Design Memo
CCC-DM-2026-094

Kitchen Exhaust Hood Design for Commercial Kitchens

Section 1

What You Need to Know

Commercial kitchens produce grease, smoke, and heat. The exhaust hood captures and removes it all. AS 1668.2 (the Australian Standard for mechanical ventilation) sets the exhaust hood rules. The NCC requires a kitchen exhaust hood when cooking equipment exceeds 8 kW electrical or 29 MJ/h gas input (NCC 2025, F6D12). Get the hood wrong, and you fail health approvals, fire compliance, and building certification.

Section 2

The Rules

  • A kitchen exhaust hood is required when any single appliance exceeds 8 kW electrical or 29 MJ/h gas, or when combined equipment exceeds 0.5 kW/m² of kitchen floor area (NCC 2025, F6D12)
  • The exhaust airflow rate must be at least 250 L/s per m² of cooking surface (AS 1668.2, Cl 5.5)
  • Hood overhang must be at least 150 mm for light cooking (Types 1–4), 300 mm for medium (Type 5), and 450 mm for heavy cooking like woks and chargrills (Types 6–7) (AS 1668.2, Appendix E)
  • Kitchen exhaust must discharge at roof level, at least 6 m from the property boundary (AS 1668.1, Section 6)
  • Exhaust exceeding 1,000 L/s must discharge vertically, unless treated filtration is used under AS 1668.2:2024
  • Fire dampers must not be installed on kitchen exhaust ducts (AS 1668.2, Cl 5)
  • Exhaust ducts need cleaning access at every change of direction and every 3 m on horizontal runs (AS 1668.2)
Section 3

What This Means in Practice

Take a mid-size restaurant kitchen with 4 m² of cooking surface. At 250 L/s per m², you need 1,000 L/s of exhaust air. That is a large centrifugal fan on the roof, connected by a dedicated duct riser through the building. The duct must be at least 1.2 mm galvanised steel (or 0.9 mm stainless steel), with all horizontal sections sloped at 0.5% toward a grease drain.

Make-up air is the other half of the equation. You need to replace 65–85% of the exhausted air to stop the kitchen going into negative pressure. Without enough make-up air, doors slam, odours pull into the dining room, and the hood stops capturing smoke. The make-up air system must interlock with the exhaust fan so both run together.

Discharge location causes the most coordination headaches. The exhaust must exit at roof level, 6 m from the property boundary. On tight urban sites, this 6 m setback can conflict with the building footprint, rooftop plant, and neighbouring windows. If vertical discharge is not practical, AS 1668.2:2024 now allows treated exhaust (filtration plus odour control) to discharge horizontally. Treatment systems add cost but solve geometry problems on constrained sites.

Section 4

Key Design Decisions

1

Prescriptive vs. Engineered Hood Design

AS 1668.2 offers two paths. The prescriptive method (Clause 5.5) uses standard exhaust rates based on hood type and cooking process. The engineered method (Clause 5.6) uses manufacturer-tested hood performance data, which can reduce exhaust volumes compared to the prescriptive method.

Trade-off: Engineered hoods cost more upfront but save on fan size, ductwork, and energy. They pay for themselves on large kitchens where the exhaust reduction cuts mechanical plant size.
2

Vertical Discharge vs. Treated Exhaust

The default is vertical discharge at roof level. Where you cannot meet the 6 m boundary setback or achieve vertical discharge, a treated exhaust system (grease filters, UV-C treatment, carbon odour filters) can allow horizontal discharge under AS 1668.2:2024.

Trade-off: Treatment systems add $15,000–40,000+ depending on kitchen size, plus ongoing filter replacement costs. But they avoid the need for a Performance Solution, which can cost $10,000–20,000 in consultant fees alone.
3

Make-Up Air Strategy

Supply 65–85% of exhaust volume as tempered make-up air. Deliver it at low velocity through perforated plenums or displacement diffusers near the hood, not through high-velocity ceiling diffusers that disrupt the hood's capture zone.

Trade-off: A dedicated make-up air unit adds $5,000–15,000. Without one, you rely on transfer air through doors and openings, which causes drafts, pressure imbalance, and poor hood capture.
4

Maintenance Access Planning

AS 1851 requires monthly inspections, with professional cleaning frequency based on measured grease accumulation. Design access hatches at every duct bend and every 3 m of horizontal duct. Place the exhaust fan where it can be serviced without scaffolding.

Trade-off: Access hatches and fan platforms add build cost, but without them, the tenant pays for expensive rope access or scaffold hire at every clean. Poor access leads to grease build-up and fire risk.
Section 5

Who Needs to Know What

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Section 6

References

  1. AS 1668.1-2015, The use of ventilation and airconditioning in buildings — Part 1: Fire and smoke control in buildings, Section 6
  2. AS 1668.2-2012 (Amendment 2), The use of ventilation and airconditioning in buildings — Part 2: Mechanical ventilation in buildings, Sections 3, 5 and Appendix E
  3. AS 1668.2:2024, The use of ventilation and airconditioning in buildings — Part 2: Mechanical ventilation in buildings (updated edition with treated exhaust pathway)
  4. National Construction Code 2022, Volume One, Part F6 — Light and ventilation (Clause F6D12)
  5. AS 1851-2012, Routine service of fire protection systems and equipment (kitchen exhaust maintenance)
  6. AIRAH Best Practice Guideline, Commercial Kitchen Exhaust Management (2022)
  7. ASHRAE Standard 154, Ventilation for Commercial Cooking Operations (international reference)

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