How Is Commercial Kitchen Hood Ventilation Properly Designed?

I begin with the cooking equipment schedule, because the hood serves the equipment, not the other way around. Each appliance is classified by duty, and the exhaust rate per linear foot of hood follows that duty — extra-heavy equipment like solid-fuel charbroilers sits at the top of the scale. Hood length must overhang the equipment on all exposed sides so the thermal plume is captured before room air currents can peel it away, and island configurations get additional airflow to compensate for the missing wall. Listed hoods carry their performance data with them, and I design to the listing rather than to rules of thumb. Demand-controlled kitchen ventilation — optic or temperature sensors that ramp exhaust and makeup air with actual cooking activity — cuts fan and conditioning energy dramatically in kitchens with variable loads, and I specify it wherever the budget allows. Every hood gets a fire suppression system listed for commercial cooking, interlocked to shut off fuel and power to the equipment it protects, because the suppression system and the hood are one life-safety assembly.

Makeup air design is pressure design. I calculate the kitchen's total exhaust — hoods, dishwasher exhaust, general room exhaust — and provide dedicated makeup air for the bulk of it, tempered to keep the kitchen workable. The air is introduced where it helps rather than hurts: through perforated ceiling diffusers away from the cooking line, or through a makeup air plenum integrated with the hood that washes air down in front of the appliances without crossing the plume. What I never do is rely on transfer air alone or on operable windows. Transfer air from dining is part of the balance — it keeps the kitchen negative — but it cannot carry the whole load without dragging dining room comfort down with it. And makeup air units need their own heating and cooling in most climates, sized honestly: a kitchen exhausting 10,000 cfm in a hot climate without cooled makeup air becomes unworkable by the second week of summer. The controls interlock exhaust and makeup so the balance holds at every fan speed, including demand-control turndown.

The grease duct is the most regulated duct in the building, and I treat it accordingly. Welded liquid-tight steel construction, pitched to drain back toward the hood or to a grease collection point, cleanouts at every change of direction and at code-required intervals, and a fire-rated enclosure or listed wrap system wherever the duct passes through the building. Clearances to combustibles follow the code, and I hold them in the drawings — not as a note for the contractor to figure out, but as dimensioned routing the structure is built around. The exhaust fan goes at the discharge end of the system, pulling the duct negative so leaks draw inward. Upblast discharge on the roof, high enough and far enough from intakes, windows, and property lines per code, with a hinged fan base for cleaning access. Fire protection ties it together: the suppression system, fuel gas and power shutoffs, exhaust fan interlocks, and any damper or airflow proving switches are commissioned as one sequence. The fire marshal witnesses that test, and I make sure it passes the first time by testing it myself first. • Classify every appliance by cooking duty and size hood exhaust to the heaviest duty under each hood • Provide dedicated makeup air at 80 to 90 percent of exhaust, tempered for the climate • Route grease duct vertically and directly with welded liquid-tight construction and fire-rated enclosure • Place the exhaust fan at the discharge end pulling the system negative • Commission suppression, shutoffs, and interlocks as one life-safety sequence