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Sizing Commercial Kitchen Exhaust Hoods From Hood Type and Cooking Duty
A Type I hood handles grease-laden vapour, and its exhaust rate depends on two things: the style of hood and the hardest-working appliance under it. For hoods that are not performance listed, the model mechanical codes set a minimum in CFM per linear foot of hood. The International Mechanical Code, Chapter 5 tabulates those minimums from the appliance duty ratings in ASHRAE 154. A wall canopy needs 200, 300, 400 or 550 CFM/ft for light, medium, heavy and extra-heavy duty. A single island canopy has no wall to contain the plume and needs 400, 500, 600 or 700, which is two thirds more than a wall hood at medium duty (500 against 300). A double island canopy, which captures from both sides, needs 250, 300, 400 and 550. Eyebrow hoods are limited to light and medium duty at 250 CFM/ft, and backshelf and pass-over hoods run 250, 300 and 400 for light, medium and heavy with no extra-heavy duty allowed.
Cooking Duty Sets the Rate for the Whole Hood
ASHRAE 154 rates cooking equipment by the plume it throws off: its heat, its surge and the grease it carries. Light duty covers ovens, steamers and kettles. Medium duty covers ranges, griddles and most fryers, and heavy duty covers charbroilers and woks. Under an unlisted hood the highest duty appliance sets the rate for the full length, even where lighter equipment shares the hood. Extra-heavy appliances that burn wood or charcoal also bring added fire protection rules under NFPA 96, the standard for ventilation control and fire protection of commercial cooking operations, so involve the fire authority early.
End Overhang and Total Hood Length
The model codes require a canopy hood to extend at least 6 inches beyond the cooking surface on its open sides, and more overhang improves capture, which is why some manufacturers list hoods for a 12 inch minimum. Rates are per linear foot of hood, so the overhang counts toward the length. Eight feet of appliances under a wall canopy with 6 inches at each end makes a 9 ft hood, and a medium duty wall hood then needs 9 × 300 = 2,700 CFM (1,274 L/s).
Listed Hoods Can Run Below the Code Table
A hood tested to UL 710 carries a label with the minimum rate at which it captures and contains the plume. Listed rates usually sit below the unlisted table, with typical wall canopy designs between 200 and 300 CFM/ft over medium duty, so the Listed Hood tab lets you enter the label rate directly. Manufacturers commonly recommend a margin of 5 to 25 percent above the listed minimum. A proximity or compensating hood is sized this way, because its capture depends on its own design rather than a table.
Fan and Duct Size
The fan has to deliver the total airflow against the real static pressure of the grease duct, its elbows and the discharge. The mechanical codes keep duct velocity between 500 and 2,500 ft/min. In the example, 2,700 CFM at 1,500 ft/min needs a round duct of about 18 in, and the allowed window runs from about 14 in at 2,500 ft/min to 31 in at 500 ft/min. For a trial size at any other velocity, use the duct sizing calculator and then check the result against the grease duct rules that apply to your project.
Makeup Air and the Pressure in the Kitchen
A hood removes thousands of CFM of conditioned air. Without replacement the room goes negative, doors become hard to open and gas appliances and water heaters can backdraft, so run the combustion air calculator for any fuel-fired equipment in the same space. Most of the replacement comes from a dedicated makeup air unit, estimated at 85 percent by default here, with the rest drawn as transfer air from the dining room. Introduce it gently near the hood, since fast air blowing across the hood face spoils capture. The makeup air calculator sizes the unit, and the ventilation heat load calculator shows what tempering it costs through a Canadian winter.
Frequently Asked Questions
Multiply the hood length, including end overhang, by the exhaust rate per linear foot. For an unlisted wall canopy the code minimum is 200 CFM/ft over light duty, 300 over medium, 400 over heavy and 550 over extra-heavy appliances, and the highest duty appliance sets the rate for the whole hood. Eight feet of medium duty appliances with 6 inches of overhang at each end makes a 9 ft hood, so 9 × 300 = 2,700 CFM (1,274 L/s). Always check the figure against the code and fire authority that apply to your project.
Makeup air has to replace what the hood removes, with the kitchen left slightly negative to the dining room so odours stay in. Most of it normally comes from a dedicated makeup air unit, estimated at 85 percent of the exhaust here and adjustable, with the rest arriving as transfer air from adjacent spaces. Introduce it gently near the hood, because fast air across the hood face spoils capture. In a Canadian winter the air must also be tempered, which is usually the largest operating cost of the system. Use the makeup air calculator with the total exhaust from this tool.
A listed hood has been tested to UL 710, and its label states the minimum exhaust rate at which it captures and contains the cooking plume. Listed rates are often lower than the code table, with typical wall canopy designs falling between 200 and 300 CFM/ft over medium duty against the unlisted minimum of 300. An unlisted hood must use the code table. Choose the Listed Hood tab and enter the label rate when you have a listed hood, and expect manufacturers to recommend a margin of about 5 to 25 percent.
The mechanical codes keep velocity between 500 and 2,500 ft/min (2.5 to 12.7 m/s), and NFPA 96 sets the same 500 ft/min floor so that grease laden air keeps moving to the fan. At 2,700 CFM a round duct running at 1,500 ft/min is about 18 in in diameter, and the allowed range spans roughly 14 in at 2,500 ft/min to 31 in at 500 ft/min. The fan must deliver the full airflow against the real static pressure of the duct run.