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Community and Q&A

Limited Air Return with Garage Furnace

aunsafe2015 | Posted in Mechanicals on

I’m thinking about buying a 1970s two-story colonial in the pacific northwest.  It has a gas furnace located in a closet in the garage, right next to the door that leads into the house. The ductwork runs through a vented crawlspace to feed floor registers on the first floor, and a trunk line runs up to the second story (bedrooms) to feed floor registers on the second floor.  I believe all of the ductwork is metal.  I have no idea what the static pressure looks like.

There is only one air return, and it is probably at least slightly undersized.  It is on the first floor, in the ceiling, right next to the door that leads into the garage (so the air return duct is quite short… probably no more than 6-8 feet from the vent in the house to the furnace in the garage).

Anybody able to speculate how big of a problem the single first-floor return would be for overall comfort, especially in the 2nd floor bedrooms?  For a 1970s house, I’m not expecting perfection, and I know there would be a fair bit of temperature difference between the first and second stories.  But I’m just curious if those more knowledgeable than me see stuff like this and think, “pretty standard for a 70s house, it would be fine,” or if they think “that’s something that would need to be fixed immediately.”

For the record, I don’t see any easy way to add an air return to the second floor.  The expensive fix would be putting a separate system for the second floor in the attic (also vented, so not an ideal place), and replacing the first floor system with one sized for only the first floor. Small slim-duct style mini splits would be great.  But to do 2 of them, including all new ductwork on the second floor, would probably be $30k or more where I live.

Thanks for any input!

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  1. seabornman | | #1

    It seems to be the standard in houses of that vintage, and can work fine as long as doors have undercuts and supply ducts are adequate.

  2. GBA Editor
    Kiley Jacques | | #2

    Your situation--furnace located in an attached garage--made me think this post would be of interest to you, as it stresses the importance of air-sealing between the garage and living spaces to ensure a healthy environment: Continuous Air Barrier Between Garage and House.

  3. walta100 | | #3

    Just to be 100% clear you need to understand code today would not allow this home to be built in this way because people have died from the hazarded that having the furnace and water heater in the garage pose.

    If you chouse to except the risk and live with the hazards understanding them and avoiding dangerous situations is the key to survival.

    1 You must never run the engine of the car in the garage for a minute longer than necessary to park the car. Understand the furnace will pull air from the garage and distribute it throughout your home. Carbon monoxide poising is a real possibility, detectors are must have devices. Also, pesticides gasoline and other chemicals stored in your garage could pollute your house. Consider building a detached shed to store such items.

    2 If your furnace and or water heater are not raised 14 inches or so above the floor, consider doing so when it comes time to replace them. The fumes from gasoline are heavier than air and will pool on the floor and if the water heater were on the floor, it could ignite the pool of fumes from refueling a lawn mower. Note it is not uncommon for cars to leak gasoline fumes.

    Please be safe.


    1. aunsafe2015 | | #4

      Walta, thanks for the response. At least in my area, I'm seeing houses built as recently as this year that have the air handler in the garage.

      Perhaps you thought I was saying the air return vent itself is in the garage? The air return is in the house, but very close to the door that leads from the interior of the house into the garage.

      Not doubting, just trying to understand what aspect of this is against code given that I've seen it recently in pretty new construction.

      And regardless, I think you are spot on about being careful about garage fumes, so I appreciate that tip.

  4. walta100 | | #5

    I understand most of the return air will come from the home but return ducts are notoriously leaky 20% is not uncommon if it is under 5% it is a miracle. Everything you bring in your garage will get inside your house.

    What the local code says and how it is enforced varies widely. Just because you are allowed to do a silly thing does not make it a good idea.


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