Venstar Venstar ACC0410 Add-A-Wire Accessory for Programmable Digital Thermostats

Adapt a 5-Wire Thermostat to a 4-Wire System: Allows 5-wire thermostats to use only 4 wires; Also allows 5 wires to work like 6 wires; Use it to fix a broken wire; Add cooling to a heat only system; Easier than running new wire