Walk through an attached garage in Branchburg or Hillsborough built during the 1980s or 1990s and there is a decent chance of finding a heat register in the ceiling or a return grille cut into the wall that backs up to the furnace room. Half of Branchburg's housing stock and the largest share of Hillsborough's homes date to that stretch, when running a branch off the main trunk to warm a workshop corner or a laundry alcove in the garage was a common shortcut, and nobody on the job gave much thought to what it meant to mix garage air with the air moving through the rest of the house.
What the code says about garage air
New Jersey's residential code is direct about this point. A furnace or air handler that supplies the living space is not allowed to supply air to, or pull return air from, an attached garage. The only exception is a system built to serve the garage alone, a separate piece of equipment from whatever heats the bedrooms and the living room. Where a duct has to pass through the garage, or through the wall or ceiling shared with the house, it has to be built from sheet steel at least No. 26 gage, with no seams, registers or openings cut into it along that run. A register tied into the house trunk and sitting in a garage ceiling does not meet that standard, however well it was installed decades ago.
Why the separation matters
The reasoning behind the rule is straightforward. A garage holds a running car, a lawn mower topped off with gas, a grill's propane tank, or a space heater left on overnight, and all of those produce exhaust or fumes that a return grille or a leaky duct seam can pull straight into the supply air feeding the rest of the house. Carbon monoxide produced by an idling engine is colorless and odorless, which is part of what makes it dangerous, and a duct connection between a garage and the living space gives it a direct path to a bedroom register. The fire separation matters on its own, too. The wall and ceiling between an attached garage and the house are built to slow a fire that starts in the garage, and an uncapped duct opening punches straight through that separation, metal boot or not.
How to spot a register or return cut into the garage
A technician checking this looks for a short list of things. A supply register set into the garage ceiling or high on a garage wall, fed by a branch that ties back to the same trunk serving bedrooms rather than a dedicated garage unit. A return grille low on a garage wall with thin flex behind it, or no real duct at all, just an open stud or joist bay carrying air back toward the furnace room, which pulls garage air into the return side of the system every time the blower runs. A duct passing through the garage wall wrapped in standard flex instead of sheet steel, or a sheet steel duct with a seam or a damper cut into it somewhere along the garage run. Any one of these is worth flagging, since the fix is the same either way: close the connection and restore a sealed boundary between the garage and the duct system.
What air duct repair does to close it
Air Duct Repair on a job like this usually means capping the register or return at the garage side, cutting the branch back to the main trunk, and sealing the penetration through the shared wall or ceiling with solid sheet steel rather than tape over a hole. Where the existing duct running through the garage is flex or a lighter gage of metal, that section gets replaced with sheet steel at the gage the code calls for, since a repair that leaves a thin duct in place has not closed the separation, only moved the weak point further down the run. The rest of the house system gets checked at the same time to confirm no other garage branch or return is tied into the trunk unmentioned, since a house with one undocumented garage connection sometimes has a second one left over from an earlier addition or a workshop wired in later by a prior owner.
If the garage still needs its own heat
None of this means a garage has to go without heat. A heater sized and installed to serve the garage alone, separate from the equipment conditioning the house, meets the code's exception, and choosing that equipment is a decision that belongs with a heating contractor rather than a duct company. Air Duct Installation covers running new duct correctly when a system is added or extended, built with the right material and without a connection looping back into the house trunk, but sizing and selecting a dedicated garage heater sits outside what a duct visit addresses. The two jobs often get discussed in the same conversation, one closing the old connection and the other opening a new one built the right way from the start.
What decides the cost of closing a garage duct connection
What this kind of repair costs depends on how many registers or returns are tied into the garage, how much of the duct running through that space has to be replaced to meet the sheet steel requirement, and how the shared wall or ceiling is built, since some assemblies are easier to re-seal than others. A written estimate after a look at the garage and the trunk line it ties into lays out what gets capped, what gets replaced, and whether a separate garage heater is worth raising with a heating contractor, rather than guessing at the scope before a wall or ceiling panel is opened.