I want to add 5th wire to my HVAC control with the Add-a-wire gizmo. Conceptually I understand their instruction diagram, but not completely when it comes to my HVAC layout. The HVAC is single stage forced gas heat and AC, currently controlled by a 4 wire thermostat. All very standard. But what do I do about the yellow wire? On my unit instead of being nicely tied to appropriately labeled terminals, the compressor is fed directly from thermostat's yellow wire. I was hoping someone experienced with these systems can dumb it down for me.

HVAC diagram wires at the HVAC official add-a-wire diagram

  • This appears to be a splitter in a plastic box. Simply jumping The Y and G terminal on the control board will do the same thing.
    – Mnc123
    Commented Feb 19, 2015 at 2:24
  • @Minc123 I don't think that's correct. Wouldn't jumping Y and G mean you couldn't run the fan without turning on the AC? My impression was that the two accessories communicate to determine whether or not to jump the wires. If the thermostat only turns on the fan, then it would not jump and the fan circuit would be completed over the blue wire?
    – Jason
    Commented Sep 4, 2019 at 17:30

1 Answer 1


You'll start by wiring the furnace and A/C as follows:

  • Red from the furnace connects to Red from the thermostat cable and Red from the ADD-A-WIRE™ device.
  • White from the furnace connects to White from the thermostat cable.
  • Green from the furnace connects to Green from the ADD-A-WIRE™ device.
  • Blue From the furnace connects to White from the A/C condensing unit, Brown from the ADD-A-WIRE™ device, and Yellow from the thermostat cable (this is C).
  • Red from the A/C condensing unit connects to Yellow from the ADD-A-WIRE™ device.
  • Blue from the ADD-A-WIRE™ device connects to Green from the thermostat cable.

ADD-A-WIRE™ Furnace Wiring

Next you'll connnect the wires to the thermostat as follows:

  • Red connects to the R terminal.
  • White connects to the W terminal.
  • Yellow connects to the C terminal.
  • Green connects to the ADD-A-WIRE™ Diode 'Y'.
  • Yellow from the ADD-A-WIRE™ Diode 'Y' connects to the Y terminal.
  • Green from the ADD-A-WIRE™ Diode 'Y' connects to the G terminal.

ADD-A-WIRE™ Thermostat Wiring

WARNING: Modifying wiring for HVAC equipment could void the warranty, and/or cause damage, and/or injury, and/or death. Proceed at your own risk. Also, I don't know anything about the internal workings of this device. If this wiring causes any of the above mentioned problems, don't come looking for me. I'm just some random guy on the internet, who you chose to take advice from.

  • you are the best!! One followup question, am I reversing polarity on the A/C condensing unit with the new wiring? It seemed like existing red was ground and white was hot. Or is it 24vAC so doesn't matter? Thank you so much for posting an awesome diagram, which you are well known for :)
    – Elijah
    Commented Jan 20, 2015 at 17:13
  • The wires going to the A/C unit should just be used to close the contactor, so it shouldn't matter which is which. If you'd like, you can connect the white to the yellow wire from the ADD-A-WIRE™ device. Then you'll connect the red wire to blue, brown, and yellow bundle.
    – Tester101
    Commented Jan 20, 2015 at 17:46

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