I only have four wires: Red, Blue, Yellow, and Green. My old thermostat jumps the R to RC and leaves C empty.

I have a new thermostat that wants 5 wires, but I don't have C. I am not sure what the circuit diagram is here, so I don't know if it's possible to make a new configuration that will give full functionality.

Here is my configuration:

Rc (jumper to red) R Red Y Blue C - W White G Green

What does add-a-wire do anyway? There is no extra wire so I would have to tear up my wall to run a new wire. I don't want to sacrifice independent fan control. What is the easiest way I can rewire this for a new thermostat?


1 Answer 1


According to the ADD-A-WIRE™ Owners Manual, you should be able to use the product to achieve your goal. I've never used this product, and do not endorse it in any way. I have no idea if it actually works, I'm simply going on the claims made by the manufacturer.

This is the diagram from the manufacturer.

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This is how I'd wire it up.

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  • R and W stay the same, at both the furnace and the thermostat.
  • Switch the BLUE wire to C, at both the furnace and thermostat.
  • Connect the ADD-A-WIRE™ device in the furnace as described in the installation instructions, but connect the BLUE wire from the ADD-A-WIRE™ device to the GREEN wire in your cable.
  • Connect the BLUE wire from the ADD-A-WIRE™ Diode 'Y', to the GREEN wire from your cable at the thermostat.
  • Finally, connect the GREEN wire from the ADD-A-WIRE™ Diode 'Y' to the G terminal of the thermostat, and the YELLOW wire from the ADD-A-WIRE™ Diode 'Y' to the Y terminal of the thermostat.

WARNING: Use of this product is at your own risk. I have never used this product, and will take no responsibility for any damage it may cause. Be sure to read all manufacture documentation before using this device.

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