I have a 3 ton Trane Weathertron 1200 XL that froze the indoor coil to where it completly blocked all airflow. When I discovered it I turned the system off but kept the blower running until it thawed out. I opened the air handler and removed the filters to make sure it was completely thawed then turned the system back on and the heating resumed as normal. I called my AC Tech and he said it sounds like a defrost control problem where the system would initiate a defrost cycle but not terminate it. He came out and tested the defrost control. It is a demand type system that uses a temperature differential between two sensors. The control board checked ok by doing the "Forced Defrost" and the blinking light on the board indicated normal. The sensors were removed and resistance checked ok at three different temps. The refrigerant pressures checked good and the filters are clean. He left saying everything is normal. That night it froze again! The next day he again checked the defrost control and cycled the reversing valve several times but could not duplicate the problem. Again it froze up.
I live in the mountains in TN and we tested it during the day with 40-45 temps. It gets in the high 20's at night when the coil freezes.
Primary heating is with wood using a fireplace insert. The heat pump is a backup but I leave the indoor blower running continuous to help circulate the heat. Thanks in advance for any help.