What really sets the cost of a kitchen exhaust

A fit-out company asked me to design the exhaust for a production bakery in the Perth metro area: two electric fryers, two gas rack ovens with their own steam fans, a pot washer, and an open-plan hall of about 970 m³ with a flat metal roof. The previous tenant had left a canopy, two ducts and a roof intake behind.

The first layout was the one most kitchens get: a canopy, a long grease duct across the ceiling to wherever the roof looked clear, a fan, a stack. It complied. It was also more expensive than it needed to be, and it carried three assumptions that the Standards do not support. Here is what changed and why.

Numbers are from the design stage of one job and are indicative, not a quotation. The client and the site are not named. Clauses are quoted from AS 1668.2-2012 with Amendments 1 and 2 (the edition NCC 2022 calls up) and AS/NZS 5601.1:2022; check the edition your building surveyor uses.

1. Put the fan above the hood
2. Size the hood to the line, not the old canopy
3. Treat the roof outlets as one group
4. Check whether a 3 m stack is required
5. Gas ovens change the air, not just the gas
6. Make-up air is the other half
7. Check every number before it is priced

DECISION 01Put the fan above the hood

Horizontal grease duct is the most expensive metre in a kitchen exhaust. It has to be welded or grease-tight, carry an access door at every bend and roughly every 3 m, hang from the roof structure, fall back towards the hood so grease drains, and be cleaned for the life of the kitchen. A vertical riser straight into a roof fan above the hood needs almost none of that: grease runs back to the hood gutter, and the cleaner reaches the whole riser from two doors.

Same hood, same airflow. The difference is where the fan sits.

The catch is the roof: purlins, the neighbour's condensers and the fresh air intake decide whether a fan can sit there. Allow two 45° offsets in the riser to clear a purlin and the position almost always works. Check it on the roof, with the structure drawings, before anyone draws a duct route.

DECISION 02Size the hood to the line, not to the old canopy

AS 1668.2-2012 Table 3.1 sets the minimum exhaust for a wall canopy as Q = K × P × H, where P is the exposed perimeter of the hood and H is the height of its lower edge above the cooking surface. The airflow follows the hood, not what sits under it. A hood that is longer than the appliance line pulls more air for as long as the kitchen runs, and so does the make-up air that has to replace it.

Height matters just as much. Appendix E4 requires the lower edge to be at least 2 m above the floor and no more than 1.2 m above the cooking appliance. A canopy hung at 2.5 m for tall combi ovens has to come down to between 2000 and 2115 mm over fryers with a 915 mm pan. "The canopy is already there, just connect it" rarely survives that check. Cutting an old canopy down is possible, but it means re-welding, new filters and a fabricator's declaration, so price it against a new hood rather than assume it saves money.

DECISION 03Treat the roof outlets as one group, and move the intake

Kitchen exhaust is Type B effluent. Above 1000 L/s it is classed as objectionable (Table 3.3), and the Note to that table says outlets within a single 6 m radius count as one discharge for the flow rate. On this job the grease fan, the pot washer fan and both oven steam risers sat within about 3.5 m of a common centre, so the group was 1672 L/s and every outlet in it, even the small oven risers, had to meet Clause 3.10.3:

• Vertical discharge at 5 m/s or more. Measured at the outlet or terminal, not in the duct. 1200 L/s through a 400 × 350 riser is 8.6 m/s, but a roof fan with an outlet larger than 1.2 / 5 = 0.24 m² would fail. The oven risers ran at about 3 m/s, so they got open reducing nozzles sized for about 6 m/s, which still passes if the oven fan delivers 10 % less than its data sheet.
• At least 6 m from any property boundary, street boundary, outdoor air intake or natural ventilation opening.

The instinct is to spread the outlets apart to avoid the grouping. It does not help: the grease fan alone is already over 1000 L/s, and spreading them adds duct and roof penetrations. Keep the outlets together and move the intake instead. The existing roof intake sat about 2 m from an oven flue and inside the 6 m of the new group, which means the kitchen, and the ovens' burners, would have breathed their own exhaust. It came out, and the new make-up air intake went to the far corner of the roof, 6.9 m from the nearest outlet.

