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Heat Pump Water Heater in Basement of Cold Climate: freezing?

lifeisgoodsteve | Posted in General Questions on

Hi All,

I’ve decided to build a small basement for a mechanical room and some storage (happy to have it cold storage too for meat freezer and wine) and wonder how how feasible it is to use a heat pump water heater  in a basement in a cold climate?

Would any heat generated from mechanical room be enough to prevent temps going below freezing in winter with heat pump running?

Would it be recommended or worth it to run radiant floor heat in the basement just in case of temps getting too cold or not worth it financially?  Trying to be smart with construction costs as definitely on a budget.

If use electric water heater, any heat from mechanical room can rise and add some warmth to house during the heating months.

Background:
Western Montana (Bitterroot Valley)
Electrical rates are cheap between $.068 – $.076 / khw (mostly hydro)
Radiant floor heat planned in slab

Thanks!

Steve

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    Remember that heat pump water heaters (HPWHs) scavenge heat from their surroundings to heat the water, so they will make the immediate area around themselves colder than it would otherwise be. Any heat scavenged in the winter would likely have originated with your primary heat source too (radiant floor's boiler, etc.), so keep that in mind. Where HPWHs shine is when you can use them in a situation where their slight cooling effect helps you.

    Are you in an area with cheap natural gas service? If you're in a cooling dominated climate, natural gas is often more efficient overall in terms of the total energy used to heat the water. If that's not an option for you, or you just want to be all electric, then you'll need to put enough heat into your mechanical space to prevent freezing (I'd recommend keeping the space at a minimum of 50*F). Your HPWH should work fine in that case. I would not expect the other equipment in the mechanical room to keep it warm enough on it's own from just thermal losses in the equipment, and remember that an HPWH makes it COLDER, not WARMER in the space.

    Bill

  2. lifeisgoodsteve | | #2

    Hi Bill,

    Thanks. Yes, in general I'd be happy with a space that's colder due to the HPWH as it'd be better for cold storage of a meat freezer, wine, vegetables, etc..

    No natural gas here, so it's electric or propane.

    If it would seem that I'd need to heat the space to keep it warm enough not to freeze, then it seems better to simply use an electric water heater.

    1. Expert Member
      BILL WICHERS | | #3

      If your alternative is electric resistance heat, then yes, you'd be better off just using a regular electric water heater. You don't gain anything using electric resistance to heat the space so that an HPWH has a source of heat to scavenge to heat the water.

      Bill

      1. Expert Member
        PETER G ENGLE PE | | #8

        That's only true in the depths of winter, which is the only time that things might freeze. The rest of the year, the HPWH provides both cooling and dehumification to the space which seems like it would benefit this OP.

        Worst case, it uses the backup resistance heating strips occasionally, at which time you're in the same boat as a resistance heater. The rest of the time, you're better off.

  3. brad_rh | | #4

    I'm a little surprised to see a concern about a basement freezing. The ground is basically a heat sink/source around 50F. Zephyr knows more than I do but I would think that if the walls above ground are well insulated it would be fine.
    In central CO (slightly warmer than the Bitterroot valley, but not much) I have my heat pump in an attached garage/shop. At the coldest, it can be 40F in there. If the basement is big enough you could split it, duct the HPWH and have a cold room and a warmer room, which is what I did.

    1. lifeisgoodsteve | | #5

      Hi Brad,

      That’s what I had been thinking with the ground’s constant temp. Yes, above it would be insulated and heated.

      So your garage doesn’t get below 40 even with heat pump running?

      Thanks,

      S

      1. brad_rh | | #9

        Correct, 40 with HPWH running and around zero outside

  4. Expert Member
    Akos | | #6

    Average hot water usage works out to around 2000BTU/h. With a heat pump, about 1/3 of that comes from electricity and the rest from the space around. Overall, that is a pretty low load. Depending on the size of the space, it will cool the basement a bit, with such low amount of cooling it won't be much.

    Since it is a small space insulate the walls and keep the slab uninsulated. This way the subsoil can help to maintain the space within a couple of degrees of it, well above freezing.

    If you have an HRV/ERV, you can also put a stale air pickup in this space. This will help in keeping the air down there cleaner and exhaust radon. Since this will draw in warm air from the house above, about a 70CFM pickup will supply enough warm air to cover the heat load of the HPWH, you can tweak the flow rate to get the temperature you want.

  5. lifeisgoodsteve | | #7

    Thanks Akos. If the static temp of the insulated walls basement is enough to support a heat pump without additional heating other than a backup of pulling a little air from 1st floor, then that sounds intriguing. It'll be the mechanical room so could definitely do a HRV pickup there.

    Found this on a google search. I clearly haven't figured out how to refine my GBA searches to get the info I'm looking for. It's basically a discussion on how many degrees basement temp would drop with a hpwh.

    https://www.greenbuildingadvisor.com/question/how-many-btus-does-a-heat-pump-water-heater-exhuast-cool-down-the-area-around-it

    1. Expert Member
      Akos | | #10

      Calculating how many degrees a basement will drop from the HPWH is tough unless you do a whole house energy analysis.

      Assuming that the basement is cut out of a crawlspace of some kind. If that crawlspace is reasonably air sealed, no 100 year old single pane windows, and there is some insulation at least in the rim joist area, the basement won't freeze. The HPWH on its own can't pull enough heat out of a large space like that. It will be colder, but if you are looking for cold storage anyways, that is not an issue.

      If you crawlspace is leaky, that is a very different story as all that cold air will now settle into the basement you just dug.

  6. Expert Member
    NICK KEENAN | | #11

    You need to insulate your basement. Unless you have insulation and air sealing between the first floor and the basement, the basement is part of the conditioned space of the house and you're heating it whether you intend to or not.

    Heating the basement doesn't really cost you any energy, it just reduces the amount of heat loss through the floor from the first floor. It also makes the first floor more comfortable. And the whole point of heating is comfort.

    Most heat pump water heaters won't work below about 50F. In order to extract heat from the air the coil has to be colder than ambient temperature. But if the coil is below 32F ice will form on it and block the flow of air. So around 50F is the lowest practical temperature.

    1. brad_rh | | #12

      The min temp for a GE geospring is 45

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