The standard number of wires on a digital thermostat with a lighted LED display that controls a single-stage heating and cooling system is five. If your system has six wires, it's because it features second-stage heating, second-stage cooling or heat-pump cooling but not all three. The extra wire signals the additional function to come on. If your system has second-stage heating and cooling as well as a heat pump, then the thermostat needs eight wires, not six.
Thermostat wire colors are supposed to tell you which wire goes on which terminal, and in five-wire thermostats, the color code is fairly standard. The color of the sixth wire that controls second-stage heating or cooling or the heat pump reversing valve isn't standardized. However, if you follow the color code for the five basic terminals, you can choose any color for the sixth wire as long as it isn't already being used for another function. Orange and blue are two common choices; however, blue is the usual color for the common wire, or C wire, so if you use blue for the extra terminal, you should use black for the common wire.
Find a 6-Wire Thermostat Diagram
Before you hook up your thermostat, it's a good idea to check a six-wire thermostat diagram. You can find one online on a number of sites, or you can check the manual that comes with your new thermostat.
You'll find that five wires have well-defined functions. The white wire connects to the W terminal and controls the heat, and the yellow wire, which connects to the Y terminal, controls cooling. Attach the green wire to the G terminal to control the fan compressor. The red wire connects to the Rc and Rh terminals, which are usually bonded by a jumper wire, and the blue wire connects to the C (common) terminal. This red/blue wire pair provides the power for the system.
You have to know what extra function your system has before you can hook up the sixth wire. If it's second-stage heating, the wire goes on the W2 terminal. If it's second-stage cooling, the wire goes on the Y2 terminal, and if it's a heat pump, the wire goes on the O/B terminal.
Hooking Up Wires
Your wires may already be installed in the wall, but if they aren't connected to an old thermostat, you can't be sure if the wire colors follow the color code. The best way to check is to uncover the system control panel and note the colors of the wires connected to the terminals. You can switch them around if necessary or simply note the colors and duplicate the configuration on the thermostat.
If you have to run a new cable, use an eight-wire cable with 18-gauge, unshielded solid copper wires. You'll have two extra wires in the package, but that's OK. Just don't use them. It would be neater to use a six-wire cable, but these usually aren't available.
Turn off the breaker controlling the heating system before connecting wires to the control panel and transformer or to the thermostat. The electricity flowing through the wires is only at 24 volts, so you can't get a serious shock, but you can damage the system if you short wires while the power is on.
The Blue Wire Is Special
In a basic system, blue is the color of the common wire, or C wire, which provides a return from the thermostat to the system transformer. Black is also an acceptable color for this function, and if you use it for the C wire, you'll leave the blue wire free to connect to the O/B terminal, which controls the reversing valve on the heat pump.
However, the orange wire can also be used for the heat pump, so if the blue wire is already connected to the system transformer, you don't need to change it. Just hook it up to the C terminal and use orange for the heat pump. You can also use the orange wire for the W2 or Y2 terminal, which control second-stage heating and second-stage cooling respectively, although it's just as common to use brown for this purpose.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.