Kitchen Exhaust and Makeup Air Balance Deficit

Every cubic foot a kitchen exhausts has to come back in from somewhere, and the deficit between exhaust and dedicated makeup air is the quantity that decides where. SOME DEFICIT IS INTENTIONAL: a kitchen is deliberately kept slightly negative to the dining room so odours and grease-laden air stay where they belong, and a small transfer across that boundary is design rather than fault. What matters is the part NOT accounted for, because that air comes through the building envelope, under exterior doors, and down any flue that will pass it -- and the flue is the one that matters. THE CONSEQUENCES ARRIVE IN AN ORDER. A modest deficit in a tight building produces a large negative pressure, because pressure is the deficit divided by how leaky the building is, and a well-sealed building has very little leakage to work with. That pressure shows up first as door opening force: an exterior door in a tight negative building can exceed the thirty pound egress limit, which is a life safety problem before it is a comfort one and which an inspector can measure. Next it shows up as combustion appliance backdrafting -- a water heater or a boiler in or near the kitchen can spill flue gases into the space, and the test is done at worst case with every exhaust fan running and every makeup path closed. Then other exhaust systems reverse: restroom fans stop exhausting and start blowing restroom air into the restrooms. AND THE HOOD ITSELF STOPS WORKING, which is the consequence people least expect. A hood starved of makeup air cannot move its rated exhaust -- the fan is on a system curve, and raising the resistance by making the room negative moves it back along that curve -- so capture degrades and grease-laden vapour escapes into the space. A kitchen with a capture problem is often a makeup air problem wearing a hood problem's clothes, and adding fan speed to a starved hood makes the negative pressure worse rather than better. The fix is makeup air, and the amount is the deficit less the intended transfer. An air balance, not a design. It does not size the hood or its exhaust, which follows from the appliance line-up and the hood style; it does not size or select the makeup air unit, temper it, or compute the heating and cooling that tempering requires, which on a cold day is a very large load; and it does not distribute the makeup air, which matters as much as its quantity -- makeup air delivered as a draft across the cooking surface disrupts capture as effectively as having none. The leakage coefficient is a crude single-parameter model of a building, and a measured blower-door result is the only sound source for it. It does not perform the combustion safety test, which is a field measurement. The applicable mechanical code, the hood and makeup air manufacturers' data, a measured air balance, and the mechanical designer govern.

deficit = total exhaust - dedicated makeup air; the uncontrolled shortfall is that deficit less the intended transfer from adjacent space; negative pressure = the shortfall / the building's leakage coefficient; door force = that pressure x the door area, against the 30 lbf egress limit.

The kitchen air balance identities by name. It balances air: it does not size a hood or its exhaust, size or temper a makeup air unit, or distribute the makeup air, which matters as much as its quantity. The leakage coefficient is a crude single-parameter building model and a measured blower-door result is the only sound source for it. The combustion safety test is a field measurement. The applicable mechanical code, the hood and makeup air manufacturers' data, a measured air balance, and the mechanical designer govern.

Two subtractions and one division on an air balance the reader measures; no code table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: hood_exhaust_cfm, other_exhaust_cfm, dedicated_makeup_cfm, intended_transfer_cfm, building_leakage_cfm_per_pa, door_width_ft, door_height_ft, total_exhaust_cfm, deficit_cfm, deficit_percent, uncontrolled_cfm, makeup_shortfall_cfm, pressure_pa, door_force_lbf, door_area_ft2

• An air balance, not a design it does not size the hood, the makeup air unit, or its temperingthe mechanical designer
• The leakage coefficient is crude a measured blower-door result is the only sound sourcea measured air balance
• Distribution matters as much as quantity makeup air delivered across the cooking surface disrupts capturethe hood manufacturer's data