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I hopefully have a quick question. I am thinking of getting a Ecobee or Nest smart thermostat, but doing my checks/research before getting one for this house. I rent, so I can only do minimal changes.

The current system is a heating only system, and only has a W, G, and Rh wire connected (with a jumper between Rh and Rc) in the thermostat. The cable used has 3 other unconnected wires, Yellow, Blue, and Brown. Since I don't have a common wire, I was curious to see if any of the other wires happen to be connected. I used a voltimeter to check each one. Using the Rh wire as the pair.

If I connect the G and Rh, i get 27v. If I connected the Yellow, Blue, or Brown unconnected cable, I get 16 volts.

So I am curious, can any of these 3 wires be used as is for a common wire. They have voltage going through them, just not the full 24volts that a normal common wire has.

The furnace, is under the house in a very tight crawl space, and i have a very hard time getting close to it. I did see it was made by Bryant. I can get closer to it, but didn't have a lot of time making my way to it in a half crouch. It looked fairly newish. Definitely in the last 10 years, or at least looks like it. Obviously once I can get to the furnace, I can just see where the cables terminate. I believe its also a Gas furnace.

I just wanted to see if I could skip all that hassle and deduce everything from just the voltimeter readings.

Anyone have any ideas? I did a search, but none of the other questions were quite what I am asking for.

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    The 16V is meaningless phantom voltage caused by capacitive coupling. Blue is the one everyone uses. – Harper Sep 5 '17 at 18:39
  • @Harper thank you for the response, you were indeed correct. I went down there after all, and opened it up. the Blue/Yellow/Brown wires were not even connected. But the Blue wire was actually a lot shorter than the other wires at the furnace end and doesn't quite reach the terminals. I used the Brown wire instead (for Copper = C). I will label them accordingly at the thermostat with the stickers that come with the new thermostat (At least the Nest does when i set it up at my parents place). If you would like, you can answer the question, and I will give you the answer. – Daegalus Sep 5 '17 at 20:01
  • You need not check for voltage - check the other end of the wires - look at the blue wire on the other end - is it connected to anything. If yes - it is probably a terminal strip and it probably has a C label on the strip or the documentation for that furnace shows the terminal strip and declares which one is the common (C). Make sure you label BOTH ends of your wire - as the COLOR codes for these things Follows a set Standard. – Ken Sep 5 '17 at 23:54
  • Brown for C is the best choice. Y yellow is for AC compressor; B blue is for heat pump change over valve. – Stavr00 Oct 30 '17 at 18:20

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