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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?

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marked as duplicate by ChrisF Apr 30 at 8:29

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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.

enter image description here

This is how I'd wire it up.

enter image description here

  • 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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