Gree has a system that claims to have full heat down to -31, so I'd say just keep researching and you'll probably be fine.
Doing a quick search, they're using 'standard' R410A refrigerant (PDF):
* https://www.greecomfort.com/assets/our-products/multi-plus-u...
the same as everyone else. It seems just that they though it worth the engineering effort to push a little further than most other companies.
See anything with the label "For Extreme Conditions":
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'm planning on getting rid of it altogether and doing the heat pump(s) most of the year, the wood stoves in winter and reclaiming a lot of headroom in the basement and an entire utility room that the furnace currently takes up.