Dampers in dryer, grease and hazardous exhaust ducts

Code conflict: building official approval neededAll three editions

The building code would require a damper here. IMC 504.2 lets a dryer duct pass through a rated assembly when the duct is galvanized steel or aluminum of the IMC 603.4 thickness and the assembly keeps its rating, but it prohibits dampers in the duct, so the two codes conflict. Reroute the duct if you can; otherwise build the duct to IMC 504.2, protect the penetration so the assembly keeps its rating, and confirm the protection with the building official.

Fire damper: prohibitedAll three editions

Fire damper: prohibited. Dampers are prohibited in dryer exhaust ducts, so use the code alternative: protect the duct as required by IBC 714.5. IBC 717.6.1, IBC 714.5, IMC 504.2

Full result in the tool: Dampers prohibited in this duct

Not permittedAll three editions

IBC 717.5.1 requires a fire damper at every duct through a fire wall, and the mechanical code prohibits fire dampers in clothes dryer exhaust. There is no code-prescribed way to take a dryer duct through a fire wall. Reroute the duct, or obtain approval of an alternative from the building official.

Fire damper: prohibited · Smoke damper: prohibitedAll three editions

Fire damper: prohibited. The mechanical code prohibits fire and combination fire/smoke dampers in this exhaust duct, and the building code waives them here. IBC 717.5.3 Exc. 5, IMC 504.2, IMC 504.11 (2021 and 2024), IMC 504.10 (2018)

Smoke damper: prohibited. Dampers that would block the exhaust flow are prohibited in this duct. IMC 504.2, IMC 504.11 (2021 and 2024), IMC 504.10 (2018)

Also: Protect the shaft penetrations with the subduct method: steel subducts at least 0.0187 inch thick extending 22 inches up inside the shaft, and an exhaust fan at the top of the shaft running continuously on power that meets IBC 909.11. IMC 607.5.5 Exc. 2, IMC 504.2, IMC 504.11 (2021 and 2024), IMC 504.10 (2018)

Full result in the tool: Dampers prohibited in this duct

Fire damper: prohibited · Smoke damper: prohibitedAll three editions

Fire damper: prohibited. The mechanical code prohibits fire and combination fire/smoke dampers in this exhaust duct, and the building code waives them here. IBC 717.5.3 Exc. 5, IMC 505.5

Smoke damper: prohibited. Dampers that would block the exhaust flow are prohibited in this duct. IMC 505.5

Also: Protect the shaft penetrations with the subduct method: steel subducts at least 0.0187 inch thick extending 22 inches up inside the shaft, and an exhaust fan at the top of the shaft running continuously on power that meets IBC 909.11. IMC 607.5.5 Exc. 2, IMC 505.5

Full result in the tool: Dampers prohibited in this duct

Duct enclosure required, no dampersAll three editions

A commercial kitchen grease duct that penetrates a ceiling, wall or floor must be enclosed all the way from where it penetrates to where it discharges, in a rated shaft, a field-applied enclosure or a factory-built enclosure. The enclosure is rated at least as high as the assembly penetrated and at least 1 hour, and serves only that grease duct. Fire and smoke dampers must not be installed in grease ducts.

Not permittedAll three editions

IBC 717.5.1 requires a fire damper at every duct through a fire wall, and the mechanical code prohibits dampers in grease ducts. There is no code-prescribed way to take a grease duct through a fire wall. Reroute the duct, or obtain approval of an alternative from the building official.

Duct enclosure required, no dampersAll three editions

Hazardous exhaust ducts that penetrate a fire-resistance-rated wall must be enclosed in fire-resistance-rated construction, built like a shaft, all the way from where it penetrates to where it discharges, unless the inside of the duct has an approved automatic fire suppression system. The enclosure must be rated at least as high as the highest-rated wall penetrated. Hazardous exhaust ducts may not have fire or smoke dampers.

Shaft enclosure requiredAll three editions

Hazardous exhaust ducts that penetrate a floor/ceiling assembly must be enclosed in a fire-resistance-rated shaft, whether or not the floor is rated. The duct may not have fire or smoke dampers.

Fire damper: prohibited · Smoke damper: prohibitedAll three editions

Fire damper: prohibited. The mechanical code does not allow fire dampers in hazardous exhaust ducts. IMC 509.6.1 (2024), IMC 510.6.1 (2021), IMC 510.7.1 (2018)

Smoke damper: prohibited. The mechanical code does not allow smoke dampers in hazardous exhaust ducts. IMC 509.6.1 (2024), IMC 510.6.1 (2021), IMC 510.7.1 (2018)

Also: Protect where the duct penetrates the shaft wall as a through-penetration under IBC 714.4.1 (tested assembly or listed firestop system). IMC 509.6.1.1 (2024), IBC 714.4.1, IMC 510.6.1.1 (2021), IMC 510.7.1.1 (2018)

Full result in the tool: Dampers prohibited in this duct