Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I’m trying to replace my thermostat with the Honeywell Smart-WiFi thermostat. Quick summary, I have an extra yellow wire which I’m making the C wire, but its not connected at the other end and I’m not sure where it should go.

This picture is with the original connections to the previous thermostat. There is no C wire. Labels are:
W = white
Y = blue
G = green
R = red
yellow wire left over, not connected
enter image description here

Here’s the photo of the new thermostat wall plate. Labels are:
C = common
R = red
W = white
Y = blue
G = green
enter image description here
I’ve since removed the R to Rc jumper loop.

Of the 5-wires that lead from the thermostat, on the A/C end they connect as follows:
thermostat blue connected to small white to small green to thick brown w/ a "C" label on it.
thermostat red connects to thicker red wire w/ label "R"
thermostat white connects to thicker white wire w/ label "W"
thermostat green connects to thicker green wire w/ label "G"
yellow is left unconnected
enter image description here

Just a note, I'm getting the feeling the blue wire is actually common because it's connected to another incoming wire that is labeled "C", but on the thermostat end, it's labeled "Y". Should I maybe just pair the yellow to the same wire labeled "C" near the A/C end?

Any help much appreciated.

share|improve this question
Trace the cable in the last photo, and see where the wires terminate. Nobody can tell you what's what, just by looking at the cables. – Tester101 Aug 10 '14 at 3:32
I did, thats why I wrote where they terminate just above that last photo. Hence the question below the photo about why is "Y" at the thermostat connected to "C" on the other end. – Ender Aug 10 '14 at 5:20
I understand you followed the thermostat wires to that junction, but now you should follow the other cable (the one with the wire labeled 'c') and see where that goes. Eventually you'll get to a terminal block, or some other device. Then you'll have a better understanding of the circuit. – Tester101 Aug 10 '14 at 13:19
up vote 0 down vote accepted

After looking closely at the images you've supplied, this is what it appears is going on in the last photo.

enter image description here

From what I know about HVAC systems, I would guess a broader view of the system would look something like this.

enter image description here

  • The transformer supplies power to the thermostat through the RED wire.
  • The BLUE wire serves as the COOL call from the thermostat.
  • The BLUE wire connects to the WHITE wire, which is connected to the coil of the contactor in the condensing unit.
  • The GREEN wire is the other side of the contactor coil.
  • The GREEN wire connects to the BROWN wire, which is the other side of the transformer (C).

When the thermostat calls for COOL, the BLUE wire is energized. This energizes the WHITE wire leading to the contactor, which pulls the contactor closed. The GREEN wire connects to the BROWN wire, which leads back to the transformer, thus completing the circuit.

Therefore, if you want to use the YELLOW wire as C. You should include it in the twist-on wire connector with the GREEN and BROWN wire.

enter image description here

You'll just want to make sure the transformer is sized properly to supply the additional current draw of the thermostat.

If it were me, I'd rewire it so that the YELLOW wire was the COOL call, and the BLUE wire was C. As that's a more common color coding.

enter image description here

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.