Good Evening, I am glad to have found this board. I scrolled through the posts to see if this had been answered previously, but did not find anything. I have a Lennox C33-24A-2f-3 furnace with an AC unit attached. My house currently has a four conductor thermostat wire in the wall. I have purchased 18-5 wire to replace it. My question is this, my current four wires are not connected to the C terminal on the board in the furnace, when I replace the wire with 5, all terminals will be populated, my concern is, does the C terminal at the furnace, by default, supply the 24VDc to control a smart thermostat, or is there something that needs to be configured on the board at the furnace.

Current wiring to furnace from thermostat. Empty C terminal.

 

It's 24vac and yes, C will power a smart thermostat without issue. Note the other thermostat cable is for the outside compressor and goes to Y and C. Two wires will go under each of these screws when you are finished.

 

Thank you for the information. Greatly appreciated.