Is a C wire required to install and operate the RTH6580WF thermostat?

Yes, a C wire is required and is the primary power source for the thermostat. Without a C wire, the thermostat will not power up. If you don't have a C wire, look for an unused wire pushed into the wall and connect it to C, verifying it is connected to the 24 VAC common at your heating/cooling system. You can also view the Alternate Wiring videos at wifithermostat.com/videos for additional guidance. (Page 13)