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Community and Q&A

Heat pumps in subzero temperatures

user-3813901 | Posted in General Questions on

Hi There Fellow Tradespeople,

When outdoor temperatures drop below -18 to -20 degrees F, and my cold climate heat pump is running inefficiently and not producing much heat, should I turn it off, or let it cruise through until temperatures rebound to within it’s range?
(Assuming of course that I’d be using my back-up heat source during an event like this.) Martin? Dana? Wanna weigh-in on this one?

Thanks All!

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  1. Expert Member
    Dana Dorsett | | #1

    What is your alternative source of heat? It matters! If your back up heat is resistance electricity the heat pump will still be beating it on marginal cost. If your backup heat is a wood stove, maybe not.

    Most cold climate heat pumps are still delivering a COP greater than 1.5 at max speed at -25C/-13F, and though not explicitly tested & rated at -20F are almost certainly still better than 1.5 @ -20C.

    If your heat pumps are listed in the NEEP spreadsheet it will specify capacity and efficiency at the lowest specified/tested outdoor temperature at both it's minimum and maximum speeds:

    (That link isn't working just this minute, but it was yesterday.)

  2. user-3813901 | | #2

    Hi Dana,

    Back up heaters for most of our projects are indeed electric resistance heat, as you reckoned. I do use the NEEP spreadsheet to size equipment ( or double check that our installer is not over-sizing). It should have occurred to me to check the COP of my Mitsu. Hyper Heat at -5F, which is I believe the lowest specified outdoor temp at both speeds on the data base. Wasn't sure how much lower the COP dropped below -5 F; so thanks for the heads-up!
    Are you finding any cold climate models/brands that aren't delivering heat below -25F, or actually shutting down? I've heard mixed reports on these phenomena.

  3. Expert Member
    Dana Dorsett | | #3

    The Hyper Heating Mitsubishis all have a specified output @ -13F.

    I have no first-person experience with them at -25F. A guy who used to post here regularly under the handle Jin Kazama heats his house with four Fujitsu xxRLS2H units, which kept up with the load at -28F.

    The Mitsubishis automatically turn off at some low temp to self protect due to the being out of range for the outdoor temperature sensing thermistor. The warmest it will turn off at will be -18F, but there are field reports of them still running well into the -20s.

    1. user-3813901 | | #4

      Right you are! -13F, not -5F as I stated.

      If we get the temps this season, I'll report back on my Mitsubishi's behavior--so far, un-challenged. Thanks Dana!

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