What Is a C Wire? (And What to Do If You Don’t Have One)

The C or common wire is the return side of the 24 V transformer. With R and C connected, a smart thermostat gets continuous power for Wi-Fi and its screen. Without it, the thermostat has to "steal" power through the heat or fan wires, which can cause furnace clicking, a blank screen, or dropped Wi-Fi.

Check behind your old thermostat for an unused wire tucked into the wall (often blue or black). If it is connected to the C terminal at your furnace, you already have a C wire. If not, see our no C wire guide for the adapter each brand uses.