Replacing a thermostat that has D, R, B/C wiring

[gbergstedt]

I have two Trane Gas Furnace / AC units and two Trane XL850 thermostats.  I also have a whole home humidifier on each of the systems that are controlled by the Trane XL850 thermostats.  The wires that are used are D, R, B/C; can I use a Net Learning Thermostat with them?

[Markjosephp]

Hello gbergstedt,

Thanks for reaching out. It sounds like you have a well-equipped HVAC setup with thermostats controlling whole-home humidifiers. When considering compatibility with a Nest Learning Thermostat, it's essential to check the wiring configuration.

The wires you've mentioned (D, R, and B/C) are typical HVAC wiring designations. However, the compatibility of a Nest Learning Thermostat depends on the specific wiring requirements of the thermostat model you're considering. There should be a wire for both of your heating and cooling systems. To have a visual, can you share with us a photo of your existing thermostat's wiring setup? Also, a photo of your system's control board.

I'll look forward to your response.

Best regards,
Mark

[gbergstedt]

Thanks for getting back to me on this.  I have two systems both are Trane and both have a Trane XL850 Smart Controller. 

Here is the wiring for System 1 (Main Floor)

Here is the wiring for System 2 (Upstairs)

[zoeuvre]

Hi folks,

@gbergstedt, chiming in—the D terminal on your current thermostat could either be a dehumidifier wire or an incompatible wire, which is considered a proprietary system. However, regardless of the D terminal being a dehumidifier or proprietary system, a pro installer is still recommended for it to be compatible with Nest Thermostats.

Thanks,

Zoe