Adding Venstar Add-a-wire to HVAC

thermostat-c-wire

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

Best Answer

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.

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