Heat Pumps and Electric Heating - Puzzler - runs in a/c mode day, switches to heat at night.

Hope someone can figure this out! We have a Trane air-to-air heat pump. I've got my t-stat set for a/c about 73 degrees. Runs fine all day when it's 80* outside, I haven't touched t-stat when I go to bed. Temp outside drops to 60* and my wife and I woke up at 3 am to 84* and the heat is still cranking!! How is this even possible when it's set on cooling? Somehow the temp drop is triggering the heat to come on, even though it's still warm inside and we want a/c. Is this installed incorrectly?

Has it always done this? Is it your heat pump that is generating the heat or your auxiliary? What does the thermostat show?

Good question, I don't believe it's in e-heat (aux) mode, but at 3am I can't really see straight! I don't think it was. It has done this every year when the nights turn very cold and the days are still hot. I went down last night and turned off the gas line just in case it was the aux. heat kicking in.

If we assume this is outdoor air trigger, then you must have one or more outdoor sensors. Try to determine what they are wired to: either your heat pump or the inside thermostat. A wire map from thermstat to air handler to heat pump is a great idea. That may give us some idea what's controlling behavior. Also, do try to find out whether your heat pump or aux are coming on. Finally what make and model thermostat and Trane are we dealing with.

Sounds like you may have a t'stat with auto changover, is that correct?

Sounds like you may have a t'stat with auto changover, is that correct?

Yes, good question. In other words, does your tstat have heat and cool mode where both can run depending on temps. If so, what are your set temps for heat and cool? I would also try cooling-only mode to see if that solves the problem.