What is a C-wire?

Technically, power flows to the thermostat from the R (red) wire. However, this is only half of the circuit. For it to flow continuously, the thermostat needs a C-wire, or “common wire,” to complete the circuit. Older thermostats typically run on batteries and turn on and off as needed, but a smart thermostat needs continuous 24V power to maintain their Wi-Fi connection and run their display.