I am helping a friend add a second story to his ranch style home. Instead of increasing the size of his existing basement HVAC system, he wants to add a second system for the upstairs. He does not want a horizontal in the attic.
We are installing 75,000 BTU, 2 ton counterflow in a central location in the second floor. I made an 18 x 26" plenum that fits between the 22" spacing on the floor joists and maintains at least 2" of clearance from all combustables. Off of this plenum are ten 6" rounds to the registers in the 4 bedrooms, laundry room and two baths. The cold air return will be in a false ceiling above the central hallway from the four bedrooms to the return of the air handler.
I would appreciate some pointers on this project. I have installed replacement systems and new conventional basement up flow systems, but this is unique. It is a similar installation to what you would see in a slab type home. Conceptually, it has been OKed by the building inspector.