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Trying to help my dad install his new Nest thermostat - for some reason his thermostat has two wires going into the C terminal. I've searched and cannot find any other question about a setup like this.

I'm wondering what kind of system may have caused this?

My thought is to abandon the C wires altogether and just install the Nest without them, since apparently it's not required.

Thermostat wiring

  • One of the wires going into the C terminal is white -- what color is the other wire? Also, what do you have for an HVAC system? – ThreePhaseEel Dec 29 '18 at 14:21
  • Not at my dad's place right now so can't check the other color. It's a forced air natural gas furnace and central air. There's also an electric air cleaner with the furnace. Would it be wrong to pigtail the two c wires together? – Darrrrrren Dec 29 '18 at 14:47
  • The current setup is the equivalent of a pigtail. – bib Dec 29 '18 at 15:16
  • @Darrrrrren -- when you get back to your dad's place, can you check and see how many thermostat wires there are at the furnace? – ThreePhaseEel Dec 29 '18 at 15:19
  • @bib, yes, but the nest only allows one wire into each terminal. To get both of these in I'd have to pigtail them externally. – Darrrrrren Dec 29 '18 at 15:32

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