Just a number I picked from domain knowledge. Almost all newer heat pumps can still efficiently pull heat out of that temperature, although some more expensive ones can go all the way down to -20 - -30F ambient air temp.
EDIT: If you're in a colder climate, in-ground coolant loops are preferred over an air transfer heat pump.
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What's special about 255K (0F) in relation to heat pumps?
Just a number I picked from domain knowledge. Almost all newer heat pumps can still efficiently pull heat out of that temperature, although some more expensive ones can go all the way down to -20 - -30F ambient air temp.
EDIT: If you're in a colder climate, in-ground coolant loops are preferred over an air transfer heat pump.
Ah I see. So it's nothing fundamental, just a limit on our current heat pump technology.