• Ecobee thermostat
  • Lennox setup (furnace, heat pump, indoor coils)
  • Installed for ~6-7 months now


Setting the Ecobee to Heat for 66F (not aux - just using the heat pump) turns on the heat pump and for ~5-10 minutes I get warm air through the vents in the house (72-73F air temp as measured by a laser thermometer). The unit outside has the fan spinning and all seems to be going great. I can also hear a sound from the indoor coils (located below the furnace) as if it's a refrigerator running, so assuming things are normal. Also, one of the connection valves (the one out of which I have a black insulated pipe going out) at the bottom of the outdoor unit reach ~130F (also measured with laser thermometer).

Once 5-10 minutes elapse, I can see that the outdoor unit fan is still spinning, but now the vents in my house register ~59-61F, effectively cooling the house more than actually heating it. The valve I am talking about above now went from ~130F to ~45-50F. The indoor coils no longer make the refrigerator-like noise, but the fan in the furnace is still working.

The Lennox diagnostic codes (LEDs inside the furnace) show E312 C2 A1600, meaning that likely:

  • E312 - restricted airflow. I replaced the filter, but the problem is still there. Tech on the phone said not to worry about that if I replaced the filter.
  • C2 - cooling stage 2.
  • A1600 - fan operating at 1600 CFM.

After ~20 minutes of blowing ~59-60F air through the vents, the temperature starts to gradually go up to ~62-63F (also through the vents).

NOTE: What I noticed is that overnight the outdoor unit stopped spinning altogether while the fan from the furnace was still blowing cold air. Setting Heat from Ecobee now had no effect at all (outdoor unit doesn't come on) while the fan was still working. Power cycling the house (turn off at fusebox) allowed me to re-start the outdoor unit (likely in lockout state - haven't verified).


What could be causing this issue? What can I do to diagnose the issue before calling a tech?

  • 1
    I take it the Ecobee is a new addition since last winter? Commented Dec 31, 2022 at 23:13
  • 1
    What is the outside temperature? If it's below about 35F, heat pumps are typically set to switch to aux heat. If it then isn't switching to aux, you won't have a heat source. Various problems can result in it not switching to the aux heat source.
    – fixer1234
    Commented Dec 31, 2022 at 23:19
  • @ThreePhaseEel nope, this came with the installation. This is a brand-new install.
    – Den
    Commented Dec 31, 2022 at 23:37
  • 1
    The fact that it works for a while and then stops working suggests it could be a bad control board. Something like a marginal component or bad solder joint that fails after it warms up. Sounds like it probably needs to be diagnosed by a tech.
    – fixer1234
    Commented Dec 31, 2022 at 23:58
  • 1
    If you can go back in time, you'll have yourself a working heat pump. :-)
    – fixer1234
    Commented Jan 1, 2023 at 0:07

2 Answers 2


My guess

the diverter valve (outside near compressor) coil fails and falls back to cooling mode. It needs continuous valve coil energized to stay in heat mode.

To check verify that the 24 volt is still going there from the thermostat.

On my Nest that would be the O/B labeled wire.

If yes, you need to check that the coil valve is working on the outside (24 volt), then switch it on and off to hear the valve working.

The coil part can be replaced without venting the coolant.

For clear communication purpose stop calling it heat pump. It is the compressor that cools or heats depending on the reversal of the coolant flow by the valve I described.

It looks something like this


  • Thank you for the thoughtful comment, @Ruskes. What's the best way to verify that 24V is still going there? Is it basic continuity of the wire from the thermostat to the compressor? For switching on and off - should I just stay outside while someone flips the thermostat in the heat mode to hear the valve working?
    – Den
    Commented Jan 1, 2023 at 0:25
  • 1
    On the thermostat use multimeter and check if there is 24 volt on the O/B, then do the same outside on the valve coil. Yes if outside have someone operate the thermostat or just disconnect/connect the O/B wire on it
    – Traveler
    Commented Jan 1, 2023 at 0:27

As it turns out, the issue was caused by a faulty thermostatic expansion valve (TXV). Due to a mechanical failure it was causing pressure build-up that then caused heat pump lock-out.

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