This thermostat has a timer feature built in that allows one to set a time before the Aux heat comes on. If the outside temp is below set point then the recovery timer starts when calling for heat. In my case outside temp below 30f then if temp isn't recovered in 1 hour then aux heat comes on. The T-stat is very capable and very flexible with settings. In the 3 years that I have had the thing I have not used aux heat. I do admit though that I have a good rate plan because I use fossil fuel as back up. My electric rate for the heat pump is 1/2 of normal from Oct. until April. It does have a separate meter and they bill that portion at the lower rate. There are so many features on this T-stat that it can accomaodate many different cost scenarios. Check out the manual in the link I posted, you may want to get one.
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