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HVAC Ducts in a Garage

We have a heated/air conditioned garage attached to our home. The furnace that heats the garage also heats other parts of the house which were added on when the garage addition was done in 1984. We had an issue with carbon monoxide when my husband ran one of his cars. It makes me wonder if having ducts in a garage that also connects to a house is up to code. We have a furnace with a humidifier as well as a sink in the garage, so not heating it would cause pipes to freeze. We live in the Philly area and have some very cold winters here.

We do have a CO detector in the garage. The car set it off and our alarm monitoring company couldn't get through to us about it because the alarm was sending info to them over our phone line. They called in the situation and we had at least half a dozen police/fire vehicles here. It was all because it took my husband a few tries to get his car aligned in the garage so he can walk around it. The highest CO level in the house was 10 which isn't dangerous, but it makes me wonder if having the house and garage connected by ducts is allowed by code.

Re: HVAC Ducts in a Garage

Big big no no. You cannot share a unit that serves a garage space with a occupied space. You can only share heating and cooling systems with chilled water, multizone mini-splits or hot water radiant or steam.

I'd seriously look at separating the ductwork and installing a new system for the occupied spaces (additions) and take this garage unit and remove the ductwork and enlarge the return air and use it to heat just the garage only. A unit for a garage space doesn't really need ductowrk. I have a grossely oversized furnace and AC serving my upstairs. I've considered installing it in my garage when I replace in in the next 2 years. When I do that I'll install it with just a short supply plenum and the return filter grill directly on the unit without any ductwork.

Re: HVAC Ducts in a Garage

I wish someone would have said something to us when we replaced the furnace back in 2004. I had a few estimates from reputable places. If the way the ducts were run wasn't up to code you would have thought that they would mention it. We have windows that need replacing and no budget for doing that and a new HVAC system (or 2). Our other HVAC system is over 15 years old and I was figuring it would need to be replaced well before this one.

We had a home inspection in 1997 before we bought. The inspector didn't say anything about one unit servicing both a garage and part of our house.

Re: HVAC Ducts in a Garage

What you could do is seal up the ducts in the garage so the furnace only heats/cools the house. (Move the thermostat to the house.) You may need to use space heaters in the garage to keep things from freezing. If you have gas supply, you could install a gas heater in the garage.

Or you could reroute the ducts to the addition so that they tap into the main house furnace instead of the garage furnace.

Re: HVAC Ducts in a Garage

"We had a home inspection in 1997 before we bought. The inspector didn't say anything about one unit servicing both a garage and part of our house"

This home inspector should have his license suspended. Everytime you see an a/c vent or register in a garage, you got to ask yourself where it's coming from.

Part of the blame in on the contractor who did the job in 1984 and the building inspector who signed off the addition.

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