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A guy in Minnesota measured the air temperature inside his house on a -27 °F day. His house was heated by a heat pump rated 24,000 BTU. Unsure if it was actually putting out 24,000 BTU at that temperature, as they get less powerful at low temperatures. The Mitsubishi SUZ-KA30NAHZ can do 32,000 BTU/h at 17°F but only 25,600 at -13°F, for example.
Whatever it was putting out, though, it did really well at heating the house. And this was five years ago.