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Community and Q&A

Basement 47 degrees in Winter

BuildingNewb | Posted in General Questions on

Hey guys,

My unconditioned, unfinished basement gets pretty cold in the winter (as low as 47 degrees). It is connected to a 3 car garage that also gets cold in the winter (semi-conditioned at 40 degrees). We are in upstate NY.

I have sealed and insulated the rim joists so I can’t for the life of me figure out why the basement gets so cold. The only combustion appliance down there is a 76k btu power vented gas water heater. 

Is it at all possible that the negative pressure caused between the combustion and power venting of the water heater is enough to draw in cold air which keeps the basement at these temperatures?

We have an excess of solar power so I wanted to replace the gas heater with a hybrid water heater (heat pump with resistance heat backup). However, in heat pump mode it’s the same as running a 4,200 btu air conditioner down there.

Do you guys think there’s any shot it would actually be warmer in the basement after getting rid of the gas water heater or should the infiltration caused by power venting and combustion be negligible?

Thanks for your time!

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  1. Expert Member
    Akos | | #1

    It sounds like you either have a lot of your basement walls above grade or still have some major airleaks. Air leaks are easy to find with a blower door, targeted air sealing the rest of the house at the same time is generally very cost effective.

    Water heaters don't run long enough to draw in a lot of combustion air, this is not like your furnace which could run non stop for months. I doubt that is the problem.

    Heat pump water heaters can be set to run in resistance only mode when cold, there is a bit of power use cost to this during those very cold months, but you'll still get the benefit of heat pump the rest of the time.

    P.S. Insulating your basment is generally one of the cheaper energy efficiency improvement. Getting as little as R10 rigid on the walls would drop your house heat use by 15% to 20%, warms up the main floor significantly and also makes the basement much more pleasant.

  2. BuildingNewb | | #2

    Thanks for the input. I definitely intend on finishing the basement within the next 2 years. I just hope the hybrid hot water heater doesn't cool the already 47 degree basement down too much more. If I have to I will switch to resistance only mode.

    I will have a ductless mini-split down there once we finish it but I know I'll be losing a lot of its efficiency since about a quarter of its output will go toward feeding the hybrid water heater with warm air.

    In hindsight, I probably should've just stuck with a gas water heater. The hybrid was half the price of the gas after incentives and we have roughly a 20% excess of solar so we figured we'd try it out.

    Also, I know of several spots where my uninsulated garage and basement connect so I'm sure that's a big contributor to the cold temps as the garage is normally 40-42 degrees when it gets really cold out.

  3. Expert Member
    BILL WICHERS | | #3

    You can put rigid foam up first on the walls you suspect are the worst offenders in terms of heat loss. An IR camera will very quickly show you what the coldest surfaces are.

    You can insulate the basement before you finish it. All you’d do to complete the finishing would be to build some studwalls over the rigid foam. If you don’t have any water issues, you don’t have to do much besides just hanging the foam, so it’s a relatively quick and easy project with a lot of return on investment — especially with as cold a your basement is!


  4. Jon_R | | #4

    +1 on finding and sealing air leaks. Stack effect pressure will pull air in.

    1. brian8smith | | #5

      What the largest typical air leaks that you find beside the rim joist and the foundation wall itself? Is it ok to seal the rim joist inside and externally at the bottom of the siding or will that trap moisture?

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