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Oxford study proves heat pumps triumph over fossil fuels in the cold
(www.nationalobserver.com)
This is a most excellent place for technology news and articles.
It could probably be done, but then it wouldn't operate at higher temperatures. Realistically you'd probably need two heat pumps, a low temperature pump and a high temperature pump and switch between them as the temperature rises or falls. It's double the cost and double the points of failure, and for a situation that rarely happens probably not worth it.
I'd have to imagine at some point, more heat is leaving the house than entering, and given enough time, inside and outside will eventually reach a cold equilibrium.
Would depend on how well insulated it is, but yeah since homes aren't exactly built to be air tight there's realistically an upper limit to the temperature delta you're going to be able to achieve. Ultimately the question isn't how hot/cold is the inside/outside of the house, but how big a difference are you looking at between those two.