Bumping this for help. I have two Trane XL 14i heat pumps that I just tried to upgrade to Nest thermostats so I could monitor over the internet. One seems to work correctly, the other is behaving oddly.
The existing thermostat was Trane TCONT800. Looking around on internet and looking at info from Ecobee's site, I should be able to treat the existing B wire as a C wire for Nest/Ecobee, and I should connect existing O to the O/B terminal. Snippet from Ecobee site:
Existing wiring with the Trane TCONT:
How I wired into each Nest:
Here are the software settings on the Nest that works correctly, O wire heat pump:
The other unit, if configured the same way, seems to cool when the thermostat is trying to heat. So I tried switching the software setting, heat pump to B wire, and then the unit doesn't cool when the stat is instructing to heat, it just blows fan, but still no heat. It doesn't make much sense to me that two identical builder-installed units would be wired differently in my house (one unit using O wire and one using B)? What should I look at next? (I have not gone to the air handler in the attic to check the wiring yet - I was hoping to avoid needing to go up there.)
I did not disconnect power when I swapped thermostats, but I was very careful to protect each wire and not short it. However, if I shorted a wire and blew the internal fuse in the air handler, would the symptom be the behavior I'm seeing? I can also try swapping the two Nest thermostats themselves to see if one is actually sending a different command.
I also read while Googling that even on heat, the heat pump will periodically run a cool cycle to defrost. Is it possible that I was in a defrost when I tested? If so, how long is a typical defrost cycle? (How long do I need to let it run before I'm sure it is/is not correctly heating?)