Aux heat on electric Heil heat pump system will work for a few minutes, then stop blowing hot air. If I turn the heater off and back on at the circuit breaker on the unit, it will go back to blowing hot air again for a few minutes, then same problem.

Of course, this only started happening when the temperature got extremely cold outside. It seems like the aux heat is coming on for a short time, then reverting back to the heat pump. The same thing happens in emergency heat mode.

The thermostat is old - mercury switch old. When using AUX heat, the LED light remains on, even after it stops blowing hot air.

I have American Home Shield coverage on the unit, but to call a tech out still costs $100. I am thinking about going to buy a new thermostat and trying it first, but I thought I would ask here first if that may be the problem. The difference would be spending close to $100 on a thermostat and not have it work, then have to spend another $100 calling a tech, or calling a tech and have to wait a few days to spend $100 and have them tell me it's a thermostat issue.

Any help would be greatly appreciated.