I live in a older home that is around 900-950 sqft. on the main level. The ac unit is 3ton. It currently has 7, 6" supplies coming out of the unit. The supplies then connect into bigger ducts that range from 8-10''. I think is used to have a gravity system and when they replaced the furnace they just hooked back up to the old lines. They house heats and cools with no problem now, but I want to finish off the basement and replace the old ducts with a square supply. How do I know how big it has to be? The run is about 20ft long. One straight shot with 7 takeoff's would be no problem, but how big does it need to be? I would like to keep it as thin as possible. I have had 3 people look at it and one said 8x16 the other 8x24. Could I go with something 6" deep?
Thanks in advance.