General guidance. Condition can only be judged on site, and reuse is always conditional on the mechanical contractor, or the certifier the council accepts, taking the reused items into the installation they commission and certify. Clause references are to AS 1668.2-2012 with Amendments 1 and 2, called up by NCC 2022. NCC 2025, usable in WA since 1 May 2026, calls up AS 1668.2:2024, where clause numbers and some values differ. Ask the building surveyor which edition the permit uses.
- Start from the new equipment
- The canopy: size
- The canopy: height and filters
- The canopy: construction
- The grease duct
- The fan and the roof outlet
- Make-up air and the old intake
- Controls and interlocks
- What usually stays and what goes
01Start from the new equipment, not the old canopy
The question is never "does the old system work". It is "does it suit what will cook under it now". AS 4674-2004 Clause 2.5.1 makes the same point from the other side: equipment installed after the exhaust has been designed and installed must not be located so that it impairs the system. So fix the equipment list first: every appliance with its width, depth, height and kW or MJ/h. Then measure what is there.
- Canopy: length, width, depth, height of the lower edge above the floor, filter type and size, gutter and drains
- Duct: size, material and thickness, joints, access doors, route and falls, where it crosses walls or floors
- Exhaust fan: nameplate, type, motor position, speed control, condition
- Roof: fan position, distances to boundaries, air intakes, openable windows and neighbours' plant
- Supply: make-up air fan, intake position, filters, where the grilles blow
- Controls: how the fans start, and whether anything links the gas supply to the fans
02The canopy: size
AS 1668.2-2012 Table 3.1 sets the minimum exhaust for a wall canopy as Q = K × P × H: a coefficient for the type of cooking, times the exposed perimeter of the hood, times the height of the hood above the cooking appliance. The airflow follows the hood, not the appliances under it. A canopy shorter than the new line fails. A canopy longer than the line passes, but pulls more air, and needs more make-up air, for as long as the kitchen runs.
Clause 3.5 bases the table on a minimum overhang beyond the cooking surface of 150 mm for cooking process Types 1 to 4, 300 mm for Type 5 (high-heat grease and solid fuel), 600 mm for woks and 450 mm for bread and steam-producing combination ovens. An old canopy built for a line of ovens may have the length but not the overhang a wok needs, or the reverse.
03The canopy: height and filters
Appendix E4 requires the lower edge of a canopy to be at least 2 m above the floor at the operator side and no more than 1.2 m above the cooking appliance. A canopy hung at 2.5 m for tall combination ovens sits 1.6 m above a 900 mm fryer: outside the rule. "The canopy is already there, just connect it" rarely survives that check when the new equipment is lower than the old.
Clause E7.1 then sets the distance from the lowest edge of the grease filter down to the cooking surface, unless reduced, for example where fire suppression is fitted:
| Heat source under the filter | Minimum distance |
|---|---|
| Charcoal and similar open fires, including food that is typically ignited | 1350 mm |
| Naked flame, such as a gas stove | 1050 mm |
| Electric equipment, or a fixed plate or pan above a gas flame (solid grill plate, deep fryer) | 600 mm |
| Kebab cooker and salamander | 200 mm |
A canopy that was fine over electric fryers can be too low for a gas range. Appendix E5 also wants the filters rigid and non-combustible, sealed so air cannot bypass them, faces vertical or no more than 30° from vertical, draining to the gutter, and removable by hand.
04The canopy: construction
- Material and seams (E3.2): non-combustible sheet such as steel or stainless, with liquid-tight seams made by continuous welding, or grooved, lapped or riveted and continuously soldered, or sealed with a compound that grease, water and cleaning agents do not attack.
- Internal surfaces (E3.4.1): sloped no more than 40° from vertical, so condensate runs to the gutter instead of dripping onto food.
- Gutters (E3.4.3 and E3.4.4): at least 50 mm wide and 25 mm deep, with capped drain holes of at least 25 mm, the inside edge at least 150 mm beyond the appliance (300 mm for Type 5 cooking), except against a wall.
- Exhaust openings into the plenum (E3.3): no more than 500 mm from the ends and no more than 1 m apart, so the hood draws evenly.
On site, look for leaking or dented gutters, grease built up behind the filters, and fittings that stick out inside the hood (E3.4.2 allows no protrusions except fire-suppression heads; E3.4.5 wants lights flush).
05The grease duct
This is where most old systems fail, and where a good one saves the most. The duct has to be grease duct, not just a duct: galvanised steel 1.2 mm or stainless 0.9 mm, grease-tight joints, cleaning points no more than 3 m apart and at every change of direction (City of Nedlands guideline; cleanouts also Cockburn and Wanneroo). Good practice adds falls back to the hood, with a grease-tight drain at any low point. A duct with screws through it, taped joints or flexible sections longer than a short connector normally goes.
Fire separation matters too. AS/NZS 1668.1:2015 does not allow fire dampers in kitchen exhaust, so a duct that runs through another fire compartment has to be protected by fire-resisting construction instead. An old duct that passes through a neighbouring tenancy is a question for the building surveyor before it is a saving.
Taking over a kitchen with an exhaust already in it? Send photos of the canopy, the duct and the roof with the new equipment list. I will tell you early what is likely to stay.
