Do the T5/T6 Pro WiFi model thermostats require a "C" wire?
The T5/T6 pro WiFi thermostats do require a common wire, also known as a "C" wire.
The "C" or common wire allows the thermostat to power itself, instead of using batteries. It must be connected to the C or common terminal on the furnace control board. Buy a c-wire adaptor here.