Were you not paying attention when you were taking apart your thermostat? Have the warning labels on the wires rubbed off or faded over time? If you’re not a skilled HVAC professional, thermostat wiring can be both tricky and dangerous.

It might seem random, but thermostat wire colors are actually very deliberate. Manufacturers follow a thermostat wire color code chart that helps professionals understand which wires have certain functions. Without understanding the colors, you cannot properly install a thermostat.

If you’ve accidentally gone too far with your thermostat, this thermostat wiring color guide will help you understand each wire and where it should connect.


Thermostat Wire Colors

Different models of thermostats have different numbers of wires. Very simple models might only have four wires, while more complicated ones could have seven or more. In other cases, thermostats could have a few wires that are the same color, yet serve different functions.

If you are not 100% sure about your thermostat’s wiring, you should call a professional for a safe and proper installation.


Red Thermostat Wires

All thermostats will have a red wire. The color red symbolizes power for thermostats. Without a connection for your red wire, your thermostat will not have a power source.

There are different types of red wires that a thermostat could have. Specifically, there are “Rc” wires and “Rh” wires. In simple terms, the Rc wires are for cooling systems while the Rh wires are for heating systems.

Rc wires connect to your transformer’s 24-hour AC voltage. These are the only red wires you will see on thermostats that are dedicated to air conditioning systems only. You will also see them paired with an Rh wire for dual transformer systems, which work for heating and cooling.

If you have both an Rc and Rh wire on your thermostat, they will have to connect to their correct terminals. The Rc wire must connect to the Rc terminal, and the Rh wire must connect to the Rh terminal.


Blue or Yellow Thermostat Wires

Blue and yellow wires on your thermostat can be tricky because sometimes the colors switch. You’ll need to know more details about your thermostat in order to properly identify them.

Yellow thermostat wires are responsible for controlling your air conditioning system through the compressor. They connect to the compressor contractor through an air handler. You must connect these yellow wires to the Y terminal on your thermostat.

There might also be more than one yellow wire. The first yellow wire is responsible for controlling the first cooling stage. The Y2 wire connects to its own Y2 terminal and controls the second cooling stage.

If there are no yellow wires, it is probably because the manufacturer has chosen to color this wire blue instead. If you know that your system has two stages of cooling, but there is only one yellow wire, it’s also possible that the second wire is colored light blue.

If you have an advanced thermostat, like a “smart” thermostat, then it’s possible you could have a blue wire that controls the smart aspects of your technology. These wires are also called “C” wires because they’re known as the common wire. This wire allows the thermostat to connect to a power source constantly.

Blue “C” wires are most common in thermostats that connect to your heat pump. This wire must connect to the B terminal on the thermostat. “C” wires are also frequently black.

If you are confused or unsure of the coloring of your yellow and blue wires, you could trace the wires to their origins. Blue wires are almost always for heating, while yellow wires are for cooling. Calling a professional may be the smartest decision.


White Thermostat Wires

In general, manufacturers have dedicated white wires to heating systems. You will only see white wires if your thermostat controls your heating system. You may also have more than one white wire if your system is equipped with more than one stage of heating.

Your white wire goes all the way directly to your heating system, regardless of what type you have. It should connect to your W terminal on your thermostat.

If you have W1 and W2, you should connect them to their respective terminals.


Green Thermostat Wires

Green wires in thermostats control your fan for your air handler or furnace. You should be able to trace this green wire directly from your G terminal on your thermostat to your heating system.


Orange Thermostat Wires

You will only see an orange wire in your thermostat if your system has heating. These orange wires directly link to your outdoor condense that reverses valve operation from one setting to another.

Orange wires only connect to air-source heat pumps, not geothermal heat pumps. If you have one, you should connect your orange wire to your O terminal on your thermostat.

It is important to note that sometimes you will see a dark blue wire instead of orange. These colors are interchangeable, as they are both attributed to heating systems.


Seeing Other Terminals or Wires?

It’s common for thermostats to have terminals that differ from the ones listed above. If you have a thermostat and system that has emergency heating, for example, you will see an “E” terminal. This “E” stands for emergency heat, but there is no standard color code for the wire.

If you see an “X” terminal or aux-wire, this means auxiliary. The X terminal is the connector for the auxiliary heating option on a heat pump. There is also no color coding standard for this wire.

Finally, some thermostats allow you to connect to an outdoor temperature sensor. A special kind of wire that has added protection and runs out separately from the other wires. These wires are the “S1” and “S2” wires and should connect to their respective terminals, or into the “T” terminal.


Install Thermostats Professionally

As you can see, thermostat wiring colors are not only complicated but inconsistent. It is too common that thermostats do not follow one or more of the color standards. Attempting to install a thermostat on your own, therefore, isn’t the best idea.

Hiring a professional means entrusting your heating and cooling systems to an expert who is aware of the variations and reasoning behind wiring terminals and colors. To hire an expert on thermostat wire colors, call Kaiser Air Conditioning today!