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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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  • check out this Q&A https://diy.stackexchange.com/questions/10482/how-can-i-add-a-c-wire-to-my-thermostat – pmont Mar 14 '22 at 20:02
  • Additionally, instead of adding one wire for the C, it'd be better future-proofing to add an 18/5 cable. Use the old 2 wire cable to pull the new 18/5 through. – pmont Mar 14 '22 at 20:06
  • Thanks @pmont, I couldn't quite grok how to apply the thorough discussion in that link to my scenario. Given that I only have two terminals coming out of the transformer, are you suggesting I run new wire w/ several wires attached to one of the transformer terminals? – justinbelcher Mar 14 '22 at 20:39
  • Yes new 18/5 cable to replace the existing 18/2 cable. 2 of the 5 wires in the cable will connect to where your existing thermostat wires go. Red for the heater, White for the switched side to complete the circuit. It may not matter which is R or W since they both go to the 2-pole gas valve (if I'm reading that diagram right). Use blue or black for the common wire and connect it directly to the low side of the transformer (basically bypass the gas valve). Which leg on the transformer may need experimentation. It shouldn't hurt if you put it on the wrong one. The thermostat just won't power up. – pmont Mar 15 '22 at 01:27
  • Use a multimeter to be sure you're connecting to the 24 VAC side, not the 120 VAC from the breaker. As long as you're connecting to the 24 VAC side, your thermostat will be fine. – pmont Mar 15 '22 at 01:27

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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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  • That makes sense. I'm assuming the wire currently connected there is from the transformer? I saw some info about 3-wire valves having a "dummy" terminal, is that the same thing as the TR/TH? – justinbelcher Mar 15 '22 at 04:29
  • @justinbelcher -- yeah, the wire currently connected there should come from the transformer, and yeah, TR/TH is that "dummy" terminal – ThreePhaseEel Mar 15 '22 at 11:45