Kitchen Ventilation Makeup Air Calculator

Every cubic foot of air a kitchen hood exhausts has to be replaced from somewhere, and this calculator sizes that replacement — the makeup air unit (MAU) — as a deliberate percentage of total exhaust CFM rather than a full one-to-one match. Replacing only 80-90% of exhaust (the default here) is intentional: the resulting 10-20% deficit creates a slight negative pressure that keeps cooking odors and grease-laden air from migrating out into the dining room, while still staying shy of the point where doors become hard to open or gas appliances back-draft dangerously. The tool flags that tradeoff directly with a pressure-status message keyed to the deficit percentage.

The heating load for tempering that makeup air uses the standard sensible-heat air formula — BTU/hr = CFM × 1.08 × ΔT — comparing your chosen winter design temperature against the desired supply temperature, then converts that to therms and estimates a monthly gas cost assuming a flat $1.20/therm and 10 hours of daily operation, both of which you should swap for your actual utility rate and operating hours. Fan horsepower and duct diameter estimates assume a typical 1.5 in. w.g. static pressure and a low-noise 1,000 FPM duct velocity respectively — reasonable planning-stage assumptions, but real ductwork routing, filter loading, and elevation will shift the final mechanical schedule. This is a sizing starting point per ASHRAE 154/62.1 principles, not a stamped engineering calculation.