I have a heat pump with a two stage thermostat, 1st stage is heat pump, 2nd stage is electric (resistance) heat. Is it ever more economical to run the furnace with resistance heat only, not the heat pump?
I know that the outdoor temperature affects the ability of the heat pump to work, but is it feasible that below a certain temperature the heat pump becomes less effective than resistance heating?

When it is very cold, say 20's. Even if the heat pump is producing a 6 degree rise in temperature it is cheaper than resistive heat, the stat will kick that on as soon as it doesn't keep up anymore.

Thanks for the reply. May I assume that you are saying that as long as it keeps up, even if it takes a considerable amount of time, it's still more economical to run the heat pump than resistive heat?
Also, other posts on this subject have said that if the temperature goes below 25 degrees it is advisable to go to electric resistance only. It often goes into the 10-15 degree range here. Do you still have the opinion that the heat pump is still the best way to go?

As I said, 25ish is about as low as I would go. You can get a low ambient control that will lock the heat pump out at a preset temperature, this is the method I prefer (restricted mode)