I an hoping to install a new thermostat that requires a C wire. I pulled off my old thermostat, an old round dial type, and it was of course only connected to four wires. The four wires are the only wires in their sheath, but there is another set of 3 wires that were just hanging loose in the wall. I'm guessing they won't help me connect my thermostat, but I'm curious what exactly they are? wires behind my thermostat

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    Can you post a photo of the wiring at the furnace/air-handler end? – ThreePhaseEel Apr 21 '18 at 22:37
  • extra wires maybe ... were is the other end? – jsotola Apr 22 '18 at 0:18
  • User76730 answer is correct in my opinion. The good news is that if a new wire was fished in after the fact it means that the new wire is not attached inside the wall— using it to pull another one with yet more conductors shouldn’t be hard at all. (Of course that’s only a prediction—if the wire runs thru the basement ceiling and they later finished that area it may not be as easy..you should study the wire route) – Tyson Apr 22 '18 at 11:58

They are very old wires with cloth sheathing. My guess is the house did not originally have air conditioning and they needed one more wire so they just pulled an entirely new cable. If you were really curious you would have to trace them out to either the attic or crawl space. I would bet the other end is dangling near your furnace if they didn’t move it when installing air conditioning.

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