How Should Commercial Kitchen Exhaust Ducts Be Designed?

Kitchen exhaust and grease duct design selects hoods matched to the cooking equipment, calculates exhaust and makeup air rates, designs welded grease duct with code-required clearances and cleanouts, routes ductwork to code-compliant termination, and integrates fire suppression and controls. The mental model that prevents kitchen ventilation failures: exhaust, makeup air, and fire protection are one system. Engineers who design the exhaust and leave makeup air to 'building air' create negative-pressure kitchens that don't ventilate. Engineers who design both but ignore the fire suppression interface create code problems. The hood, the duct, the makeup air unit, and the suppression system have to be designed together from the equipment list outward.

The design starts with the cooking equipment schedule — appliance by appliance, with duty ratings — because exhaust rates follow the equipment, not the kitchen size. Hood selection matches the appliance type and arrangement: wall canopy, island canopy, backshelf, each with capture velocities the design must achieve. The exhaust calculation sets the fan size; the makeup air calculation sets the replacement air, tempered where climate demands it, delivered so it doesn't disrupt hood capture. The duct design is where the fire code takes over: welded black steel, liquid-tight, pitched to drain grease toward collection, with cleanouts at every turn and rated enclosures or clearances through the building. Termination matters too — the exhaust must discharge above the roof with clearances from intakes, property lines, and operable windows, because grease-laden exhaust near an air intake is both a nuisance and a contamination problem. Controls tie it together: interlocks that start exhaust with the cooking equipment, and in many jurisdictions demand-control ventilation that ramps fans with actual cooking activity to save energy.

Kitchen exhaust is one of the most inspected systems in a building, and plan reviewers know exactly where the weak designs are. These are the items that keep a kitchen project clean.

• Start from the equipment list: exhaust rates and hood types follow the actual appliances, not rules of thumb
• Design makeup air as part of the system: tempered replacement air, delivered without disrupting hood capture
• Detail the grease duct completely: welded construction, clearances or rated enclosures, cleanouts at every change of direction
• Route to a compliant termination: roof discharge with proper clearances from intakes, windows, and property lines
• Coordinate suppression and controls: hood fire suppression interface, equipment interlocks, and demand-control ventilation