On new construction here in Central NJ, it is very common for a forced air HVAC system to have one system in the basement with the ducts running through the floors and/or lower walls and a furnace in the attic with ducts running through the ceiling. Is this the best way or just the most convenient? I would think that one would want heating ducts as low as possible and AC ducts to be as high as possible. One other question-is there any problem with flex duct running through an attic, or is some other ductwork preferred. Thanks.