Kitchen Makeup Air Shortfall Produces Hood Spill and Flue Backdraft
A two-storey restaurant has a 12-ft island hood rated at 200 cfm per linear foot (2,400 cfm) and a 6-ft wall hood at 300 cfm/ft (1,800 cfm), both ducted to rooftop upblast fans, 4,200 cfm total. The makeup air unit is a dedicated rooftop unit with a 3,000 cfm direct-drive fan, no heating coil, and four 2 × 2 ft ceiling diffusers over the cook line that blow horizontally across the island hood face. The building has an atmospheric gas water heater and a Category I boiler, about 600 MBH combined, in a mechanical room with a single 12 in. × 12 in. combustion air louvre. Complaints: smoke rolls out of the island hood during the dinner rush, the front door is hard to pull open, the water heater flue spills (combustion analyser shows CO rising in the kitchen), and the kitchen sits at 58 °F in January. The building is 8,000 ft² with envelope leakage of about 0.3 cfm/ft² at 50 Pa.
1. Quantify the makeup air deficit and explain how the same deficit produces both the hood capture failure and the flue spill.
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Deficit = 4,200 − 3,000 = 1,200 cfm, 29% short of the exhaust.
Hood capture: the island hood needs 2,400 cfm across a 12 × 4 ft face, only 50 fpm face velocity - already marginal for an island hood. The MUA diffusers each pass 750 cfm through a 4 ft² face, about 187 fpm face velocity, and at 10 ft still deliver roughly 200 fpm of horizontal cross-draft (free-jet decay, Vx ≈ 6.3 V0 √A0 / x, gives about 6.3 × 187 × 2/10 ≈ 236 fpm). Capture is destroyed by cross-draft long before the exhaust volume is the issue, which is why smoke rolls out even with both fans running.
Flue spill: the missing 1,200 cfm is drawn through the envelope and any open flue. With leakage of 0.3 cfm/ft² at 50 Pa the building's leakage coefficient is about 186 cfm at 1 in. w.c.; passing 1,200 cfm needs roughly (1,200/186)^1.54 ≈ 17 Pa ≈ 0.07 in. w.c. of depression. That is more than enough to overcome the draft of an atmospheric water heater, especially with a 12 × 12 louvre providing only about 72 in² of free area against the roughly 150 in² per opening that gravity combustion air requires for 600 MBH of Category I input. The flue becomes the path of least resistance.
2. Specify the makeup air quantity, temperature, distribution and interlocks, with numbers.
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Quantity: deliver 80–90% of the exhaust, 3,400–3,800 cfm, and let the remaining 400–800 cfm transfer from the dining area. Delivering 100% at the hood would remove the kitchen's negative pressure relative to the dining room and push cooking odour into the front of house; the kitchen should stay 0.02–0.05 in. w.c. negative to the dining room, and the dining room slightly positive to outdoors.
Temperature: 3,600 cfm of 30 °F outdoor air with no heat explains the 58 °F kitchen. A heating coil sized to deliver 65–70 °F at design adds about 1.08 × 3,600 × 40 ≈ 155,000 Btu/h. That belongs in the project scope, not in a later retrofit.
Distribution: deliver at the perimeter and at ceiling level away from the hood, at low discharge velocity, keeping cross-draft at the hood face below 50 fpm - ideally below 25 fpm. Diffusers aimed across the hood face must be re-aimed, replaced with low-velocity units, or relocated at least 6–8 ft from the hood.
Interlocks: prove the hood exhaust fans with airflow switches and the MUA fan with a current sensor, and fail the exhaust if makeup air is lost - the hood must not run in a depressurized building. Add a powered combustion air damper interlocked with the exhaust fans for the water heater and boiler.
3. What verification and life-safety steps would you require before handing the kitchen back?
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- Prove the exhaust and MUA fans with airflow switches and current sensors, and confirm the interlock actually shuts the hoods down on MUA failure - test it, do not read the sequence of operations.
- Commission the combustion air path: size and install the powered damper for 600 MBH of Category I input, independent of the MUA, and re-test both appliances with a combustion analyser. CO and draft should return to the manufacturers' range; if the water heater still spills, it gets a sealed-combustion or power-vented replacement rather than another louvre.
- Install CO alarms in the kitchen and mechanical room, and treat the existing alarm history as a reportable event.
- Measure pressure differentials, not just cfm: kitchen to dining, dining to outdoors, mechanical room to outdoors. Adjust the balance until the cascade is right at design exhaust with the doors closed.
- Smoke-test hood capture at the worst case - all fans on, MUA at design, doors closed, at the dinner rush - and re-check face velocities after balancing, because correcting the MUA flow will change every diffuser's throw and may require re-aiming or replacing the outlets.