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There is a very old gas furnace in our attic. It has a two-wire transformer and no obvious terminals to expand.

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I need to run a C wire to my nest thermostat, which I'm planning to do via their nest power connector so I don't have to refish any thermostat wire.

Given the wiring diagram and available transformer terminals, where do I tie in? Just figure out where the hot 24v wire is and add the connector to that?

justinbelcher
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1 Answers1

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With your current gas valve, it's the terminal not labeled "THERM"

Generally speaking, 3-wire gas valves, like your current ones, have their terminals designated as such:

  • TH -- "thermostat", for the W wire controlling the gas valve
  • TR -- "transformer", for the return wire to the transformer
  • TR/TH -- "transformer/thermostat", a spare terminal for the power supply from the transformer to be passed thru to the 'stat

Your gas valve is labeled somewhat differently, but it's clear where the thermostat hooks up -- the two terminals labeled THERM. As a result, those can be treated as R and W, leaving the other terminal as a suitable C connection by process of elimination.

ThreePhaseEel
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