Grease Duct Clearance and Fire Damper Through a Rated Assembly
A 40-seat restaurant installs a Type I hood over a six-burner range and a charbroiler. The hood exhaust is 2,400 cfm; the replacement-air unit supplies 1,800 cfm. The 16-gauge black steel grease duct is not a listed grease duct; it is field-welded. It runs 22 ft horizontally and 18 ft vertically through a second-floor storage area framed with wood joists. To gain headroom, the contractor built a tight enclosure of 2 by 4 framing and 5/8-inch Type X gypsum board around the duct, leaving a 3-inch space, and installed a 1.5-hour fire damper at the floor/ceiling penetration. The contractor cites the board's fire rating and says the damper restores the assembly. The inspector red-tags the installation. The owner wants to keep the duct route because relocating it would require cutting a structural beam.
1. Evaluate the 3-inch clearance and the field-built enclosure under the UMC. What determines whether the clearance can be reduced?
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The 3-inch clearance is not acceptable as installed. The UMC baseline for grease ducts is 18 inches to combustible construction. A reduced clearance is permitted only where the duct is a listed grease duct installed per its listing, or where an approved engineered clearance-reduction assembly is used. A field-built box of 2x4 framing and 5/8-inch Type X gypsum board is not a listed reduced-clearance assembly; the Type X rating addresses fire resistance of the assembly, not the clearance-to-combustible listing for a grease duct. The wood joists remain combustible and are only 3 inches away. The inspector should require either restoration of 18-inch clearance, replacement with a listed grease duct with a tested reduced clearance, or an engineered assembly approved by the AHJ. The field-fabricated enclosure alone does not satisfy the code.
2. Evaluate the fire damper at the rated floor/ceiling penetration. Is any damper permitted in this duct?
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No. Fire dampers are prohibited in grease ducts because grease will accumulate on the damper and its frame, and the damper can become inoperable or feed a fire. The rated floor/ceiling penetration must be protected by an approved method that does not obstruct the grease duct, such as a listed grease-duct firestop assembly or a properly constructed shaft/enclosure, not a 1.5-hour damper. The contractor's argument that the damper "restores" the assembly misunderstands the protection concept: the penetration protection must be listed for the grease duct and the assembly, and the duct itself must remain continuous and liquid-tight. The damper must be removed and the penetration details revised.
3. Identify acceptable paths forward if the beam cannot be cut, and explain what documentation the AHJ should require.
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Acceptable paths include rerouting the duct to maintain 18 inches; using a listed reduced-clearance grease duct and installing it per the listing; building an approved engineered enclosure tested for the application; or obtaining a code modification from the AHJ with supporting engineering. The AHJ should require manufacturer's listings, engineering calculations, firestop details, and inspection of the final assembly. Cutting the beam only for convenience is not an acceptable substitute for compliance; if structural modification is chosen, it needs a structural engineer's design. The AHJ should also confirm that the hood exhaust and replacement air are balanced closely enough to avoid excessive negative pressure in the kitchen, although that is separate from the clearance violation.