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I have an new Emerson thermostat and would like to connect a c wire so it always has power. I have an extra wire that's not being used, but I don't have a C wire terminal on my furnace. I'm wondering if I can use any of the unused terminals or if I can tap into one of the wires coming off of the 24v transformer.

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The blue wire coming off of the transformer is what you want, but there might be a better place to tie into that. Look for any wires that are simply screwed to the metal chassis of the air handler. You can check with a continuity tester, but those chassis grounds are usually a connection for that other side of the transformer (the blue wire).

Looking at that schematic, I can see a few places where a wire terminates to "ground", but I don't necessarily see any terminals where the "C wire" would be conveniently exposed. Also, when you find a ground screw use a volt meter to make sure you get a good 24v there between the ground point and the red wire from the thermostat and/or transformer.

  • Lol I think I edited your answer instead of mine. – Joe Fala Apr 4 at 17:51
  • The B terminal already has a green wire on it. Can I just add another wire to that terminal? It will be ok? – Karl Apr 4 at 17:53
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The B terminal should be you common wire. It looks like you have air conditioning, one wire goes to the Y terminal and the other is common. Often marked B for heat pump applications.

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It's literally the same as the blue wire on the transformer as in JPhi1618 pointed out. Either spot will work the same. The B terminal is easier.

  • The B terminal already has a green wire on it. Can I just add another wire to that terminal? It will be ok? – Karl Apr 4 at 17:56
  • Fixed the edit... I believe you about the B wire, but that notation on the schematic is saying to "See note 5" on the right side of the label. The 5 being under the O and the B terminal is coincidence. – JPhi1618 Apr 4 at 17:57
  • There is a rectangle box on the left above all the text that lists "B/C" as a notation for Common, so makes sense. Voltage and continuity test can confirm if there is and doubt. And @Karl, yes, two of the same size wires under that screw terminal is fine - typically one on each side. One wire being larger than the other can cause clamping issues on some terminals. – JPhi1618 Apr 4 at 18:05
  • You guys are awesome. I'll try the B terminal tonight and let you know. Thanks! – Karl Apr 4 at 18:52
  • I finally got time to do this yesterday and it worked beautifully. Thanks again! – Karl Apr 8 at 19:44

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