I dont know squat but let's assume I know my drag coefficient, my distance, the number of turns, the number of air exchanges per hour, the type of filter, I'm using my efficiency of my blower, blah blah blah and it comes out I need an 18" round duct to get the adequate CFM. Do I need 2 18" units - one for the supply and one for the return? Yes, I need to talk to an HVAC expert but I'm trying to learn the basics and I wouldn't know an expert from an idiot who is going to just cut holes in my joists for the ductwork until I learn the basics.