Type I Hood & Exhaust
Grease-Laden Ventilation per NFPA 96

Type I vs. Type II: The Critical Distinction

The International Mechanical Code and NFPA 96 classify commercial kitchen hoods into two categories based on the type of cooking they ventilate:

Type I hoods are required over equipment that produces grease-laden vapors. This includes fryers, griddles, char-broilers, ranges, woks, convection ovens used for roasting, rotisseries, and any other appliance that heats cooking oil or animal fats to the point of aerosolization. Type I hoods are subject to the full scope of NFPA 96, including the requirements for fire suppression systems, grease filters, duct construction, and cleaning schedules NFPA 96, §4.1.1.

Type II hoods serve equipment that produces heat, steam, or moisture but not grease — dishwashers, steam tables, coffee brewers, and ovens used exclusively for baking. Type II hoods are governed by the IMC but are exempt from NFPA 96 requirements because they do not accumulate combustible grease deposits.

The distinction matters enormously for fire safety: a Type I hood must have a listed fire suppression system, grease-rated ductwork, and a cleaning schedule. Misclassifying a grease-producing appliance under a Type II hood is a code violation that leaves the kitchen unprotected.

Type I Hood Construction

Type I hoods must be constructed of materials that can withstand the heat and corrosive effects of grease-laden cooking vapors:

Material: Minimum 18-gauge steel (or 20-gauge stainless steel). Stainless steel is standard for commercial kitchens because it resists corrosion from cleaning chemicals and is easier to maintain NFPA 96, §4.2.

Overhang: The hood must extend beyond the cooking equipment on all open sides by a minimum of 6 inches (wall-mounted canopy) or overhang at least 12 inches on island-style installations. This overhang captures rising grease vapors before they escape into the kitchen IMC §507.7.

Height: The bottom of the hood is typically mounted 6.5 to 7 feet above the floor, or no more than 4 feet above the cooking surface for optimal capture efficiency. Mounting too high reduces capture performance and allows grease to escape.

Grease gutters: The hood includes internal gutters (drip edges) that direct captured grease toward a collection trough or drip cup. These must be cleaned regularly to prevent overflow onto the cooking line.

Listed vs. unlisted: Hoods listed to UL 710 have been tested for fire containment and exhaust performance. Many AHJs require UL 710 listing for new installations. UL 710B covers recirculating hoods, which filter and return air to the kitchen instead of exhausting it outside.

Grease Filters

Grease filters are the first line of defense against grease accumulation in the duct system. They are mounted at the hood opening and capture grease particles before they enter the exhaust duct:

Baffle filters: The most common type. Interlocking metal baffles force the exhaust air through a series of turns that cause grease droplets to impact the baffle surfaces and drain into the grease gutter. Baffle filters are listed to UL 1046 and must be arranged to provide complete coverage of the hood opening NFPA 96, §6.2.

Angle and orientation: Filters must be installed at a minimum 45-degree angle to facilitate drainage. Horizontal installation traps grease in the filter media and creates a fire hazard.

Cleaning: Grease filters must be cleaned on a regular schedule — typically daily or weekly depending on cooking volume. Filters should be removed, soaked in a degreasing solution, and thoroughly dried before reinstallation. Running a kitchen without filters or with missing filter panels is a code violation.

Exhaust Duct Construction

The exhaust duct connecting the hood to the exterior is the most critical fire-safety element of the ventilation system. Grease accumulates on duct surfaces over time, and a duct fire can spread rapidly through the building if the duct is not properly constructed:

Material: Exhaust ducts for Type I hoods must be constructed of minimum 16-gauge carbon steel or 18-gauge stainless steel. Galvanized steel is not permitted because the zinc coating breaks down at high temperatures NFPA 96, §7.1.

Welded seams: All longitudinal and transverse seams must be continuously welded, liquid-tight, to prevent grease leakage into the building structure. Screwed or riveted connections are prohibited because they create leak points and snag grease during cleaning.

Clearance to combustibles: Exhaust ducts must maintain a minimum 18-inch clearance to combustible building materials, or be enclosed in a fire-rated shaft. Reduced clearances are permitted with listed wrap or enclosure systems NFPA 96, §7.3.

Slope: Horizontal duct runs must slope a minimum of 1/4 inch per foot back toward the hood to prevent grease pooling. A flat or sagging duct accumulates grease faster and is harder to clean.

Access panels: Ducts must have access panels at every change in direction and at intervals not exceeding 12 feet on straight runs, to allow cleaning and inspection of the interior surfaces NFPA 96, §7.4.

Makeup Air

A commercial kitchen exhaust system removes large volumes of air from the building — often 1,500 to 5,000 CFM or more. This air must be replaced, or the kitchen will develop negative pressure that impedes exhaust performance, pulls conditioned air from the dining room, causes doors to slam or become difficult to open, and may even pull combustion gases back through gas appliance flues.

Makeup air units (MAU): Dedicated HVAC units that supply tempered outside air to the kitchen to replace exhausted air. The IMC requires makeup air for exhaust systems exceeding 400 CFM IMC §508.1.

Balance: The makeup air volume should be approximately 80–90% of the exhaust volume, maintaining a slight negative pressure that prevents kitchen odors from migrating to the dining area.

Short-circuit prevention: Makeup air supply diffusers must be positioned so that replacement air does not short-circuit directly into the hood, bypassing the cooking equipment. This would reduce capture efficiency and waste energy.

Fire Suppression Integration

Every Type I hood must be protected by a listed fire suppression system — typically a wet chemical system per NFPA 17A. The suppression system nozzles are mounted inside the hood plenum, aimed at each appliance, and connected to the duct plenum. Upon activation, the system discharges agent onto the cooking surfaces and into the duct entry simultaneously NFPA 96, §10.1.

The suppression system must also shut off the gas supply and the exhaust fan (or close the fire damper at the fan) to prevent agent from being drawn out of the hood before it can extinguish the fire.

Common Deficiencies

Missing or dirty filters: Operating without filters or with heavily grease-laden filters accelerates duct contamination and increases fire risk.

Grease dripping from ductwork: Visible grease at seams or access panels indicates either a cleaning failure or a construction defect (non-welded seams).

Inadequate clearance: Ductwork passing through combustible construction without the required 18-inch clearance or rated enclosure is a serious deficiency.

Equipment added without hood extension: A new fryer placed beyond the hood overhang is not captured by the exhaust or suppression system.

Makeup air disconnected or undersized: A kitchen with inadequate makeup air will exhibit poor capture, grease migration, and door pressure problems.

Practical Inspection Tips

Stand at the cooking line: Hold a smoke pencil or tissue at the hood edge. If the capture is working, the smoke or tissue should be drawn upward into the hood. If it drifts sideways, the exhaust is insufficient or the makeup air is overwhelming the hood.

Check the filters: Pull one filter and inspect the grease loading. If grease is dripping or the baffles are caked, the cleaning frequency is insufficient.

Look at the duct penetrations: Where the duct passes through walls or the roof, verify the fire-rated enclosure or clearance requirement is met. This is frequently compromised during roof repairs or building modifications.

Verify the fan: The rooftop exhaust fan should be an upblast design with a grease containment curb. Confirm the fan is running and the hinge mechanism allows the unit to be tipped for cleaning.

References