DECISION 04Check whether a 3 m stack is really required

"The exhaust has to go 3 m above the roof" is the most repeated rule on kitchen jobs. In AS 1668.2-2012 it sits in Clause 3.10.3(b), and that paragraph opens with "for a Type A effluent". Kitchen exhaust is Type B. What Type B needs is the 5 m/s and the 6 m of Decision 03, and Note 2 to Clause 3.10.2 recommends at least 2 m above a roof that people regularly walk on.

The 3 m for kitchens comes from some council food business guidelines, written on older editions of the Standard (Cockburn and Wanneroo, for example). Others simply refer to AS 1668.2 and set no height. On this job the council's construction guideline set none, and the 3 m in that council's health local law turned out to apply only to fish processing premises. Removing the stacks took out sheet metal, stay wires, a support frame, a wind check on the roof and about 25 Pa of fan pressure.

DECISION 05Gas ovens change the air, not just the gas

Most kitchen exhaust designs treat gas as the gasfitter's problem. The pipework and flues are, but the air the burners need is a ventilation design question, and AS/NZS 5601.1:2022 has specific rules for it.

A rack oven that takes combustion air from the room and sends its products up a flue is an open flued appliance (Cl 1.3.2.10). A production hall is a room, not an enclosure (Cl 1.3.29). For a building approved after WA adopted the 2013 edition, Clause 6.4.6.3.1 asks for G = gas input / room volume:

Three things catch people here. The 3 MJ/h per m³ figure that often gets quoted is the limit for flueless appliances, not flued ovens; used by mistake, it says no ventilation is needed. The 0.263 m² is effective free area after the louvre blades and insect mesh, so the louvre face is about twice that. And if the make-up air fan is the pathway, Table 6.4.9 calls for the air at low level, at or below the burners, which ceiling diffusers alone do not give; Clause 6.4.10 then requires an interlock that senses real air movement and shuts the gas if the supply fails.

On this job both pathways are shown, the mechanical one is recommended (filtered air, no permanent cold opening in a food hall, and the fan and interlock already exist), and the licensed gasfitter confirms the burner type and the pathway.

DECISION 06Make-up air is the other half of the system

Every litre the hoods remove has to come back in. AS 1668.2-2012 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. In an open-plan hall with no existing supply, that means a dedicated fan.

• How much. The Standard sets no percentage. In an open plan we supply about 90 % of the exhaust so the shortfall does not whistle through doors: 1550 L/s against 1672 L/s.
• The balance. The remaining 122 L/s, plus about 59 L/s of flue gas leaving through the oven flues (AS/NZS 5601.1 Table G.9, 0.1 L/s per MJ/h), enters through doors and openings. Clause 3.8.4 caps the pressure drop at 12 Pa; at a 5 Pa design target that needs about 0.10 m² of free opening.
• Where. Diffusers on the far side of the hall, blowing down at low velocity. A supply grille aimed at the front of a canopy, like the one the previous tenant left, pushes the plume out of the hood.

DECISION 07Check every number before it is priced

A calculation note that two contractors price differently is not finished. On this job every duty went into one design data file that the report, drawings and schedules quote, and every number was re-derived by a separate script written without the design engine: Colebrook friction, fitting losses, filter resistance, fan power, air balance, roof distances. 85 values, all within 2 %. Every clause was read in the Standard itself, and figures that come from judgement are marked as judgement.

Before the site measurements, the builder got a four-page preliminary concept: the layout, the reasons, and a short list of the documents only others hold (site survey, construction drawings, appliance models, the architect's set-out, non-public manufacturer manuals, the NCC edition). It let the job keep moving while the data came in.

QUESTIONSWhat builders and owners ask

Scope: I design and calculate as a mechanical engineer (MIEAust). The licensed mechanical contractor supplies, installs, commissions and certifies the system, and the licensed gasfitter designs, installs and certifies the gas work and flues. More on the service: kitchen exhaust, ventilation and air conditioning design in Perth.