Nest with 2 c wires

[Amie90]

I’m wiring a Nest Learning Thermostat in place of an existing Honeywell. We have an A/C and boiler system. The existing thermostat has two wires connected to the same C terminal but I’m unable to get both of them in the C terminal of the Nest. Which one should go in C and what should I do with the other wire?

 

[Patrick_Caezza · Accepted solution]

It looks like you have a 1-stage heat unit and a separate 2-stage cool unit.

You should have two separate cables going to your thermostat.

One from the heat with its wires connected to terminals C (grn), R (red), and W (wht).

The other cable will be from the cooling and will go to terminals C (brn), Rc (red), Y (yel), G (grn), and Y2 (blu).

If I am right then you don't need the green (C) wire from the heating unit and you can just tape it off.

Try this wiring

Green - C - not used

Brown - C to C

Red - Rc to Rc

Red - R to Rh

White - W to W1

Yellow - Y to Y1

Green - G to G

Blue - Y2 to Y2

Be careful and do not mix up the two red wires.

[Amie90]

Thank you so much,  that worked perfectly!

[MelbaDT]

Hey folks, 

Thanks again Patrick_Caezza for your response. 

Amie90, we're glad to hear that you've got it working by following Patrick's suggestion. Feel free to let us know if there's anything else. 

Best, 

Melba

[aatienza]

Hey Amie90,

Chiming in to check if you have additional questions or concerns as we will be locking this thread within 24 hours. 

Let us know so we can assist you right away.

Thanks,

Archie