How To Install A 6-Wire Thermostat

The standard number of wires on a digital thermostat with a lighted LED display is five. This would be a non-battery-operated unit that controls a basic furnace and air conditioner. If you need to refer to a six-wire thermostat wiring diagram to connect your HVAC system, it's because the system has one of the following features: second-stage heating, second-stage cooling, or a reversible heat pump. If your system has all of these features, then the thermostat needs eight wires, not six.

The thermostat wiring color code, which tells you which wire goes on which terminal, is fairly standard in a five-wire thermostat. However, when you add a sixth wire, the color for that wire isn't universal because it could control second-stage heating, second-stage cooling, or a heat pump reversing valve, and the standard color for each of these functions is different. You'll probably have a choice of wires because wire bundles with six wires are rare; the most common wire bundles have eight wires, which means you'll be choosing one wire and leaving two of them unused.

There's nothing stopping you from choosing any color for the sixth wire as long as that color is connected to the proper terminal in the HVAC control board. There may be some confusion about the blue wire, though. Blue is the usual color for a C-wire, or common wire, but it can also be used to control a heat pump reversing valve. When blue is chosen for the heat pump, black is usually used for the common wire. It is always a good idea to take note of the wiring on paper, or simply take a picture of the old wiring.

Before a homeowner DIYs a thermostat installation, it's a good idea to check the wiring diagram in the owner's manual that comes with your new thermostat. Here are the most common wiring configurations.

Four-Wire Thermostat Wiring

Four 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. The green wire (G-wire) connects to the G terminal to control the blower, which is located in the air handler or furnace. The red wire connects to the R terminal, which often consists of an Rh and Rc terminal bonded by a jumper wire. A two-wire thermostat has terminals for only a red and white wire, and a three-wire thermostat has an additional terminal for a yellow wire. These are mechanical or battery-operated thermostats.

On a five-wire thermostat, the blue wire connects to the C (common) terminal, making it the C-wire; together with the red wire, the C-wire provides the power for the thermostat, which is necessary for every programmable thermostat that doesn't have batteries.

On a six-wire heat pump thermostat, the blue wire may connect to the O/B terminal instead, making it the B-wire; in this case, a black wire is used for the common (C-wire). You can also use the orange wire for the heat pump and keep the blue as the common.

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 reversible heat pump, the wire goes on the O/B terminal. The following table describes the standard thermostat wiring color code.

Your wires may already be installed in the wall, but if they aren't already 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 board 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. Note that some Wi-Fi-enabled smart thermostats, such as the Nest thermostat, need both an Rc-wire and an Rh-wire because they control the heating system — which may be an electric furnace or a gas furnace — and the AC unit separately. In that case, you will have to use one of the extra wires.

Turn off the breaker controlling the HVAC system before making a wire connection to the control board or to the thermostat or before doing any other troubleshooting. 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.

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 for the heat pump condenser.

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 system. 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.