A:AnswerThe C wire is a common ground wire. Without getting into too much detail, older thermostats "cheated" and used power wires as ground wires so they didn't have to run as many wires.
Yes, you can absolutely still use this thermostat. If you have a C wire, it'll use it.
A:AnswerIf the lock is on, you can only change the function of the thermostat in the Sensi phone app or in the Apple Home app (or the Android equivalent). Pressing buttons on the unit will do nothing. If you unlock, the physical buttons on the unit work again.
