HVAC (Heat Pump) Question: I have a 2-year old Trane XR13 single stage heat pump with a two-speed air handler and electric back-up or "emergency" heat, in Central Florida. The unit works fine in cooling mode, but not in heat mode. The blower motor starts as required, but neither the condenser
fan or compressor start at all. If I swith to cool mode and then back to heat, the condenser fan and compressor will start and run for anywhere between a few seconds to a few minutes and then quit and not restart, while the blower keeps running. I've checked the thermostat wiring and even installed a second thermostat, to no avail. The reversing valve also seems to work fine (thermostat sends current to solenoid on cool mode, none on heat mode, and solenoid seems to click as necessary, you can also hear the gas pressure release when the reversing valve solenoid switches from heat to cool. Last winter I had a friend who's an HVAC tech (unlicensed) check charge, and he said it was a little low and added some freon, but it made no difference.Two days ago with about 70 degrees OD temperature I got the heat to run for several minutes after switching back from cool mode, but now with temps in the 30s to 50s, it will only run a few seconds, and only after switching back from cool mode. Yesterday with OD temp about 55, heat ran for a few minutes, then condenser entered defrost mode, seemed to complete as required, and resumed heating, but only for a few seconds. That was the end of it. Could it be some sort of faulty pressure or temp sensor, or the defrost board? If the reversing valve was sticking or partially sticking, how long would it run in heat before cutting off? What else could it be? How can I troubleshoot? Is there a way to bypass for now to run heat at least temporarily with no need for cool? Thanks.