[Declanomous]

My understanding is the limit to the heating potential of a heat pump system is the evaporator freezing over, which is why they are more popular in areas with low humidity. I think at a certain point the heat pumps spend so much time in defrost mode with the auxiliary heater running that they aren't economical.

[hx87]

Wintertime humidity is rarely if ever high enough for frost buildup to be an issue--it's the humid shoulder seasons that have this problem, but your heating/cooling needs during those seasons aren't that high anyway, so defrost cycles won't lower comfort and efficiency all that much.