Our heat pump is having trouble keeping up on colder mornings. It's not just defrost kicking in, as it's blowing cold (less than the target temperature) for an hour or two some days. The heat strip is functioning, but I believe it's too small for our conditions:

  • 35-40° outdoor temp
  • trying to maintain 65° indoor temp
  • ~1400 sq ft, Pacific NW

The unit has a 10kw heat strip, and has a list of supported models on the side (the upper end being 10kw). Assuming electrical can handle it (currently 60A breaker, 6AWG - I believe I'll need to add a 2nd 60A circuit), is there a reason I can't or shouldn't use a 15 or 20kw here? There are compatible models that look like they should work, aside from the exclusion on the label.

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  • If the label lists the 9.6kw heater as the largest available, why do you think a 15 or 20 would be ok?
    – JPhi1618
    Commented Jan 15, 2020 at 20:24
  • Because I was seeing larger compatible heat strips in the manual (though now I'm realizing they're for the larger handlers), and because the last technician who looked at the unit implied it could take a larger heat strip. Did they just install too small of an air handler for this house, and I'd have to replace the whole thing to be able to keep up on our 3 days of cold per year? Commented Jan 15, 2020 at 21:14
  • My implied point is to make sure with the manufacturer that it would work. Even if it physically fits in the enclosure, the metal thickness or insulation configuration, or who knows what might not be able to deal with twice the heat.
    – JPhi1618
    Commented Jan 15, 2020 at 21:16
  • "Did they just install too small of an air handler for this house, and I'd have to replace the whole thing to be able to keep up on our 3 days of cold per year?". Probably not. The coldest days you are referencing are probably 10 or 20 deg F colder than the design minimum temperature, which is what you unit is probably sized for, In my area, the design low temp is 22 deg F, but it occasionally gets down into the single digits.
    – SteveSh
    Commented Jan 15, 2020 at 23:24
  • How do you know the backup heat is working? If it is, I don't think you should be feeling cold air coming out of your registers.
    – SteveSh
    Commented Jan 15, 2020 at 23:31

2 Answers 2


Hold on. This doesn't make any sense.

You seem to be assuming that operation of the heat strip is a normal state of affairs and this is where your heat comes from on a 35-40 degree day. No, no, that would be entirely pointless! It would defeat the purpose of even having a heat pump. At that point you might as well just have an air conditioner added to a pure resistive electric furnace.

Something is wrong with your heat pump. I know that's not what you want to hear, but the repairman will be cheaper than the electric bills all winter long from running resistive heat. What an expensive way to heat!!!!

I mean, if you really want that, you can just install a bunch of $50 Cadet style electric baseboard heaters at 2000 watts a heater, as many as you plese. Feed them from a contactor operated by the same 24V signal that kicks the emergency heat on. Easy peasy. But you'll hate me when the bill comes.

The only other possibility is that the heat pump was wildly mis-specced for your location, which needs to be hashed out with the installer if it's a new install, or the previous owner has just been living with for years. But that seems insane; you have the worst of both worlds. The maintenance expense of a heat pump system with none of the cost/efficiency savings.


Sounds like you are using the heat pump setting instead of supplement heat. On heat pump, if it doesn't get the temperature up within a certain amount of time then the supplement coil kicks in. Was it getting really cold in the summer when you had it set to cool? If not, then the coolant needs to be checked. Personally, here in Toronto, I prefer to run the winter in supplement mode as it gets to temperature sooo much faster.

  • Auxiliary heat (as I've seen it called here, supplement heat sounds like the same thing though), is kicking in when it's cold enough - there's an icon on the thermostat (a '+' on the heating mode), and the element's warming up - it's just not enough to get past the temperature differential. There's also 'emergency heat', which runs just the heat strips and bypasses the heat pump, which I can enable but has the same problem (not enough output to get up to temp). Commented Jan 15, 2020 at 22:15
  • 1
    But "supplemental heat" (read: using the emergency heat in a non-emergency) is so much more expensive than heat pump heat, and who cares how fast it warms up when you're not in the building? Commented Jan 15, 2020 at 22:19
  • Fyi. Heat pumps are only good above 18 degrees farenheit. Colder and it is better to run supplement heat only. If it can't heat with just the supplement only then you definitely need to get someone whom knows how to do a proper heat loss calculation so the unit is sized correctly.
    – user68386
    Commented Jan 17, 2020 at 22:02

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