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Heat pump backup heat size

tkzz | Posted in Mechanicals on


I am narrowing down a new hvac system for my house, which has a design heat load of approx. 24,000 BTU/h (calculated by fuel use and by furnace runtime to confirm).

I am looking at 2 ton inverter systems. If I get an all-electric system, as opposed to dual-fuel, how big does the backup electric coil need to be?

One hvac installer has specified 10 or 15 kw but this is so big that it would cover my entire home’s heating load and then some. Is that right or should I be looking at a much smaller coil to cover only supplementary heat for the rare cold snaps / extreme weather events? If so what is a good guideline to size the coil?


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  1. paul_wiedefeld | | #1

    It depends - how cold does it get where you live?

    The cold climate heat pumps start to cut out around -15F. So if that describes your climate, you would want a full 100% backup, 8kw.

    Otherwise, your decision is different: you can either get a full 100% backup in case the heat pump breaks (it's cheap insurance, but you probably didn't backup your furnace 100% so is it worth it?) or get something smaller to cover heat loads that are slightly above the heat pumps capacity - for instance: a 26,000 btu/hr heat load would exceed the heat pump alone so could be met with heat pump + 3kw (they have limited coil sizes so it'll likely be oversized).

    A third consideration is to slightly UNDERSIZE the heat pump for heating with the goal of having it really perfectly sized for cooling. The theory is that a better sized cooling unit will handle dehumidification better. In this case, the backup resistance will cover some of the heating load in cold, but normal conditions.

    The relevant sizes of electric resistance are 3, 5 and 8kw. As there's not much of a price difference, it comes down to what goals you're trying to achieve.

    1. 2010G37X | | #9

      the price of the wire for 8kw is a lot more than the 3 and 5 kw.

      based on my experience, which is both, choosing a large back up and a small back up,
      picking the 5kw is the way to go. I am starting to lean toward the even lower 3kw backup.

      If you are looking at purely back-up then 3kw is perfect due to, why does it matter if you can only heat your house to 15C in stead of 20C when you are operating on a backup (especially, some people are starting to use battery's).

      Also my recommendation is based on a good product like Mitsubishi and you picked their hyper model, then for sure the 5kw even the 3kw can be a choice.

      Summary, it depends on the unit you are going with.

  2. Expert Member
    Akos | | #2

    Also keep in mind service loads. The 15kW heat strip needs an 80A feed, this should be fine on a 200A panel but might be tight if you also have an EV charger.

    The goal of any backup heat should be to keep your house from freezing or to help during polar vortex days, even the smaller strip should be fine for this.

    1. 2010G37X | | #10


  3. tkzz | | #3

    Thanks guys, this is helpful. The design temp here is 3F. It barely scratched that this past winter.

    I am leaning toward a 5 kw element or maybe 2 x 3kw if it can be staged... Any thoughts on staging?

    I do have a 200A panel but would like to keep some capacity for a future EV charger. I don't feel that I need the coils to keep my house at 21C on a polar vortex day on their own. In the worst case, if something happens and it's too cold to live in the house, I can shut off the water and leave while waiting for a new heat pump. Super rare and unlikely I think right?

  4. greenright | | #4

    You don’t need backup. The current crop of vapor injection ashp as mentioned will go into turtle mode around -10 -15f. Do you ever hit that?

  5. tkzz | | #5

    @greenright I haven’t bought a system yet! I thought a backup was required for insurance purposes. Could be wrong. Also hear it helps with comfort during defrost?

    1. greenright | | #6

      Defrost shuts off the indoor unit fan so you don’t get a (cold) draft. It is barely noticeable on wall warts and not noticeable on duct based systems.

      1. tkzz | | #8

        Good to know, thanks!

  6. xbcornwellco | | #7

    I like 15-20 kw for every 2.5 tons HP.

    1. Deleted | | #11


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