How to connect an unused c wire in your furnace compartment. Firstly make sure you have a C wire. In order to see if you have one, refer to this video:    • How to Tell if Your Thermostat Has a ...  
Once you follow the steps in this guide and have a C wire that is connected you can now install your choice of smart thermostat! Some thermostats even allow the use of without a c wire, by using an adapter or wire extender kit.
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