I’ve looked into heat pumps pretty extensively for my upcoming boiler replacement (looking at an air-to-water in my case, but very similar principles apply).
In my case, the 99% design temperature is high single digits Fahrenheit. For the 3.5 days/year colder than that, the plan is to have the house “coast” on thermal mass.
That’s for cases where the ambient temp is below design (where the heater can make heat but just no longer enough to keep up with the building heat loss), not for when it’s below a cutoff (where the heater shuts off entirely). In my case, that’s so far below design temp that I’d expect to never see it. (We hit -9°F in 2016 and would have to go all the way back to 1943 to find a low of -14°F.) If it happened, thermal mass would start to carry us with electric space heating keeping the house from totally freezing.
I might invest in some backup propane heaters as well (a buddy heater at Walmart is pretty affordable and is safe for indoor use). I feel like in blizzard-like conditions having electricity be your backup plan might not be too wise.
In my case, the 99% design temperature is high single digits Fahrenheit. For the 3.5 days/year colder than that, the plan is to have the house “coast” on thermal mass.
That’s for cases where the ambient temp is below design (where the heater can make heat but just no longer enough to keep up with the building heat loss), not for when it’s below a cutoff (where the heater shuts off entirely). In my case, that’s so far below design temp that I’d expect to never see it. (We hit -9°F in 2016 and would have to go all the way back to 1943 to find a low of -14°F.) If it happened, thermal mass would start to carry us with electric space heating keeping the house from totally freezing.