I have used setback thermostats for many years, but now I live in a home with a heat pump rather than a gas furnace. I understand that setting back the thermostat, then calling for heat will energize the emergency / auxiliary heat. Is the savings obtained by setback enough to overcome the resistance heat? Is it feasible to control the resistance heat with an aux. thermostat to prevent it from coming on unless the outside temp is too cool for the heat pump alone to heat the house?

I appreciate any advice... thanks.

It is best to let a heat pump run and not use a set back feature. Resistance heat is much more expensive than using the HP.

Many years ago we tried this with a thermostat that would not go into emergency heat automatically. It had a switch on the thermostat that you could turn off and on to control the resistance heat. Most folks were not happy because of two things. They were very cold because the heat pump would take a long time to bring the temperature back up to there normal setting. They were eventually forced in very cold weather to turn on the resistance heat and there electric bill shot up.

If you are talking a set back of 2 or 3 degrees it may work for you but you will have to see how many degrees of uptake it takes to trigger the emergency heat.

Honeywell makes setback thermostats specifically designed for heat pumps. What they do is turn back on the heat very early (sometimes several hours before you want it back up to temperature) so that they can heat the house back up using only the heat pump. That way, you get some savings from the setback.

Do note that if you have a very large set back, the resistance heat may still kick on to help get the house back to temperature. Also note that because the heat pump will take longer to get back up to temperature than a gas furnace, you will see a lower percentage of savings than you would have seen with a gas furnace on a setback.

All that said, if I had a heat pump in my current house, I would still put in a setback thermostat, but probably wouldn't set it back quite as much as I plan on doing this winter with my gas heat. This summer, my setback thermostat made the energy usage on my house a little less than the energy usage last summer on my apartment that was less than half the size. Same efficiency compressors, but no setback thermostat at the apartment.

Check the packaging of any heat pump setback thermostat you buy to make sure it doesn't use the resistance heat to bring the house back up to temperature (a modern one shouldn't, especially a digital one, but always good to be safe) and you should be good to go.