11.4 Commercial Exhaust, Kitchen Hoods & Building Ventilation

Commercial Kitchen Exhaust, Makeup Air and Building Ventilation

Air-balance rule: 2018 IMC § 508 requires makeup air from all sources at a rate approximately equal to the building's kitchen exhaust. It does not prescribe a universal 80–90% mechanical-supply ratio or guarantee a fixed kitchen pressure relationship.

---

1. Type I and Type II Hoods

A Type I hood serves cooking equipment that produces grease or smoke. It includes grease filters or listed grease-removal devices and is paired with a grease duct and required fire-suppression system. Fryers, griddles, ranges, broilers, and similar grease-producing appliances normally require Type I protection.

A Type II hood handles heat, steam, vapor, odors, or products of combustion where grease is not produced. Examples can include certain ovens, dishwashers, and steam equipment, subject to the code exceptions and equipment listing.

Classify the process, not the appliance's marketing name. A nominal oven used for grease-laden cooking can require Type I capture, while listed ventless equipment may qualify for a specific exception. The code official and appliance listing control.

Hood geometry must capture the thermal plume. Overhang, mounting height, side panels, cross drafts, replacement-air discharge, and exhaust rate all influence capture and containment. A large exhaust rate does not cure a poorly placed hood and can worsen building pressure.

2. Type I Hood and Grease-Duct Construction

Under the 2018 IMC, Type I hoods are constructed from steel at least 0.043 inch thick or stainless steel at least 0.037 inch thick, with joints and seams made liquidtight as required. Listed factory-built hoods follow their listing and installation instructions.

Field-built grease ducts use carbon steel at least 0.055 inch thick or stainless steel at least 0.044 inch thick—commonly described as No. 16 carbon steel or No. 18 stainless. Joints, seams, and penetrations are liquidtight with continuous external welds unless a listed factory-built grease duct system is used.

A grease duct:

• Serves the Type I exhaust system without prohibited interconnections.
• Provides access for inspection and cleaning at required locations and changes of direction.
• Is installed with the required slope toward the hood or approved grease reservoir.
• Maintains clearance from combustibles, commonly 18 inches for an unprotected field-built duct, unless a listed enclosure, reduced-clearance system, or shaft construction permits otherwise.
• Discharges outdoors at the location and separation required by the code.
• Uses an exhaust fan suited to grease service and arranged for access and cleaning.

Do not conceal a field-welded grease duct before inspection. A pinhole or internal weld that traps grease can fail under normal service and during a fire.

3. Fire Suppression and Controls

Cooking appliances under a Type I hood require the approved automatic fire-extinguishing protection specified by the adopted building and fire codes. Modern wet-chemical systems are listed under UL 300 for the protected appliances, hood, plenum, and duct.

On system actuation, fuel and electrical energy to protected cooking appliances are shut off as required, except energy needed for listed operation of the fire-extinguishing system. Manual actuation is also provided. The exhaust and makeup-air response follows the suppression-system listing, hood design, and adopted code. Do not memorize a blanket statement that makeup air must always stop: some listed arrangements allow supply air to continue when it does not interfere with extinguishment, while exhaust operation is commonly maintained.

After any equipment relocation or appliance change, verify that nozzle type, aim, flow point, detection, and fuel/electrical interlocks still match the listing. Moving a fryer outside a nozzle's protected zone invalidates the arrangement.

4. Makeup Air Under IMC § 508

Commercial kitchen exhaust removes thousands of cubic feet of building air each minute. Without replacement, pressure can make doors difficult to operate, reduce hood capture, increase infiltration, and backdraft natural-draft combustion appliances.

Section 508 requires makeup air from all sources approximately equal to exhaust air for all building exhaust systems. The replacement can come from:

• A dedicated outdoor-air makeup unit.
• Transfer air from adjacent conditioned spaces where transfer is permitted.
• Outdoor air delivered by the building HVAC system.
• Approved gravity openings.
• A combination of these sources.

Mechanical makeup air starts and operates simultaneously with the exhaust system. Its location and discharge pattern must not reduce hood capture or disturb the grease plume. For a hood exhausting 4,000 CFM, begin the building air balance near 4,000 CFM of total replacement from all permitted sources, then account for design exfiltration or infiltration and verify pressure in the completed building.

A design may intentionally supply 80–90% through a dedicated hood unit and obtain the balance as transfer air. That is a project decision, not the universal code formula. The air-balance schedule should identify every exhaust and replacement path.

Section 508.1.1 limits the temperature difference between makeup air and the conditioned space to 10°F unless the added heating or cooling load does not exceed available HVAC capacity. “Tempered” therefore depends on climate and system capacity, not only on a fixed winter discharge temperature.

5. Ventilation Calculations

Occupied buildings need outdoor air for people and building-related contaminants. Under the common ASHRAE 62.1 ventilation-rate procedure, breathing-zone outdoor airflow begins with:

Vbz = Rp × Pz + Ra × Az

where Rp is the people outdoor-air rate, Pz is design population, Ra is the area rate, and Az is zone floor area. Zone air-distribution effectiveness and system ventilation efficiency are then applied as the standard requires. Use the occupancy category and current adopted standard; do not reuse an office rate for a dining room or workshop.

Exhaust air from toilets, janitor rooms, kitchens, and hazardous processes is not automatically suitable for recirculation. Transfer-air direction should move from cleaner to less-clean spaces. Confirm intake separation from exhaust outlets, combustion vents, plumbing vents, loading areas, and other contamination sources.

6. HRVs and ERVs

A heat-recovery ventilator principally transfers sensible heat between outgoing and incoming airstreams. An energy-recovery ventilator transfers sensible heat and some water vapor through an enthalpy wheel or membrane.

An ERV can reduce the incoming latent load in humid weather, but it is not a standalone dehumidifier. Moisture transfer direction depends on vapor-pressure difference, wheel or membrane properties, airflow, leakage, and control sequence. Frost protection, condensate, filtration, cross-contamination limits, purge arrangements, and maintenance access belong in selection.

Recovery equipment must not transfer prohibited grease, smoke, toxic material, or other contaminants into supply air. For each application, verify the code's energy-recovery allowance and the equipment listing.