[dudul]

OK, so that's one, very well understood case. Yes, a heat pump will not work all the time. I have one and it stops working around 20F and the furnace takes over. So what? I live at the US/Canada border and my furnace runs maybe ~2 months during the year. This is still tremendous savings.

[bluGill]

Big savings, but is it big enough to be worth the extra expense of a heat pump vs used using the furnace year round. The times when the heat pump works are the times when you least need it, since other activities of life are adding heat to the house too.