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Sizing commercial kitchen exhaust hoods per ASHRAE 154 and NFPA 96
Commercial kitchen exhaust hoods (Type 1 hoods, for grease-laden vapours) are sized based on hood configuration and cooking appliance duty classification rather than a one-size-fits-all rule. ASHRAE 154 provides airflow rates per linear foot of hood, varying by hood type (wall-mounted canopy, island canopy, eyebrow, or proximity/compensating hood) and cooking duty (light, medium, heavy, or extra-heavy).
Wall-mounted canopy hoods, the most common type, generally require the lowest airflow rate per foot because the wall behind the hood helps contain the thermal plume. Island canopy hoods, exposed on all sides without a wall for containment, require roughly 50% more airflow per foot to capture the same plume. Eyebrow hoods mounted directly on dishwashers or ovens have specific lower rates due to their tight capture geometry, while proximity (compensating) hoods use a different design philosophy entirely, supplying some replacement air directly at the hood face to reduce total exhaust and makeup air needed.
Cooking duty classification matters more than equipment count
NFPA 96 and ASHRAE 154 classify cooking equipment by the heat and grease-laden vapour it produces, not simply by type. Light-duty equipment (ovens, steamers, kettles) produces minimal grease-laden vapour. Medium-duty equipment (ranges, griddles, most fryers) is the most common category in typical restaurants. Heavy-duty equipment (charbroilers, woks) produces substantially more grease-laden smoke and heat, requiring proportionally higher exhaust rates. Extra-heavy-duty (solid fuel, wood or charcoal-fired equipment) requires the highest rates and often additional fire suppression considerations under NFPA 96.
End overhang and total hood length
NFPA 96 requires the hood to extend beyond the appliance line on each open end, typically a minimum of 6 inches for wall-mounted canopy and eyebrow hoods, and often 6-12 inches for island canopy hoods, to ensure adequate capture of the thermal plume at the appliance edges. This overhang adds to the total hood length used in the airflow calculation, not just the appliance footprint itself. Always verify minimum overhang requirements against the current NFPA 96 edition adopted by your local fire code authority.
Don't forget makeup air
A properly sized exhaust hood removes hundreds to thousands of CFM of conditioned kitchen air. Without dedicated makeup air, this creates excessive negative pressure that pulls in unconditioned air through every available gap, makes doors difficult to open, and can cause backdrafting of gas appliances and water heaters. Use the makeup air calculator immediately after sizing your hood exhaust to complete the kitchen ventilation design.
Frequently Asked Questions
CFM = Hood Length (ft) × Airflow Rate per Foot. Medium-duty appliances (most cooking equipment) under a wall canopy hood typically use 300 CFM/ft. Light-duty (ovens, steamers) uses around 200 CFM/ft. Heavy-duty (charbroilers, woks) needs around 400 CFM/ft or more. Add appropriate end overhang (typically 6 inches each side) to the appliance line length before calculating. Always verify against current NFPA 96 and your local fire code. Use the makeup air calculator next to size replacement air.
Typically 80-90% of the exhaust airflow is supplied through a dedicated makeup air unit, with the remainder from transfer air. Without adequate makeup air, the kitchen becomes excessively negative, causing door problems, pilot light extinguishing, and back-drafting of combustion appliances. In Canada, makeup air units must temper incoming outdoor air to avoid cold winter drafts, significantly increasing both capital and operating cost. Use the makeup air calculator based on your exhaust hood's total CFM.