Adding Venstar Add-a-wire to HVAC
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.




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.




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.