Take your existing thermostat off the wall — it's usually easy to do — and turn it over. You'll see wires of many colors, and one of those might be blue. If so, look at the terminal to which it's attached, and you should see the letter C. The blue wire, or C-wire, is known as the common wire. It's there to provide power to the thermostat.
Older thermostats usually don't have a C-wire because they either don't need power or, if they do, they get it from a battery. Modern thermostats are a different story. They have programmable functions, Wi-Fi connectivity, lights and an LED screen that all need power, and wiring them to the 24-volt transformer on the heating/cooling system control panel is more efficient than using a battery. Power comes into the thermostat on a wire connected to the transformer, which is usually red, and returns to the transformer on the C-wire, which is usually blue, but it can be black.
Thermostat Color Code
When doing HVAC wiring, technicians usually follow a standard color code for the wires, but they aren't as rigorous about it as electricians are about electrical wires. You shouldn't always assume that the colored wires at the thermostat are performing their standard functions. The best way to make sure is to check the letter on the thermostat terminal to which each wire is attached. If you have wires sticking out of the wall but no thermostat, check the colors of the wires attached to the terminals on the control panel of your heating/cooling unit.
If everything is hooked up according to industry standards, the wires serve the following functions:
— The white wire turns the auxiliary heater off and on. It connects to the W terminal.
— The yellow wire connects to the compressor on a heat pump. It connects to the Y terminal.
— The green wire controls the fan. It connects to the G terminal.
— The orange wire controls the reversing valve that turns a heat pump into a cooling system. It connects to the O terminal.
— The brown wire usually controls the second-stage heat function on systems that have it. It connects to the W2 terminal.
— The red wire supplies primary 24V power from the transformer. It connects to the R terminal. Systems with both heating and cooling functions have two terminals, marked RH and RC. These may each have a red wire or they may be connected by a jumper wire in such a way that they only need one red wire.
If you see a blue wire, it's usually providing a return path for the red wire, and it's connected to the C terminal. On some systems, such as Rheem and Rudd, you'll find a B terminal that performs the same function as the O terminal. On these systems, the blue wire connects to the B terminal, and a black wire is used for the common.
Wiring a New Thermostat
Most contemporary thermostats, such as Nest thermostats featuring internet connectivity, have a common terminal. If the wires in your wall include a blue wire, you're all set. Just connect it to the C terminal and it becomes the C-wire. However, if you have a Rheem or Rudd system, you may see a black wire as well as a blue one in the wire set. If so, the black one is probably the C-wire, but you should check the wire configuration on the system control panel to make sure. You won't break anything or get a shock if you swap wires between the C and B terminals, but the thermostat won't work.
If you want to install a new thermostat that has a C terminal, but you don't have a blue wire because your old thermostat didn't need one, you'll have to install a blue wire. You can either run that wire separately from the system control panel to the thermostat or, preferably, run a new cable that includes all the wires you need. Use 16- or 18-gauge wire as specified by the manufacturer of your heating/cooling system.
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.