To replace a three-wire thermostat, connect each of the three wires to the right connection. The three wires are red, white, and blue or yellow, depending on the manufacturer. If they are connected incorrectly, the thermostat can burn out.
Red is the universal color indicating a hot wire. When replacing a three-wire thermostat, connect this wire to the R connector. This wire allows power to enter the unit. Connect the white wire to the connector labeled W. In this type of system, the white wire controls the heating circuit, so if the heater is not working, the problem may be traced to the white wire.
Connect the blue or yellow wire to the B or Y connector, depending on the type of system. These colors are interchangeable; they serve the same purpose and only differ based on manufacturer design, with yellow used more commonly than blue.
If you find a connected labeled G or C, the thermostat has an area where a common ground can be connected. Battery-operated thermostats do not typically have a common ground, but if you find one, connect it to the electrical circuit's common ground. In most cases, the ground wire is black, but it can also be green.