Old thermostat was only using 3 wires, connected to W, G and R-Rc jumper. no C wire. Does have extra wires not being used at each end. Big question is what to I connect to at the furnace for a power source for the new wifi thermostat? There is only Y, and Com 24V Terminals available on the furnace board. Can I use Com 24v at furnace terminal and C wire at thermostat?