C wire for a smart thermostat

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.

You might be in luck, but it depends on how the Honeywell thermostat works. I know Ecobee3 lets you power the thermostat using the Rc and C terminals, but I'm not sure if Honeywell does the same. However, this solution might still work, even if the Honeywell is not the same as an EcoBee3.

If you look at the documentation for the boiler, you'll see this.

Which shows that the boiler exposes additional wiring, that's intended specifically for powering a thermostat.

You can see these two wires in the photo of your boiler.

If the Honeywell can be powered using Rc and C, then here's what you'll have to do.

1. Install an 18 AWG (at least) 2-wire (at least) cable between the boiler and the thermostat(s).
2. Connect one of the wires to the wire labeled R in the boiler (as highlighted above).
3. Connect the other wire to the wire labeled C in the boiler (as highlighted above).
4. At the thermostat, remove the jumper between R and Rc.
5. Connect the wire that is connected to the R wire in the boiler, to the Rc terminal on the thermostat base.
6. Connect the wire that is connected to the C wire in the boiler, to the C terminal on the thermostat base.
7. Connect the other thermostat wires as they were on the old thermostat.

If the Honeywell thermostat cannot be powered using Rc and C, then you may still be able to get it working. Follow the same procedure as above, but do not connect the wire between the R wire in the boiler and the Rc terminal at the thermostat. Also, leave the jumper in place between R and Rc. This may or may not work, depending on how the system is currently wired.