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Furnace / hot water heater closet location and size

Rian Bart | Posted in Energy Efficiency and Durability on

I am at climate zone 3.

I initially planned to put furnace and hot water heater in the attic. Since it is more energy efficient to put furnace in the conditioned space, I found a couple corners (bedroom closet and bathroom corner)

Questions:

1. Can a furnace fit into 2′ deep closet?
2. What’s the minimal size required for the furnace closet and access limitation?
3. Is there any issue placing a furnace in a bedroom closet?

For hot water heater, is it also more energy efficient to place it in the conditioned space? If not, I probably will leave it in the attic (my garage is too small – 20 * 18).

Thank you!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Rian,
    Q. "Can a furnace fit into 2' deep closet?"

    A. No. I'm not saying it's technically impossible -- someone has probably done it, because you see such things in mobile homes -- but you don't want to do it. You need plenty of room to facilitate installation, and you need room for maintenance personnel to get in there. Moreover, you have to imagine duct layouts -- how will you run the main supply duct leaving the supply plenum? How will you route the return ducts to the return plenum?

    Q. "What's the minimal size required for the furnace closet and access limitation?"

    A. Most mechanical rooms are way too small. I think such a room should be at least 6' x 8' -- bigger is better -- but my sympathies always lie with the service personnel, not the homeowner who is giving up living space. Duct layouts determine mechanical room location.

    Q. "Is there any issue placing a furnace in a bedroom closet?"

    A. Yes. Furnaces are noisy.

  2. Expert Member
    Armando Cobo | | #2

    Rian,
    You cannot install a furnace in a bedroom closet. Most of my home designs have dedicated hallway closets for furnaces and WHs (unless I’m using tankless) to place the WH within 30’ of the furthest faucet and minimal duct runs. The size of the closet depends on the furnace, but in most cases you can install a furnace and a WH in a 36” deep x 60” wide closet. Of course, its better if you have more room, like Martin said. You can install double 24” doors for access and maintenance and the equipment must be seal combustion.
    If you want to install just the furnace, a 36”x36” hallway closet can be fine for a small furnace. Also, consult with your HVAC contractor for equipment specifications.

  3. Michael Chandler | | #3

    We routinely install heat pump air handlers in 30" deep insulated interior closets, generally with 6'8 tall 8'0 wide insulated doors that open wide to allow access to the HVAC, demand water heaters and solar hot water equipment. As Martin points out the noise is an issue so we line the closets with OSB or Thermo-ply and insulate the back of that with foam or fiberglass before drywall. The doors get foam on the back and have bump-up seal on the floor as well as top and sides and center. Even so the noise is still an issue for light sleepers. Consulting with the HVAC contractor and others involves (electrician, plumber) is critical, it's not just the HVAC cabinet. The condensate drain, duct work, access to zone dampers and balancing dampers and general supply and return air duct design all need to be accommodated in the HVAC space design. We got it wrong on one house in particular by six inches and there was really nothing to be done but re-frame the wall. Inches count here. Big doors rule.

    The drawing below shows a 22x22 counter flow air handler cab on a zoned bypass duct system with a demand water heater and a 120 gallon drain-back solar all packed into a 28" x 10'4" closet. The project manager, plumber, solar, HVAC, and electrical folks all had to work together very smoothly to make this work well.

  4. Danny Kelly | | #4

    "Is there any issue placing a furnace in a bedroom closet?"

    You also need to be concerned with proper combustion air and make up air. Even some sealed combustion appliances have minimum combustion air requirements.
    Our mechanical code also requires that the door has an automatic closer installed if in a bedroom closet.

    1. 4airconcern | | #5

      Please explain to me what makeup air is. I have a furnace in a bedroom closet with a louvered door that i want to get rid of bcuz of noise. There is an air return coming from the living room to the furnace. There is a 3 inch pvc air intake and a 3 inch metal exhaust pipe. Can i go ahead and put a solid door with weatherstripping and door sweep to get rid of the noise problem? The model # NTC7075BFA3. btu input 75,000 btu/hr.

      1. Expert Member
        Dana Dorsett | | #6

        >"Please explain to me what makeup air is."

        Makeup air for an atmospheric drafted furnace is to keep the exhaust vent from depressurizing the space, causing spillage of exhaust into the space (plus combustion air).

        For the burner to work properly in atmospheric drafted equipment the pressure at the exhaust has to be approximately the same as the combustion air intake pressure. Air also needs to be mixed in with the exhaust product (which contains a LOT of H2O) to raise the dew point of the gases going up the stack in order to avoid corrosive condensation in the flue.

        The volume of air necessary for stack dilution is usually greater than the combustion air, and restricting the dilution air while the burner is running depressurizes the room, risking some of the exhaust product entering the room via the dilution hood.

        Though pretty common, a 75K is actually a pretty big furnace for an insulated normal sized house, as I noted in your other thread. Don't replace the door, replace the furnace for something right-sized and much quieter (like a modulating ducted mini-split heat pump), if it's in the budget. It's a lot more expensive than buying a door and creating makeup air vents in the exterior wall, but it's guaranteed to be a lot quieter if you pick the right unit.

  5. 4airconcern | | #7

    Please explain to me what makeup air is. I have a furnace in a bedroom closet with a louvered door that i want to get rid of bcuz of noise. There is an air return coming from the living room to the furnace. There is a 3 inch pvc air intake and a 3 inch metal exhaust pipe. Can i go ahead and put a solid door with weatherstripping and door sweep to get rid of the noise problem? The model # NTC7075BFA3. btu input 75,000 btu/hr.

    1. Expert Member
      Dana Dorsett | | #8

      Makeup air is necessary for the exhaust to work correctly, as explained in response #6. You won't be able to simply seal up the closet door unless you create an appropriately sized makeup air path to the outdoors or another room.

      Another way to lower the noise is to lower the blower speed (that furnace has 4 speeds), but that won't affect burner noise.

      To really kill of the noise, replacing it with a right-sized ducted mini-split heat pump would lower the noise dramatically (even with louvered door), and would allow you to seal up the closet, since there would be no exhaust or combustion air or makeup air requirement. It's a high budget solution, but it would be nearly silent- you might not hear it at all if you replace the door and weatherstrip it. (Can you hear your kitchen refrigerator from your bedroom?)

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