An extra unused wire in the wall can be used as the C wire for your new thermostat. The extra wire will also need to be connected to the C (common) terminal at the heating and cooling system. This will provide power to your new thermostat.
Please note that the C-wire is only needed for Wi-Fi connected thermostats [click here to see our Wi-Fi thermostats]. Most standard thermostats do not need a C-wire.
Please refer to the videos below for more information:
What Is a C-Wire?
What if I Don't Have a C-Wire?
A C-Wire Adapter replaces the need for a C-Wire. The C-Wire Adapter is included in newer Honeywell Wi-Fi Thermostats. If you own an older thermostat model, you can purchase a C-Wire Adapter here.