[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

We are designing the ventilation for a restaurant kitchen with a 3.0 m long, 1.2 m deep wall-mounted canopy hood over a cooking line with fryers, griddle and ranges.

• How is the exhaust airflow calculated: per metre of hood, by face velocity, or from the appliance heat load?
• How much make-up air should be supplied, and how should it be introduced?
• What duct velocity and other code checks apply?

We would like to compare the US (IMC, NFPA 96) and European (EN 16282, VDI 2052) approaches.

[MEP Center Engineering]

For a quick, defensible figure use the airflow per metre of hood for the duty of the appliances (IMC method) and check it against face velocity; for a detailed design use the appliance heat-load method of EN 16282 / VDI 2052. Supply make-up air for about 80 to 90 % of the exhaust at low velocity so the kitchen stays slightly negative.

1. Methods

• IMC (US): minimum exhaust per linear foot of hood depends on hood type and appliance duty (light, medium, heavy, extra-heavy). For a wall-mounted canopy over medium-duty appliances it is 300 cfm/ft (about 465 l/s per metre).
• Face velocity check: about 0.25 to 0.5 m/s across the hood face area for canopy hoods, higher for heavy duty.
• EN 16282 / VDI 2052: exhaust is built from the convective heat and moisture of each appliance, its simultaneity and the hood arrangement. It often gives lower values for light cooking and higher values for heavy charbroiling.
• ASHRAE 154 follows the hood-type and duty logic used in the IMC.

2. Worked example

• Hood: 3.0 m = 9.84 ft, wall canopy, medium duty (fryers, griddle, ranges).
• Exhaust: 300 cfm/ft × 9.84 ft ≈ 2950 cfm ≈ 1.39 m³/s (5020 m³/h).
• Face velocity check: 1.39 / (3.0 × 1.2) ≈ 0.39 m/s. Acceptable.
• Make-up air at 85 %: ≈ 1.18 m³/s (4270 m³/h).
• Exhaust duct at about 7.5 to 8 m/s: 1.39 / 8 ≈ 0.17 m² → for example 450 × 400 mm (0.18 m², 7.7 m/s).

3. Code checks

• NFPA 96 requires a minimum grease duct velocity of 500 fpm (2.5 m/s); design velocities of 7 to 10 m/s keep grease in suspension, but check noise and pressure drop.
• Canopy overhang: at least 6 in (150 mm) beyond the cooking surface on open sides, and the hood lip no more than 4 ft (1.2 m) above it (IMC).
• Grease ducts: continuous welded or listed, fire-rated enclosure where they pass through other compartments, access doors for cleaning, falling back to the hood.
• Fire suppression over cooking appliances, interlocked to shut off fuel and stop make-up air as the system requires.

4. Make-up air

• Introduce it through low-velocity diffusers or perforated plenums away from the hood edge; keep air velocity near the hood below about 0.25 m/s, or the plume is disturbed and spills.
• Temper it in winter. In large kitchens consider demand control (temperature and optical sensors) to reduce exhaust at low load.
• Interlock exhaust and make-up fans; the exhaust starts first and stops last.

Common mistakes

• Sizing by air changes of the kitchen volume instead of by hood and appliance duty.
• Make-up air through ceiling diffusers blowing straight at the hood.
• Grease duct velocity too low on long horizontal runs.
• Ignoring that the hood length must cover the whole cooking line plus overhang.

Related: Commercial Kitchen Hood Exhaust Airflow: Quick Sizing, Kitchen Hood Wet Chemical Systems: Design Basics.

Check the edition of the code that applies to your project.

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