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:

Worked Example: Sizing a Grease Duct for Velocity

The exhaust airflow comes from the hood listing or the mechanical design, not from the duct. The duct only has to carry that airflow at an acceptable velocity: Velocity (FPM) = CFM ÷ duct area (sq. ft.). The floor is the 500 FPM minimum. A common design range is roughly 1,500 to 1,800 FPM at full flow, which keeps the duct compact and leaves room to slow the fan down without falling under the minimum.

Take a hood exhausting 2,400 CFM with an 1,800 FPM target. Required area = 2,400 ÷ 1,800 = 1.33 sq. ft., or 192 sq. in. A 16 × 12 duct is exactly 192 sq. in., so velocity is 1,800 FPM. If a demand-control system turns the fan down to half flow, 1,200 ÷ 1.33 = 900 FPM, still above 500. The largest duct that would hold 500 FPM at full flow is 2,400 ÷ 500 = 4.8 sq. ft.

The slope works the same way. At the 1/4" per foot minimum, a 20-foot horizontal run has to rise 20 × 0.25 = 5 inches from the hood or drain end to the far end. Check that the ceiling space has room for that rise before the duct is fabricated, because a welded run cannot be re-pitched in the field.

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. Grease-laden exhaust requires 16-gauge carbon steel or 18-gauge stainless steel duct, 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.

Frequently Asked Questions

What is the difference between a Type I and a Type II kitchen hood?

A Type I hood serves cooking equipment that produces grease-laden vapors, such as fryers, ranges, griddles, broilers and woks, and its duct must be built as welded grease duct under NFPA 96. A Type II hood serves equipment that produces only heat, steam and odors, such as dishwashers and steamers, and can typically use standard HVAC duct construction.

What gauge steel is required for a commercial kitchen grease duct?

NFPA 96 requires Type I grease duct to be at least 16-gauge black carbon steel or 18-gauge stainless steel. All seams and joints must be continuous, liquid-tight external welds with no mechanical fasteners through the duct wall, so a grease fire stays contained and the interior stays smooth enough to clean.

How much clearance does a grease duct need from combustible construction?

A grease duct needs at least 18 inches of clearance to any combustible construction. A listed duct enclosure system tested to UL 2221 reduces the required clearance to 3 inches, which is how grease duct is usually routed through existing buildings where 18 inches is not available. The alternative is enclosing the duct in a fire-rated shaft.

Where are access panels required on a grease exhaust duct?

Access panels are required at every change of direction and at intervals of no more than 12 feet along the grease duct run. Each panel must be the same material and minimum gauge as the duct, with a gasketed closure, and large enough for cleaning equipment to reach the interior, typically 20 by 20 inches on duct wider than 20 inches.