Thanks, Mike... I was on the phone with Nest last night. They told me the XV80 is not compatible, to which I replied that I already have one connected. I'll dig into the C wire question... Thanks for the insight.

I just noticed you have a black wire going to the B terminal on the thermostat, but I don't see where it goes to the control board. Is this furnace connected to a heat pump?

Did the other thermostat have a C wire?

No heat pump, just a gas furnace. Yes, the other XV80 has a C wire, though I will add that the Nest on that unit draws power from the Rc wire. Here's a better shot of the panel that shows the "B" connection on the far right. The blue wire is running to the compressor, FYI.

Do you have the manual for your Honeywell thermostat? If yes, what is the B terminal? Normally it is the heat pump control.

The B terminal on the thermostat is connected to a black wire. The the B terminal on the control board is blue. Have you figured that out?

I looked at XV80 documentation I found on line. I see a terminal on the control board labeled B/C. The note says it is the common connection. You should be able to find the the product data document if you search the model number. You may be able to use the B wire for the C connection. You need to be careful and not fry your new thermostat.

The R and Rc (normally tied together) is the 24 V supply. The C is the ground. The WiFi radio draws most of the power. If there is no ground then the 24 V circuit is not complete. The Nest will then power itself with the internal lithium battery which lasts about a day.

Mike... Thanks. It's hard to see in the photo, but the black wire is in there, connected to the B terminal and running to the thermostat. The blue wire is running to the compressor... I *believe* it's substituting for the black wire, reasons unknown. You can see the unused black wire wrapped around the sheath near the center of the photo. We just bought this house, so everything is a mystery for now.

The thermostat is an XT302C, manufactured by Honeywell, but branded as Trane. I imagine it was provided with the XV80 on installation. See the photo below for the wiring hookup schematic in the manual.

Thanks for the insights... Based on what you say, I'm not clear what's happening with the XV80/Nest combination that IS working... I'll need to go home and take a closer look. The Nest battery dies in about 5 minutes, so it's clearly drawing power from the line.