We have a fairly new trane heat pump system and recently replaced the thermostat to a ritetemp 6036. Everything has been working fine for months until one day I walked in the house and the heat wasn't working, the target temp was 73 and the actual was 60. My husband flipped the breaker off and on and then turned the t-stat back on and it worked fine through the night. But we did notice that the unit outside didn't appear to be running but heat was being circulated throughout the house. Once the temp reached its target it would shut off and back on once it dropped a bit like normal. A couple days later it was back to not coming on again, so I flip the t-stat off and back on and it works. How do I know if the system is running on aux heat or normal heat?

If you have electric aux heat, set the tstat OFF. Check how fast your electric meter is spinning then get someone to turn the tstat up to call for heat. If the meter spins faster, you most likely have electric secondary heat. Alternatively, check for an electrical disconnect box next to the air handler that does not turn the air handler off. It will be for the heat unit. And then too, if you are getting heat out the ducts and the outdoor unit isn't running, almost certainly you are running on the backup heat.