Help with smart thermostat wiring
Most all the newer furnaces there will be a 'Y' terminal at the furnace.  However, this 'Y' terminal does nothing at the furnace location. On the board, it is a connection point only. Essentially, the Y signal originates at the thermostat, travels on the yellow wire, passes straight through the furnace out to the starting contactor at the A/C condenser. That contactor at the condenser also needs the B/C from the furnace.  These High tech thermostats will also need a common, B/C wire as well as the normal control wires.