Where can I connect the C wire on the A/C side?

Question

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

Here’s the photo of the new thermostat wall plate. Labels are:
C = common
R = red
W = white
Y = blue
G = green

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

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.

Answer 1

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

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

• 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.

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.