A C wire, also known as a common wire, is a wire that provides continuous power to a thermostat. It is usually blue or black and is common for 24-volt power to the thermostat. Unlike other wires connected to a thermostat, the C wire doesnt control heating or cooling functions. Instead, it only provides constant power to the thermostat. Most smart thermostats today that are Wi-Fi enabled or have a color touch screen may require constant power through a C wire. If youre considering a smart thermostat, you should check your HVAC system compatibility to see if you need a C wire for your smart thermostat.

If youre installing a new thermostat, but your existing one is missing a C wire, you have the following options:

• Check for an unused wire wrapped around the wire bundle or cut down just behind the faceplate and the other end of that wire unused near the control board. If there is an unused wire, you can use it as a C wire to install your new thermostat by connecting it to the C terminal at your thermostat and at your HVAC equipment.
• Use a C wire adaptor.
• Change your G wire to be a C wire.

If youre unsure whether your thermostat has a C wire, you can turn off the power supply and verify if there is a wire connected to the C terminal on your thermostat. If youre installing a Nest Thermostat, the Google Home app will often tell you during setup if you need to connect a C wire.