Ductwork Through Fire-Rated Assemblies

Why this matters

A duct is a hole in a rated assembly that carries air on purpose, and air is exactly what a fire separation exists to stop. In a single-family house the place this bites is the wall and ceiling between the garage and the house, where a length of flex duct or a return grille someone cut into the furnace closet quietly connects the living space to the room where cars idle and gasoline is stored. In apartments and condos it bites at dampers: a residential tech replacing equipment in a unit is often the only person who touches that ductwork for a decade, and the one change that defeats a damper takes ten minutes. Neither failure shows until the day it matters.

The one question every duct change gets

Before any duct is added, rerouted, replaced or extended, ask: does this duct cross a rated line, and does my change touch whatever protects the crossing? The rated line is the garage separation in a house, and in multifamily work it is any rated wall, shaft, corridor or floor-ceiling the duct passes through or opens into.

The rest of this card is that question run against two jobs that resolve in opposite directions: one where the crossing was already broken and the tech's job is to restore it, and one where it is intact and the tech's job is to leave it alone and keep it reachable.

The garage separation: what the IRC asks of a duct

The separation itself, what it is made of and where it runs, belongs to its own card; The Garage-to-House Fire Separation. For ducts, three rules carry most of the weight:

• Material and openings. IRC R302.5.2 (2021 edition, as adopted) requires ducts in the garage, and ducts penetrating the walls or ceilings that separate the dwelling from the garage, to be at least No. 26 gage sheet steel or another approved material, with no openings into the garage. Flex duct is not 26 gage steel. A supply register in the garage fed from the house system is an opening.
• No return air from the garage. IRC M1602.2 (2021 edition) lists the garage among the spaces return air may not be taken from. A furnace in the garage with a grille cut into its closet, or a return duct with a leaking joint in the garage, pulls garage air, including carbon monoxide and fuel vapor, into the house supply.
• The penetration itself is still protected. The gap where the steel duct passes through the gypsum is filled as R302.5.3 and R302.11 item 4 describe, with an approved material that resists the passage of flame and combustion products.

The IRC does not call for a fire damper at the garage separation. The protection is the steel, the absence of openings and the sealed gap, which is why a flex connector or a cut-in grille removes it completely.

Multifamily: the dampers you will find

In buildings under the IBC and IMC, rated walls, shafts and floor-ceilings that ducts cross are commonly protected by dampers, and a residential tech meets four kinds. Their differences are their own card (see related: Why a Fire Damper and a Smoke Damper Are Different Devices); one clause each here:

• a fire damper, closed by heat at a fusible link, in a rated wall or shaft;
• a smoke damper, closed by a signal from detection or the fire alarm;
• a combination fire/smoke damper, both in one device;
• a ceiling radiation damper, a listed device at a register or diffuser in the ceiling membrane of a rated floor-ceiling or roof-ceiling, so the register's opening does not let heat into the joist space.

What a residential tech must not do

These are the changes that defeat a damper while the system keeps working, which is why they persist:

• Remove, wire open or screw open a fire damper's blades or curtain, or remove its fusible link, to cure noise or a pressure drop.
• Remove the damper to gain airflow when a new air handler comes up short.
• Move a register off its ceiling radiation damper, or connect a new flex run to a ceiling opening that bypasses the damper above it.
• Cut a new branch tap between a damper and the barrier, so air finds a route through the rated line that the damper does not close.
• Pull a damper out of the plane of the wall. A listed damper is installed within the wall or floor with its sleeve and retaining angles per its listing; a damper hung in the duct a foot from the wall protects nothing at the wall.
• Disconnect or reroute a smoke damper's actuator or its control wiring. That belongs to whoever maintains the fire alarm interface.
• Bury the access door behind new drywall, a larger cabinet or a bigger air handler.

Each of these is someone else's assembly. Where one is unavoidable, the work stops and the owner of the assembly decides; see related: How to Coordinate a Penetration with the Trade That Owns the Assembly.

Why inspection access is your problem too

Fire and smoke dampers must be reachable. The mechanical code as adopted (IMC 607.4 in recent editions) requires an access door sized for inspection and maintenance, and many jurisdictions require it labeled. NFPA 80 (fire dampers) and NFPA 105 (smoke dampers), in the editions your fire code adopts, call for operational testing within one year after installation and then at a regular interval, every four years in most occupancies and every six years in hospitals. That duty binds the building owner through the adopted fire code, not you.

