Wiring difference

[Francispierre]

Hi, my home has two furnaces and AC systems. I've had a Nest thermostat on the downstairs one for about a year and want to add another to the upstairs. However, the installation app says the wiring is not compatible. I don't understand how that can be the case in the same house that was built and wired all at once. Any suggestions? The labels I have upstairs are RC, W, RH, B, O, Y, and G.

[luciaaa]

Hi, @Francispierre,

Thank you so much for posting in the community. I’m happy to assist you with your wiring configuration.

Based on the information you've provided, your system appears to be compatible with Nest thermostats and has the necessary wiring. To determine the specific compatible model, please answer the following questions:

• Is there a jumper from Rh to Rc?
• Do you have a conventional or heat pump system?
• Could you please share a picture of your wiring configuration?

Additionally, the B wire typically connects to the O/B terminal on the Nest; it's possible it's actually a common (C) wire instead and should be connected to the C terminal.

For identifying if your B wire is a common wire (C wire), you can refer to this article: How to tell if a B wire is really a common or C wire.

If a common wire is inserted into any connector other than C on your Nest thermostat, you risk blowing a fuse on your HVAC system and/or damaging the thermostat. If you're unsure about the type of wire you have, it's strongly recommended that you contact a Nest Pro technician. 

Let me know if you have any other questions.

Best regards,

Ana