop together.

For toilets, the maths is simpler. A bathroom block with 8 WC pans needs 8 × 25 L/s = 200 L/s of exhaust. This air goes straight outside through a dedicated duct. It cannot pass through the return air system or mix with supply air for other spaces.