Installing a thermostat is more difficult if you are not familiar with the color codes of thermostat wire. These color codes are vitally important in connecting the right wires to the corresponding terminals. Failure to properly link these can lead to failures in the system, including short circuits, which can lead to fires. Therefore, taking the time to learn all of the possible combinations of wires and colors is essential to keeping the family safe.
While most people use the same basic code for each device, you should be aware that some people may have rewired their circuitry, or built their own devices. If that's the case, these codes are no longer applicable to your home wiring. If you're in doubt, check how the system is wired in your house before adding new features.
The Basic Colors
The most essential colors are red, green, and yellow, although you may find up to eight different wires in a heat pump thermostat, or as few as five in a more common hot water system. The red wire is connected to terminals beginning with the letter R, which include R, RH, and RC units, depending upon how the thermostat is being used. This terminal provides the connection between electricity supply and system circuitry, so it is vital that this wire is connected correctly.
The yellow wire is often seen in air conditioning units and attaches to the compressor. Yellow wires connect to the Y terminal and the wire is sometimes spliced, being connected to a compressor and the condenser, although this is not necessary. Follow the guidelines on your device for the correct terminal connection method.
The green wire connects to the G terminal and is used to control fan relays, triggering the blower fan. This system can be triggered by programming the thermostat to reach a certain temperature. Once this degree is reached, the G terminal will be given power, causing the blower to start up.
The Other Colors
There are a number of other colors that are used in different systems. These are not likely to be found in every thermostat, so check with your manufacturers' instructions.
The white wire connects to the W terminal and triggers the heating. The W terminal will link directly into the furnace or boiler. There may be a W2 connection, which links to an auxiliary form of heat.
The black wire may be linked to the C terminal, which controls the transformer. This is most often seen in electronic thermostats where only 24 volts are required. C terminals are connected to the common wire, which is most often black, but can also be other colors.
Dark blue wires connect to the B terminal, found in heat pumps, although this may be an orange wire connecting to the O terminal in some makes. Most brands use the O pump, as this connects to a valve used to trigger cooling.
Other wires may be auxiliary power supplies, which are linked to the Aux terminal, or E wires, which link to the E terminal and provide emergency heating.