Venstar Wireless Add-A-Wire Thermostat AccessoryThe Add-A-Wire can be used in applications where additional wiring cannot be run, the Add-A-Wire accessory can be used to add a wire to the thermostat. The accessory is also used to fix a broken wire, to make one thermostat work like two, add cooling to a heat only system, or add a common wire to a 4-wire system (the green and yellow wires of the Diode 'Y'may not be connected to the R or C terminal of the thermostat). Features: Easy to install Easier than running new wire Allows 5 wire thermostats to use only 4 wires Also allows 5 wires to work like 6 wires Replaces: Totaline P474-0410, Carrier 33CSACC-AW, Bryant TSTATBBPACC-AW