Commercial Kitchen Exhaust Ductwork: Grease Ducts and Type I Hoods

Commercial kitchen exhaust ductwork is one of the most regulated and highest-risk duct installations in the building trades. The combination of grease-laden exhaust, high temperatures, and open flames creates a fire hazard that has caused some of the most devastating restaurant and commercial kitchen fires in modern history. NFPA 96 (Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations) exists precisely because improperly installed or maintained grease exhaust duct kills people and destroys buildings. This guide covers what contractors need to know.

Type I vs Type II Hoods: The Critical Distinction

Every commercial kitchen hood falls into one of two categories, and the duct requirements differ dramatically between them:

Type I hoods are installed over cooking equipment that produces grease-laden vapors — fryers, ranges, griddles, broilers, and woks. The exhaust carries a combination of grease particles, combustion products, and heat. Type I duct is specifically designed to contain a grease fire. These are the applications that require grease duct construction per NFPA 96.

Type II hoods are installed over equipment that produces heat, steam, and odors but not grease — dishwashers, steamers, holding ovens, coffee equipment, and some bakery equipment. Type II exhaust does not carry grease and is not subject to NFPA 96 grease duct requirements. Standard HVAC duct construction is typically acceptable for Type II exhaust.

The distinction matters enormously for duct material, gauge, clearance, and maintenance requirements. Misidentifying a Type I application as Type II and building standard ductwork is both a code violation and a fire hazard.

NFPA 96 Grease Duct Requirements

For Type I grease exhaust, NFPA 96 Section 7 specifies:

Why Welded Seams Are Required

Standard HVAC ductwork uses mechanical connections — slip and drive, TDC flanges, Pittsburgh locks — because they are fast to fabricate and assemble. For grease exhaust duct, these mechanical connections are prohibited at duct seams because:

• Mechanical seams create crevices where grease accumulates and carbonizes, which are nearly impossible to clean completely.
• A grease fire inside the duct reaches temperatures of 1,400°F to 1,800°F. At these temperatures, mechanical fasteners can fail and allow sections to separate, spreading the fire.
• The cyclic thermal expansion from repeated heating and cooling loosens mechanical connections over time, creating leak paths for grease vapors to escape into the building structure.

Continuously welded seams eliminate these failure modes. The weld bead must be on the exterior of the duct — the interior must be smooth for cleanability.

Clearances and Fire-Resistant Enclosures

The 18" combustible clearance requirement makes routing grease duct through existing buildings extremely challenging. Many older restaurant renovations require enclosing the duct in a fire-rated shaft or using a listed duct enclosure system that provides reduced clearance to combustibles.

Listed enclosure systems (UL 2221 listed) reduce the required clearance to combustibles from 18" to 3" by providing a tested fire-resistant assembly around the duct. These systems typically consist of a galvanized enclosure with mineral wool fill, tested to the conditions of a grease duct fire. They allow grease duct routing in locations where 18" clearance is not achievable.

Access Panels: The Maintenance Requirement

NFPA 96 requires access panels at every change of direction and at maximum 12-foot intervals along the entire duct run. Access panels allow cleaning crews to reach the interior of the duct during scheduled cleanings. A duct run without adequate access cannot be cleaned to code, which voids the fire suppression system's effectiveness.

Access panels must be of the same material and minimum gauge as the duct, with gasketed closures that maintain the duct's fire rating. Access panel dimensions must be large enough for the cleaning equipment to access the interior — typically 20" × 20" minimum for duct above 20" width.

Commissioning and Annual Inspection

NFPA 96 requires inspection by a qualified inspector after installation and at scheduled intervals based on cooking volume: monthly for high-volume operations like wok cooking or charbroiling, quarterly for moderate-volume cooking, and semi-annually for low-volume operations. The inspection covers grease accumulation, duct integrity, fire suppression system function, and access panel condition.

PMX Ductwork fabricates stainless steel straight sections, elbows, transitions, and transitions in any dimension for commercial kitchen exhaust systems. Stainless steel duct meets NFPA 96's 18-gauge minimum requirement. Configure your kitchen exhaust sections in 18-gauge or heavier stainless with instant pricing.

Key Takeaways for Commercial Kitchen Exhaust Duct

Commercial kitchen exhaust ductwork is among the most heavily regulated ductwork in the building code. Stainless steel is required for grease-laden exhaust, welds must be continuous and grease-tight, clearances from combustibles are non-negotiable, and access panels for cleaning are mandatory. Skipping any of these requirements creates both a code violation and a fire hazard. When in doubt, reference NFPA 96 and consult with your AHJ early in the design process.