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I'm replacing an old thermostat from the early 90's with a new wifi thermostat.

When removing the old one, it only used R,G,Y,W wires, and the new wifi thermostat requires a C.

After fully removing the old thermostat, I saw there were wires tucked away in the wall. One was orange and the other blue. I'm assuming (just based off pictures) the orange is the C, but what could the blue one be?

Also, is there a way I can test the wires to determine which is a C? As of right now, I have the power turned off at the breaker for both the furnace and AC.

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  • Can you find where that bundle ends up in the furnace control box and see where they are connected? – ratchet freak Aug 28 at 15:33
  • @ratchetfreak I'm going to be honest, I've never done anything HVAC related. I'm not 100% sure where my furnace control box is. I opened the side of my furnace (where the wiring and the blower is) but I didn't see anything that resembled these wires coming from the wall. – Timmy Aug 28 at 15:54
  • Blue is the preferred wire color for C. A thermostat C wire is a totally new requirement, so a random 10 year old furnace won't have it wired up at the furnace end. You'll have to connect it there too. – Harper - Reinstate Monica Aug 28 at 16:23
  • Can you post a photo of the wiring inside your furnace/air-handler? – ThreePhaseEel Aug 28 at 23:21

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