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I've got a high efficiency natural gas furnace, and a wood stove in the same room. I get wood for free, so I'd like to use it to supplement heating with natural gas. The problem is a severe lack of air circulation, that particular room gets very hot, but the air doesn't circulate to the rest of the house, because the furnace's air circuit is closed.

What I'd like to do is disconnect the cold air return from the air intake of the furnace, and have the furnace intake hot air from the wood stove. The returns can still send air back to the furnace room, but not directly to the furnace. I can close the door to that room, to effectively seal it and use it as a buffer/pre-heater for the furnace air.

I don't see any issues with this, as I'm not breaking the furnace circuit in any way, just adding a large buffer area. Any excess combustion air that the wood stove requires will be pulled in through the air returns (I may eventually add a fresh air supply for the wood stove if I have draft issues), and the furnace can be run in fan-only mode to pull just air from the wood stove, or run in heat mode, but will operate more efficiently as the air it is heating is already very warm. I don't foresee that I will generate negative pressure, as there are air returns, so I can't see how this could be a concern of pulling exhaust gases out of the wood stove.

Are there any issues with this that I don't see?

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    Is this same "room" a utility closet or is the wood stove in actual living space in the house? Why not open/remove the door to this room and put a fan in to blow the hot air out? Does the stove have any sort of "hot air out" ducting, or does it just rely on convection to move the heat away from it?
    – FreeMan
    Commented Sep 2, 2022 at 16:08
  • Unless you plan on disabling the natural gas, think fans is the best. Imagine hot/quite warm intake air for the furnace will mess up sensor readings in the furnace. Think it is a bit complicated than heating up warm water on a stove.
    – crip659
    Commented Sep 2, 2022 at 16:18
  • @FreeMan This is a utility closet/furnace room that includes the furnace, water heater, and the newly installed wood stove (chimney was always there, previous owners capped it). Adding a fan doesn't do much as there's no way for air to replace what the fan blows out - the air returns are sealed to the furnace
    – Brydon Gibson
    Commented Sep 2, 2022 at 17:01
  • Is this downstairs in a 2 story house? If so, could you put a large (say 18x24) vent in the second story floor to allow heat to rise out of the utility room?
    – RetiredATC
    Commented Sep 2, 2022 at 20:23

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Perhaps an HVAC expert can chime in, (I'm not) . However I think this may not be a good idea. The burning of the wood produces some fumes that are not good for humans. Most of the time these go up the chimney and out of the house with the hot rising air. Adding a return that could act as a vacuum for the heated air may cause these fumes to back up in the room as well and possibly spread through the entire house. Perhaps there is a formula for the amount of return air flow as opposed to the venting of the fire. I'm just suggesting this may not be a safe idea. More info is needed.

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    Backdraft, down draft, and perhaps induced reverse draft are common terms for the phenomenon you describe. It's definitely something to be concerned about. The furnace blower will creative a relative low-pressure zone in the room; if it's easier for air to be pulled down through the chimney than from elsewhere in the house then that's exactly what'll happen.
    – Greg Hill
    Commented Sep 2, 2022 at 16:04
  • Perhaps add a heat exchanger of sorts. Bring air from outside by fan, run closed pipe by fire, heat the air, outlet the air by the furnace air return? The dangers then do not affect the air going to furnace
    – Chris
    Commented Sep 2, 2022 at 18:11

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