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Heat pumps can't take the cold? Nordics debunk the myth
(techxplore.com)
This is a most excellent place for technology news and articles.
Which is why you have a backup system. I have a net zero house in Saskatchewan, Canada. My Carrier heat pump will operate to -15C, and switches to electric heating coils in the air handler if it's colder outside. It's a rather extreme climate here, but in most other places, you'd be fine with some baseboard heaters as backup.