We just got a new WiFi capable thermostat that, unfortunately, seems to use a form of power stealing in order to charge itself. The problem here is that our house is actually quite well insulated; the unit doesn't run for very long, so the thermostat keeps turning off the WiFi as the battery falls too low.

When I first installed the new thermostat, I found the blue wire that was listed as likely being the common wire; however, when I connected it, the thermostat indicated that there wasn't any power and prompted me to remove the wire. So I went up to the furnace, hit the breaker, and popped open the panel. I can see the blue wire appears to just be wrapped around the outside of another wire rather being connected to the furnace.

So... we have this confirmed: Yellow wire for cooling, white wire for heat, green wire for the fan, and the red wire for power. I can only confirm because the thermostat, for all the WiFi woes it seems to have, is working to control the temperature just fine right now.

Here is a picture of the wiring at the furnace; is this enough for anyone to tell me where I should be connecting that blue wire, or should I go ahead and call an HVAC technician out?

Picture of wiring at furnace

1 Answer 1


First of all, don't make any assumptions about wire colors. There is no standard, everyone does it differently.

Secondly, there is a very clear C terminal in your picture, at the bottom right of the control board. It also looks like you have an air conditioner connected.

enter image description here

This "blue" wire you're talking about looks like it's not connected at the board. If my assumptions on the wires are correct, then all you need to do is connect the blue wire to the "C" terminal, and you'll have a C wire at the thermostat.

If I'm wrong about anything, or just for more background, you can refer to our go-to question: How can I add a "C" wire to my thermostat? (or read any of the other questions with the tag).

  • I knew what the wires at the thermostat did, but I was worried about the wiring at the unit itself. I'm not all that knowledgeable about electrical wiring (any wiring really) so I didn't want to go pugging things in without asking about it. Your arrows on my picture actually really helped me better visualize what was going on (I was confused by the extra white/red wire but it makes sense now). Thanks a lot for the quick answer. Plugged it in, added it to the thermostat, turned the unit back on, and it detected constant power right away. Thanks!
    – Michael J.
    Jan 10, 2014 at 1:19
  • 1
    Don't forget to insure that the transformer has the capability to supply the newfangled thermostat.
    – Tester101
    Jan 10, 2014 at 11:02

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