If the wiring for your room thermostat needs to be replaced, this quick guide should help you figure out how it's done without taking the chance of making serious mistakes. This article explains the color-coded wiring layout for both 2 and 3 wire systems.
Two Wire Systems
Two wire systems generally have one red and one white wire.It is especially important that these wires are connected to the correct location, as hooking them up backwards could result in burning out sensitive system components.
Red - The red wire is the "hot" wire. It is the wire that supplies power to the thermostat, allowing it to operate. In a two wire system, the red wire is also redirected to form the power control for the fan motor. The red wire connects to the R connection on the thermostat, so all you have to remember is that R stands for "red."
White - If R is for red, then W must be for white, and that is where the white wire will connect. On a 2 wire system, the white will be responsible for creating a circuit when the temperature triggers a change. On 2 wire systems, the thermostat simply turn the unit on or off, and the fan motor operates in tandem without an individual controller circuit.
Three Wire Systems
Three wire systems are more common today than a few years ago. As with a 2 wire system, the connection are labeled according to which color wire connects to it. There may also be a fourth wire, labeled C, and we will talk it about it in a moment. For now, let's look at the wires which make it all work.
Red - Red is the universal color of the hot wire, or the one which the power enters the unit through. It should connect to the R label, just as before.
White - The white wire connects to the W connector. In a 3 wire system, the white wire controls the heating circuit. If the heater is not operating correctly, the white wire may have a bad connection, or no connection at all.
Blue or Yellow -The blue wire is the wild card. On some systems, this will be a yellow wire. On most thermostats, the connector it attaches to is almost always labeled Y for the yellow wire. In some rare cases, the connector may be labeled with a B, but this is not a common occurrence. The blue and yellow colors may be interchanged, according to what is available. These two wires serve the same purpose, and only vary according to manufacturing designs.
The G Connector
If your Thermostat has a connector labelled as C or G, it is intended to allow a common ground to be connected. Thermostats that operate with battery power will not usually have a common ground. If there is a C connector available, it should be connected to the common ground of the electrical circuit, which is usually a black wire, but may also be green on some models.