7.2 Type I Exhaust Ducts, Makeup Air & Fire Suppression

501.2 still says commercial kitchen exhaust is built to Sections 506 through 509, and Type I exhaust shall be independent of all other exhaust systems except 506.3.5. That independence sentence is an M2 staple. What changed in 2024 is the map: kitchen fire suppression now lives at 507.2.11. Section 509 is Hazardous Exhaust Systems — labs, incompatible materials, explosion relief — taught in hazardous exhaust. An answer key that still says “IMC 509 = hood suppression” is a 2021 memory, not a 2024 book.

Grease duct materials, joints, fasteners, test

506.3.1: grease ducts shall be steel not less than 0.0575 inch (1.463 mm) (No. 16 gage) or stainless steel not less than 0.0450 inch (1.14 mm) (No. 18 gage). Exception: factory-built grease ducts listed and labeled to UL 1978 and installed per 304.1.

506.3.2: joints, seams, and penetrations shall be a continuous liquid-tight weld or braze on the external surface. Exceptions: listed sealing devices at penetrations; internal welds that are formed or ground smooth and accessible; UL 1978 factory ducts. 506.3.2.1 limits joints to butt, welded flange not more than ½ inch deep, or overlapping telescoping/bell joints that drain (overlap difference not more than ¼ inch, overlap length not more than 2 inches). 506.3.3: bracing is noncombustible and sized for gravity and seismic loads. Bolts, screws, rivets and other mechanical fasteners shall not penetrate grease duct walls. A self-tapper “just to hold the wrap” or a screw-through cleanout strap is a 506.3.3 fail.

506.3.2.5 requires a field leakage test before use or concealment. Concealment includes shafts and wraps that hide all sides. Test the entire system, including the hood-to-duct joint; sections are allowed if every joint is tested. UL 1978 factory welds are exempt; field joints are not. Light test (506.3.2.5.1): a lamp of not less than 1,600 lumens, open so it emits equally perpendicular to the walls; pass = no light visible on the exterior. Water-spray test (506.3.2.5.2): a pump of not less than 1,200 psi simulating cleaning; pass = no cleaning water on the exterior.

506.3.4: design velocity in the grease duct shall be not less than 500 feet per minute, except transitions not more than 3 feet long that connect mismatched hood or fan openings and that do not trap grease.

Slope, reservoirs, cleanouts

506.3.7: construct the system so grease cannot collect. Slope not less than ¼ unit vertical in 12 units horizontal (2 percent) toward the hood or toward a 506.3.7.1 reservoir. Where a horizontal grease duct exceeds 75 feet, slope not less than 1 unit vertical in 12 units horizontal (8.3 percent). Factory-built ducts follow the listing. A 90-foot flat run “because the steel guy ran out of rise” fails 506.3.7.

506.3.7.1 reservoirs (when you do not drain back to the hood): built as required for the duct; on the bottom of the horizontal run or the bottommost riser section; full width and not less than 12 inches long and 1 inch deep; bottom slopes to a drain; cleanout on the side or top that reaches the reservoir.

506.3.8 / 506.3.9 cleanouts: no openings except those required for operation and maintenance. Inaccessible sections: cleanouts not more than 20 feet apart and not more than 10 feet from changes in direction greater than 45 degrees. Doors are steel at least as thick as the duct, liquid-tight, no fasteners that penetrate the duct, gaskets rated not less than 1,500°F. Horizontal cleanouts are those same 20-foot / 10-foot numbers; bottom openings only when nothing else works, with internal damming so grease does not pool; not closer than 1 inch to a duct edge; opening not less than 12 inches by 12 inches (or 12 inches on one side if that rectangle will not fit); at reservoirs; and within 3 feet of a horizontal discharge fan. 506.3.12: enclosure access at each cleanout, labeled “ACCESS PANEL. DO NOT OBSTRUCT.”

Enclosure, independence, fans, termination

506.3.6 (no enclosure required): Type I grease ducts and exhaust equipment 18 inches to combustibles and 3 inches to noncombustible construction and gypsum wallboard on noncombustible structure. Exceptions: UL 1978 ducts, listed exhaust equipment per 304.1, and ducts continuously covered on all sides with a listed ASTM E2336 field-applied enclosure (clearance per that listing).

506.3.11: a Type I grease duct that penetrates a ceiling, wall, floor, or any concealed space shall be enclosed from the penetration to the outlet terminal. In-line fans that are not outdoors are enclosed as required for the duct. The enclosure serves that single grease duct and shall not contain other ducts, piping, or wiring. Three legal enclosures: IBC shaft (506.3.11.1), ASTM E2336 field wrap continuously on all sides (506.3.11.2), or UL 2221 factory-built enclosure assembly (506.3.11.3). Rating: not less than the assembly penetrated and not less than 1 hour. Through-penetrations use ASTM E814 or UL 1479 firestops with F and T ratings equal to the assembly. Fire dampers and smoke dampers shall not be installed in grease ducts. Exception: no enclosure if the duct penetrates only a nonfire-resistance-rated roof/ceiling. Partial wrap used only to “buy” clearance at one stud bay fails 506.3.11.2. Field fail: a welded riser disappearing into a combustible attic with no shaft, no E2336 wrap, and no listing.

