We're about to start a kitchen remodel. Since the ceiling with be open for chunks already, it seems like a good time to upgrade the very outdated return air system. Before I talk to our contractor, I want to make sure I'm understanding basic design.

The current configuration is an "F". Large duct running north/south in the garage into the furnace. Two return air "ducts"--actually, just unlined joist cavities, running east/west across the house, and "connected" to the main duct by holes in the wall that roughly line up with holes in the duct. I doubt this was even up to code in 1958, when the house was built. Joists are 2 x 8s, spaced 16" apart, so the joist cavity is roughly 7.5" x 14.5". Furnace is 80,000 BTUs, AC unit is 3 ton.

So, using the AC system for calculating, I think I need ducts that can deliver 1,200 CFMs. If I use 14" x 6" rectangular ducts, that's 320 CFM per duct, so I need 4 total return ducts (connecting to an appropriately large duct in the garage). Does it work this way? That is, can I use multiple ducts for cumulative CFM?