There is no circuit board in the furnace.I have attached two photographs (hopefully). One shows the unit itself and the other is a . The furnace model is Nortron Model 21B18 and is about 8 years old. Whoops - looks like the schematic will come in a following reply.
I think the schematic will follow!
OK, from this wiring diagram it looks like your blower is controlled by the FR relay. So, the thermostat wire attaches to G on the furnace and then powers up this relay which turns on your blower motor. This is what happens when you turn the thermostat to the fan "on" position.
What I don't see is where the Y wire from your stat attaches for air conditioning. It's not in the wiring diagram, at least not that I see.