I just had new evaporator coils and a new thermostat installed. This was all due to a leak in the evaporator coils. Now I have nice new shiny aluminum coils. Once the system was filled with gas and the AC fired up it was amazing. It was cooling the house like it hadn't in years. Really cold air blowing out. The issue is that now the heat doesn't "fully" work".

Here is what I mean by "fully". When I switch the thermostat to heat and then crank the heat up a few degrees higher than the room temp I can hear the thermostat click and then the heat starts blowing. A quick check shows the air is coming out of the vents at about 80 degrees F (about 5 degrees warmer than room temp). When I walk outside to the heat pump the pipe going in is hot to the touch. To hot to keep your hand on it. So all seems well and good until about 4 or so minutes later. At that point the air suddenly starts blowing cooler out of the vent (about room temp air) and eventually the pipe outside that was VERY hot starts to cool down. Not cold like when the AC is on, but not hot at all.

My AC guy isn't sure what is going on. He has replaced the thermostat once already. The outside unit is a Trane. Before this repair, while the AC was not cold the heat worked just fine. Now I have a AC that is amazing and no heat. Is it possible the wires are incorrect in the thermostat? The replacement thermostat (Honeywell TH3210D) does not have the exact same wire labels as the old one (Simple Comfort SC2200). The old thermostat has

B O W2 G E Y1 Y2  

The new thermostat has


Unfortunately I don't have a picture of the old way it was wired, but the new one has wires going into


There are no jumpers.

Any ideas? I would think if the compressor was having an issue that the cold would not work either. It also is strange that when the heat first comes on it appears to work, but then stops a few minutes later.

  • What's the ambient outdoor temperature when you're running the heat?
    – Tester101
    Commented Apr 18, 2017 at 17:35
  • Hi Tester101, When testing today it was probably about 76 degrees F. A week ago when this all started the outside temp was in the 60's and still the same issue.
    – Blue
    Commented Apr 18, 2017 at 17:56
  • Did your A/C fellow do the install of the new parts? If so, keep him working on it until the heat works properly again. Sure looks like miswired thermostat, and the outside pipe temperature drops due to a local overheat-cutoff switch. Commented Apr 18, 2017 at 18:48
  • HI Carl, Yes, the AC technician did all of the work. He says he is going to ask around to see if anyone has any ideas on what might be going on. One thing that struck me as curious is that the thermostat has a wire going to E, but no wire going to AUX. I would have thought the opposite to be true in that the wire would go to AUX. Though I am completely out of my league in so far as understanding these systems. Just going crazy looking at youtube videos to try and help troubleshoot. Ridiculous I know. Would a stuck or bad reversing valve cause this issue?
    – Blue
    Commented Apr 18, 2017 at 19:05
  • What's the model number of the heat pump? When they replaced the coil, did they also replace the metering device (expansion valve)?
    – Tester101
    Commented Apr 19, 2017 at 0:45

1 Answer 1


You need to find a real heat pump service person. It's hard to trouble shoot a system without being there. Did he put in a correct refrigerant charge amount and the correct type refrigerant? Is the new coil the same size (BTU) capacity as the old coil? Does it have the same type expansion valve? Has anyone used gauges, if so what do the pressures do when the unit heats then begins to cool off? Does the compressor change it"s tune level? What happens to the temperature of both the small and larger line when this heating and cool off happens? It's not rocket science you just need to know what is happening with the system and what should be happening at any given time in the "on cycle".

  • If the cooling is working "amazing" as the OP said the charge and gas type are fine. Ether the thermostat is not connected correctly or the reversing valve is faulty.
    – Ed Beal
    Commented Feb 15, 2018 at 15:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.