We recently converted our house to 100% heat pumps. The models we have are efficient down to like 0F and continue to work down to -15F (lower than the record low here). So this tech is novel but already consumer available. We have Fujitsu Halcyon XLTH models and commercial units that are less common for residential.
They all “continue to work” at cold temperatures but the question is with what and how much you’re going to supplement their insufficient heat with when it gets that low.
COP is still decent at low temps. That is the point. They are 80% efficient at their lowest temp rating. And, at least where we live 99.9% of days will be 100% efficient.
I guess 80% efficiency for a few days a year is OK. In The Midwest we can go below zero Fahrenheit for a month or more at a time and while I happily live with 80-90% furnace efficiency with natural gas as a fuel, I certainly would never want to do that with electricity as a power source (given the historic price difference between the two).
You need a separate hot water tank and a 3-way valve, when hot water is requested the hot water from heat-pump will get diverted into the hot water tank (puffer).
