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Trying to add a C wire for a new Nest thermostat. Currently only have a Red and White wire. There are no other terminals available for the C wire on the furnace transformer. Red go to one end of the transformer and the white one come from the other transformer terminal that went through the relay. Thanks in advance

Joseph
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  • Do you have only heating or only cooling (AC)? Thermostats that control multiple systems have a wire per system. The 2-wire you're describing sounds like a single system, possibly humidifier or doorbell. See the various other C wire questions here for more info. – Fredric Shope Nov 22 '21 at 21:32
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    What make/model is your furnace? Can you post photos of the wiring at both the furnace and the thermostat ends? – ThreePhaseEel Nov 23 '21 at 04:41
  • https://diy.stackexchange.com/questions/10482/how-can-i-add-a-c-wire-to-my-thermostat The answer on post has a lot of information that you're looking for. – brett_x Nov 26 '21 at 18:22

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If your furnace does not have the C wire from the control board, you may want to consider just buying a transformer and powering your Nest from it and using the Red & White to send the control signals:

https://sensi.emerson.com/en-us/support/adding-a-24-vac-external-transformer

Essentially all this talk about the C-wire is running a 24 VAC power line to the Nest Thermostat so it can charge the battery and keep running.

If your furnace control board doesn't have those lines, you can create external power to the Nest (using the external transfomer) and drive the control lines individually.

Leroy105
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