My 2nd generation nest didn’t need a C wire but I think the new Nest thermostat is requiring a C

[Etrotochaud]

I had a second generation Nest Thermostat and was forced to upgrade to the new Nest Thermostat. My old wiring did not require a C wire and it seems like this new one is prompting me to have a C wire. Is this correct or is there a way to figure out if something else is wrong?

[Aurora_Webb · Accepted solution]

Yes, that’s correct. The new Nest Thermostat (2020+ models) often requires a C (common) wire to provide continuous power, especially if your HVAC system can’t supply enough power through the existing wires.

To check:

1. Remove the old thermostat and look at the wiring.

2. See if a C wire is present. If not, the new Nest might prompt for it.

3. If you don’t have a C wire, you can:

   • Use a Nest Power Connector (sold separately) to simulate a C wire.

   • Or have an electrician/installer run a C wire from your HVAC control board.

Without a C wire or power adapter, the new Nest may struggle to charge or function reliably.