Restaurant Kitchen Makeup Air and Hood Capture
A new 60-seat restaurant installs a Type I hood over a gas-fired charbroiler and fryer bank. The hood exhaust is rated 3,000 cfm. The design provides 2,400 cfm of makeup air delivered through ceiling diffusers located directly over the cooking line, with the remaining 600 cfm intended to infiltrate through the dining room and entry. During the first week of operation, cooks report smoke rolling out from the hood ends, the kitchen stays under negative pressure relative to the dining room, and the entry doors are hard to open. The owner asks the contractor to simply increase the exhaust fan speed. The contractor proposes raising exhaust to 3,600 cfm without changing makeup air.
1. Evaluate the contractor's proposal to increase exhaust only. Will it solve the capture problem? Explain.
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Increasing exhaust alone will likely make the problem worse, not better. The root cause is an imbalance between exhaust and makeup air, compounded by makeup air placement.
Reasoning:
- Pressure balance: If exhaust is 3,000 cfm and makeup is 2,400 cfm plus unreliable infiltration, the kitchen is already negative. Raising exhaust to 3,600 cfm with makeup unchanged increases the negative pressure, increasing the pressure difference across the hood and drawing more air from the dining room and entry.
- Door force and infiltration: Greater negative pressure increases door-opening force and pulls unconditioned air (and odors) through the dining room. Infiltration is not a reliable or controllable makeup air source.
- Makeup air location: Supply diffusers directly over the cooking line can disturb the hood's capture and short-circuit into the exhaust, reducing capture effectiveness. Makeup air should be introduced so it does not blow across the cooking surface or into the hood capture zone.
- Correct approach: Balance exhaust and makeup air so the kitchen is slightly negative relative to the dining room but not so negative that doors are hard to open. Provide makeup air close to the exhaust volume, and reposition supply so it does not disrupt capture. Then verify capture with a smoke test.
2. What should the contractor verify about the hood and exhaust system beyond airflow volume before concluding the design is adequate?
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Airflow volume is only one variable. The contractor should verify:
- Hood capture and overhang: The hood must extend beyond the cooking equipment edges by the required amount and be at the required mounting height above the cooking surface. Insufficient overhang or excessive height allows smoke to escape at the ends.
- Hood type and listing: Confirm the hood is listed for the cooking appliances served (Type I for grease-laden vapor) and that the exhaust rate matches the listing or the design method used.
- Duct and fan: Verify the exhaust duct is sized for the airflow, the fan is rated for grease-laden air, and the duct route and cleanouts comply.
- Grease filters: Confirm filters are installed, properly sized, and listed, since missing or misinstalled filters change capture and create fire hazards.
- Fire suppression interface: Confirm the hood suppression system interlocks with fuel and that the exhaust fan operates as designed during cooking.
- Makeup air delivery: Verify makeup air quantity and location so it does not interfere with capture.
Only after these are verified should airflow be adjusted, and any change should be documented and re-tested with a smoke test.