Send photos and the equipment list06The fan and the roof outlet
Check the old fan against its curve at the new airflow and pressure, not the nameplate. The new hood may need more air, or the same duct may need more pressure. Grease exhaust fans normally keep the motor out of the airstream and have a speed control so the airflow can be set at commissioning.
Then check where the air leaves. AS 1668.2-2012 treats kitchen exhaust above 1000 L/s as objectionable, and outlets within a single 6 m radius count as one discharge for that flow (Table 3.3). Clause 3.10.3 then requires each outlet to discharge vertically at 5 m/s or more and to sit at least 6 m from property boundaries, street boundaries, outdoor air intakes and natural ventilation openings. Below 1000 L/s, Table 3.4 sets a separation that grows with the flow, from 1 m under 200 L/s to 5 m under 1000 L/s. So an outlet that was fine for a small café can fail once the new kitchen pushes the group past 1000 L/s, and a neighbour may have added an intake since. Some councils also set a discharge height; see getting a kitchen exhaust approved in WA.
07Make-up air and the old intake
Clause 3.8.3 requires make-up air to be supplied directly when the outdoor air already reaching the kitchen cannot offset the hood exhaust, and Clause 3.8.5 requires the supply fan to run whenever the exhaust runs. Clause 3.3.2.1 adds that supply outlets must not reduce hood performance or cause excessive cross-draughts over the appliances.
- The old intake. If it sits inside the separation the new discharges need (6 m once the group exceeds 1000 L/s, Table 3.4 below that), it moves. That is compliance, not cost. On the bakery job the existing roof intake was about 2.6 m from a new oven outlet and 2 m from an oven flue; the design moves the intake to a position 6.9 m from the nearest outlet.
- The old grilles. A supply grille aimed at the front of a canopy pushes the plume out of the hood. It comes out or gets moved to the far side of the kitchen, blowing down at low velocity.
- The old supply fan. Keep it only if its capacity, filters and intake position suit the new exhaust.
08Controls and interlocks
Old systems often start from a single wall switch. The new design sets the logic: the supply fan runs with the exhaust (AS 1668.2-2012 Cl 3.8.5), and where gas appliances rely on a mechanical air supply, AS/NZS 5601.1:2022 Clause 6.4.10 requires an interlock that senses real air movement and shuts the gas if the supply fails. The engineer writes the control and interface requirements; the licensed gasfitter installs and certifies the gas side, and the electrician wires it.
09What usually stays and what goes
| Item | Often reusable when | Usually replaced when |
|---|---|---|
| Canopy | Same line length, compliant height over the new appliances, welded, gutters intact | Too short or much longer than the new line, hung too high, no gutters |
| Grease duct and riser | Right position, grease-tight, correct thickness, cleaning access, cleanable | Wrong route, low points, screws, tape or flexible sections, unprotected through another fire compartment |
| Exhaust fan | Curve meets the new duty, motor out of the airstream, sound condition | Too small for the new duty, motor in the airstream, too loud for nearby homes |
| Roof outlet position | Vertical, clear of boundaries, intakes and openings by the required distance | Inside that distance, or horizontal on a group above 1000 L/s |
| Make-up air intake | Outside the separation every discharge needs, and clear of flues | Near the new outlets or a flue |
| Supply grilles | Far side of the kitchen, low velocity | Blowing across the hood |
From design practice. Every job is measured and calculated; the table is where to look first, not a verdict.
Do not put reuse in the budget as a saving until it is measured. The saving is real when the riser, the roof penetration and the fan stay. It disappears when a canopy has to be cut down and re-certified, or an intake has to move. Price the reuse option and the new option side by side. For what drives the total, see 7 design decisions that set the cost of a kitchen exhaust.
QUESTIONSWhat builders and owners ask
Can I reuse a kitchen canopy left by a previous tenant?
Yes, if it suits the new appliance line: length and overhang to AS 1668.2-2012 Clause 3.5, lower edge at least 2 m above the floor and no more than 1.2 m above the cooking appliance (Appendix E4), grease filters at the right height above the cooking surface (E7.1), and sound construction. A canopy longer than the new line needs more exhaust and make-up air for as long as it runs.
Can the old grease duct be reused?
Only if it is built as grease duct and sits in the right place for the new hood: galvanised steel 1.2 mm or stainless 0.9 mm, grease-tight joints, cleaning access no more than 3 m apart and at every change of direction, and a grease-tight drain at any low point. Have it cleaned and inspected inside before deciding.
Does an existing kitchen exhaust need approval again for a new business?
Often. A new food business has to register with or notify the council, and a changed fit-out is usually assessed again under Food Standards Code Standard 3.2.3. Many WA councils, such as Fremantle, Canning and Mandurah, ask for written certification that the exhaust meets the performance criteria, and Rockingham requires approval even for refurbishments and minor works. Check with the council's environmental health team.
What usually has to be replaced in an old kitchen exhaust?
Canopies sized or hung for different equipment, supply grilles that blow across the front of the hood, and air intakes inside the separation the new discharges need (6 m once the group exceeds 1000 L/s). A welded riser in the right place and a fan whose curve meets the new duty are often worth keeping.
Scope: I assess existing equipment and design kitchen exhaust systems as a mechanical engineer (MIEAust). The mechanical contractor installs, commissions and certifies the system, including any reused items, and the licensed gasfitter installs and certifies the gas work. More on the service: kitchen exhaust, ventilation and air conditioning design in Perth.