I'm trying to troubleshoot my 12-yr old Trane heat pump unit that has a problem with freezing-up the indoor evaporator coil. The unit is the first-level zone of a two zone system with the evaporator/blower in the basement and the condenser/compressor outside. During the winter, a local HVAC contractor installed an AprilAire humidifier into the ducting with a by-pass installation. I turned the AprilAire off at end of the winter. When I turned-on the cooling side of the heat pump, within 6 hours, the indoor evap col iced over. Th eblower was still blowing but there was no air flow. I tuened off the system and thawed the evap coil, cleaned the coils carefully, put in a new filter, and in approx 6 hrs it will start icing the coils again. The blower motor works fine and was replaced a year ago. In addition, when the evap coil ices-up, the refrigerant line (covered in black insulated foam) going out to the outside condensor starts sweating like crazy.
What is happening? I have good air flow past the evap coil (when it's not iced-over). Help?