506.3.5: a separate grease duct for each Type I hood, unless all of these are true: interconnected hoods on the same story; in the same room or adjoining rooms; interconnecting ducts do not penetrate fire-resistance-rated assemblies; and the system does not serve solid-fuel appliances. 507.2.2 adds that extra-heavy-duty (solid-fuel) Type I hoods shall not cover heavy-, medium-, or light-duty appliances and shall discharge to an exhaust system independent of other exhaust systems. You cannot manifold a wood grill into the fryer riser.

506.5.1: Type I fan housings are built as grease ducts, except fans listed to UL 705. 506.5.1.1: the motor is outside the exhaust airstream. 506.5.3: discharge shall not impinge on the roof, equipment, or the structure; a vertical-discharge fan needs an approved drain at the lowest point of the housing. 506.5.4: upblast fans serving Type I hoods shall be hinged, have a flexible weatherproof electrical cable, and have a restraint that limits swing; the grease duct shall extend not less than 18 inches above the roof. 506.5.5: exhaust equipment 18 inches to combustibles unless listed for less.

506.3.13 termination: above the roof, the discharge opening is not less than 40 inches above the roof surface. Through an exterior wall only where smoke, grease, and odor are not a public nuisance or fire hazard, not where the IBC requires protected openings, not within 3 feet of any opening in that wall. Location: not less than 10 feet horizontally from the same or contiguous buildings, adjacent buildings, and adjacent property lines; not less than 10 feet above adjoining grade; not less than 10 feet horizontally from or 3 feet above air intakes. Exception: 5 feet horizontally from those targets where the outlet discharges away from them. 506.5.6 Exception: a vertical-discharge fan may be 2 feet from a parapet if the parapet is not higher than the top of the fan discharge opening.

Makeup air — 508, not “whatever the RTU has left”

508.1: makeup air shall be supplied while commercial kitchen exhaust serving commercial cooking appliances operates. The amount of makeup air from all sources shall be approximately equal to the exhaust air for all exhaust systems in the building. Makeup air shall not reduce the effectiveness of the exhaust system. Gravity, mechanical, or both are allowed. Mechanical makeup air shall start and operate simultaneously with the exhaust. Intakes follow 401.4. A supply grille that blows across the hood lip and knocks the plume into the dining room fails 508.1 even if the cfm schedule balances on paper.

508.1.1: kitchen HVAC shall handle the latent and sensible load of the makeup air, or dedicated makeup shall be conditioned so the difference between makeup temperature and the kitchen setpoint is not greater than 10°F. Exception: makeup supplied to a compensating hood need not be conditioned. 508.1.2: makeup ducts connecting to or within 18 inches of a Type I hood follow the Chapter 6 metallic-duct rules listed there; insulation in that 18-inch zone is noncombustible or listed. 508.1.3: the drawings shall show a design outdoor-air balance: all exhaust, all replacement, and net exfiltration; replacement equals exhaust plus net exfiltration. 508.2: compensating hoods are labeled with the minimum exhaust and/or maximum makeup that still captures.

507.1.1 interlock: the Type I exhaust fan shall start when any appliance that needs that hood is turned on, or the appliances shall be locked out when the fan is off. A heat or radiant sensor shall start the fan not more than 15 minutes after the first appliance is on. The interlock shall not kill standing pilots and shall not depend on any component of the fire-extinguishing system. 507.1.7 / 507.1.7.1: before final approval, prove 507.2.10 or 507.3.4 exhaust, 508 makeup, and capture with all appliances hot and all makeup and recirculated air running — visual smoke or steam.

Fire suppression — 507.2.11, UL 300, shutoff

507.2.11: a Type I hood shall have an approved automatic fire suppression system complying with IBC Section 904.13 and the International Fire Code. (2024 IBC/IFC renumbered commercial cooking systems to 904.14; the IMC pointer still says 904.13. Open the IBC/IFC cooking section, not 2024 IMC 509.) IBC/IFC 904.14: the system shall be recognized for the equipment and exhaust arrangement protected. Preengineered automatic dry- and wet-chemical systems shall be tested to UL 300 and listed for the application. Other types shall be listed for commercial cooking. Install per the IBC/IFC, NFPA 96, the listing, and the manufacturer. Recognized installations include NFPA 12 (CO₂), 13 (sprinkler), 750 (water mist), 11 (foam-water), 17 (dry chemical), and 17A (wet chemical). Exception: UL 710B recirculating systems installed per IMC 304.1.

904.14.2 System interconnection: actuation shall automatically shut down the fuel or electrical power supply to the cooking equipment. The fuel and electrical reset shall be manual. A technician who “jumps the microswitch so the line can finish brunch” has disabled a required interlock. 904.14.1: a manual pull at or near a means of egress from the cooking area, not less than 10 feet and not more than 20 feet from the kitchen exhaust system, 42 to 48 inches above the floor, clearly identifying the hazard. (Sprinklered cooking lines are excepted from the manual pull; a relocated pull needs the fire-code official.)

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