I'm not the pro on this but I am currious. It seems to me, not remembering the formular to find out how many square inches there are in a circle, that you have more square inches of exhaust the you do for supply!
I believe the only numbers that are relevant would be the size of the duct from the furnace because that is all the volume you can have, and the amount the volume of all the 12- 6" ducts combined.
You have an 8" X 24" supply or 192 square inches of opening that feeds 12- 6" openings. (I don't know the number of square inch's but, someone will)
Much like connecting a 3" hose to a 1/2 " spicket, you will only get as much volume as the smallest connection.
Do you have any dampers installed in this system so you can control the air flow and cut it back where it is not needed?
It's just a thought, and I am probably wrong but, like I said, I'm currious.
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