Your part is narrower and real: do not be the reason the next test cannot happen. A replacement that blocks the access door converts a four-year inspection into a demolition job, and the building owner finds out when the inspector writes up a damper nobody can reach.

Hazard at any damper: a fire damper with an intact link is a spring-loaded curtain held open by a piece of solder. Keep hands and tools out of the blade path whenever you reach through an access door, and never test a link by heating it.

Case A: the crossing was already broken

An HVAC crew is replacing a gas furnace in the attached garage of a 1999 house. A bonus room sits over the garage, so the garage ceiling is 5/8 in Type X under habitable space. Running the gate:

• Does a duct cross the rated line? Yes, twice. A supply branch to the bonus room rises through the garage ceiling, and the return comes in from the house through the common wall.
• Does the existing work protect the crossings? No. The supply branch is flex duct for its whole run, including the section through the garage ceiling. And a prior installer cut a 14 by 6 in return grille into the side of the furnace closet facing the garage, presumably to quiet a starved return.
• What is restored: the flex through the ceiling is replaced by 26 gage steel duct from inside the garage through the ceiling and into the bonus room joist space, with the gap between the steel and the gypsum filled with an approved material. The grille is removed, the opening patched to match the closet wall, and the return path is made adequate from the house side instead. Every joint in the return duct and the furnace cabinet's return side that sits in the garage is sealed, because any return leak in the garage is a garage-air intake.

Why the grille mattered more than it looked: that grille sat on the negative side of the blower. With the blower running, the closet was below garage pressure, so it drew air from the garage directly into the supply going to every room, including the bedroom over the garage.

Hazard specific to this job: nobody runs a vehicle in the garage while the crew works, the furnace is fired only after the return is sealed and the cabinet is closed, and a CO reading in the bonus room and at the supply register nearest the garage is taken with the new unit running, with the CO monitor on the tech's belt the whole time the appliance is firing.

Case B: the crossing is intact, so the job is to stay out of it

A tech replaces the air handler in a closet inside a top-floor unit of a four-story condominium building. The return duct leaves the closet and passes into a rated corridor wall, where a fire damper sits in the plane of the wall with an access door on the closet side. The living room ceiling is a rated roof-ceiling assembly in this building, and its supply registers sit on ceiling radiation dampers.

• Does a duct cross the rated line? Yes, the return at the corridor wall, and each ceiling register.
• Does my change touch the protection? The new air handler is 4 in wider than the old one. Set in the old position, its cabinet would cover the damper's access door. The owner also asks to move a supply register 18 in to clear a new kitchen pendant.
• What the tech does: the air handler is set on the opposite side of its platform with a short transition, leaving the access door fully openable, and the tech confirms it opens and the damper and its link are visible through it before closing up. The register move is declined. Moving it means relocating the ceiling radiation damper to a new opening in a rated ceiling the association owns, installed per that damper's listing, and patching the old opening to the design. That decision belongs to the association and its contractor, and the tech writes it up so the owner can take it to them.

What changes the answer: if the building's records show the ceiling is not part of a rated assembly (some buildings have an unrated roof-ceiling over the top floor), the register move becomes ordinary work. That is a fact you get from the association's drawings, not one you infer from the absence of a label on the register.

How to verify you got it right

• Every duct you installed or touched that crosses the garage separation is steel of at least 26 gage, with no register, grille or open joint on the garage side.
• No return opening or return-duct leak remains in the garage.
• Every damper you passed is still in the plane of its wall or ceiling, its link or actuator untouched, and its access door opens fully.
• The record names each rated crossing you touched and what you did or declined there.

References

• International Residential Code, R302.5.2, R302.5.3, R302.11 and M1602.2, 2021 edition, as adopted by your jurisdiction.
• International Mechanical Code, Section 607 (ducts and air transfer openings), as adopted.
• NFPA 80 and NFPA 105, in the editions adopted by your jurisdiction's fire code, for damper inspection and testing.
• See related: Why a Fire Damper and a Smoke Damper Are Different Devices; The Garage-to-House Fire Separation; Penetrations in a Rated Ceiling.