I feel the residential heating by ASHP is not specially challenging in N-America,if the widgets ,supplied locally, could only somehow manage -15degC with reasonable COP (>>1,5).
In N-Europe practically all AC (R410) at heating mode cope easily with that
minus15C ... some even "well" with -25C.
Below one collection of test reports (note, defrosting included in the results) in Finnish:
Ilmalämpöpumppuvertailu - VTT:n testiraportit | Scanoffice
and further one sample of ME`s MUZ/MSZ FD35VABH -model:
http://www.scanoffice.fi/uploads/vtt...S-08820-10.pdf
Look at pdf.page 9/11 (POWER W vs. TEMPERATURE C(elsius)
The corresponding COP vs.temp. (defrosting incl.!) shown at .pdf 7/11
It is for sure the pumps do not enjoy the conditions -20C ... -30C.
The deep-cold POE-oils should be returned back to the compressor at random basis (forced defrosting mode,trigged like egg-clock by software) ,eventhough there is no frost at the evaporator due to extra low humidity in the outside air (RH <30%)!