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How do I use the green wire as a C wire as suggested in most installation instructions for the newer wi-fi thermostats? They all reference terminal strips at the furnace, all I have is the 3 wire nuts on a older Lennox ES5 series. Thank you.enter image description here

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    Can you post the wiring diagram for your furnace and/or a model number? – ThreePhaseEel Dec 26 '17 at 2:49
  • There isn't a cut-off wire hiding in that thermostat cable of yours I take it? – ThreePhaseEel Dec 26 '17 at 4:25
  • Also, do you have a full wiring diagram for the innards of your unit? Something's fishy here -- it looks like you have an electrically heated AHU with a heat-to-fan relay in it?! – ThreePhaseEel Dec 26 '17 at 4:27
  • No 4th wire. Thats all I have for diagrams. Its all original as installed in 1976. Guessing I will have to run another wire as a C-wire, still not sure where to connect at the furnace end. – Paul Dec 26 '17 at 4:38
  • can you replace the existing thermostat cable? – ThreePhaseEel Dec 26 '17 at 5:11

It would appear from the pictures that the blue and orange wires (wire nutted to the red thermostat wire) should go back to the transformer for one of the two 24 volt wires. The other 24 volt wire on the transformer is the common to hook up to the thermostat. There should be 2 other wires that are line voltage coming out of the other end of the transformer. Don’t mess with those. Depending on the transformer there may be multiple taps, without seeing the transformer it would be difficult to tell which ones to use unless the transformer is marked.

Great background on thermostat wiring is at: How can I add a "C" wire to my thermostat?

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  • That appears correct. Thank you. The black wire that the green is currently connected to goes to the blower relay. Can this wire be disconnected? Is the blower going to function correctly? I do not care about being able to manually run the fan. – Paul Dec 26 '17 at 18:15
  • Got it all figured out, thanks for the help. I ran the green to the common side of the 24v transformer (yellow wire seen to right of wire nuts), ran the black wire from the blower relay (previously connected to green) to the white thermostat/red relay signal wire. Everything seems to be working perfect. – Paul Dec 26 '17 at 21:21

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