My house has an 80% efficiency gas furnace in an interior mechanical closet with the return air path below the furnace and through a grille under the closet. However, since the furnace is not a sealed combustion model, the whole mechanical closet itself requires combustion air for the furnace to use, which is provided via a huge duct connecting the closet to the attic above it. Last winter, I found that this was a major source of cold air infiltration when the furnace is off, and I weatherstripped the door and sealed the furnace to its platform to prevent the cold air entering the mechanical closet from leaking into the house.

However, while those measures prevented cold exterior air from entering the house through those channels, I've found that it's still entering the house through the unit itself: apparently down through the filter and out the return air grille. I can feel the cool breeze coming through the return air grille when the unit is off.

Is there any way for me to solve this problem without interfering with the furnace's operation or replacing it with a sealed combustion unit? That's eventually in the cards anyway, but I'd prefer to solve the problem now, if possible.

  • Is the cold air coming from the room or the return? How well is the return grill area sealed-off from the room? I suspect a gap between the return duct and the floor.
    – Mazura
    Commented Nov 9, 2014 at 6:46
  • 1
    There is no return duct; there's a space below the furnace that's sort of just a return air plenum. Like this: 911furnace.com/newheaterprices/…
    – iLikeDirt
    Commented Nov 9, 2014 at 14:09

2 Answers 2


Judging from the previous picture, I thought it returned from the lower floor through the bottom of the furnace. If it looks just like that new pic; a side tap with plenty of room, you could put a low-leak damper there that the blower will pull open all by itself when it runs (I'm having trouble finding the kind with a small weight attached...). This will block most of the flow in the ductwork, which is exchanging heat and causing a draft (half of the problem) where it's run in unconditioned space.

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How is the ductwork insulated that I assume runs through the attic? Your plenum chamber should have self-stick insulation on it, if it's in an unconditioned space as should any ductwork.

There is also a noticeable lack of sealant on the system allowing unconditioned air into it (the other half of the problem). My new best friend is duct sealant mastic in a bucket; tape needs the proper surface prep and can does eventually fail.

Steps one and two for addressing rigid ductwork in unconditioned space:


Another possible problem is that you have not truly sealed the mechanical room from the rest of the house including, however small, the return area.

Things to consider:

  • Installing a new system may require a completely different plenum; you should seal it for now but maybe wait on insulating it.

  • Jacket loss on equipment is horrendous, even on a well sealed and buttoned-up install; I prefer it in conditioned space. It's more efficient and less considerations need be taken: freezing temperatures; insulation is necessary.

  • You've used the word pascals, which means you know more about it than I do, however I'd of been quite satisfied blocking the air intake and installing a louvered door to include the room into the 'house'. If it is completely spray foamed (hermetically sealed) I'd be concerned, otherwise...meh. (Provided return air is not taken from the mechanical room.)

702.1 All air from indoors.

Combustion and dilution air shall be permitted to be obtained entirely from the indoors in buildings that are not of unusually tight construction. In buildings of unusually tight construction, combustion air shall be obtained from the outdoors in accordance with Section 703, 705, 706 or 707. -iccsafe.org

  • This intake will disappear (along with most of the problem) once you put in a HE system. Decide when and what you're going to spend money on. Recommended order: 1. duct sealant 2. HE furnace 3. spray foam insulation.

Having to install a damper is not a common solution. Attack the root of the problem; air infiltration and heat exchange.


The answer is no. This furnace is a non-sealed-combustion model that's meant to have combustion air circulating in the space. Since my combustion air supply is ducted into the furnace closet from the attic (exterior space, effectively), the only options are:

  • Live with it
  • Block off the duct and get combustion air from inside (unacceptable; the house is too tight)
  • Replace the furnace with a sealed-combustion model (or an electric heat pump or other unit that requires no air connection to the exterior).

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