Central Air: Figuring out duct size for supply/return trunks
The easiest way to size your duct work is with a ductulator, but most people don't have one so heres another easy way you can do with a calculator.
the formula is V X A = CFM.
V stands for velocity like in feet per minute, the standard residental design for trunk ducts is 900 fpm so V = 900
A is the area of the duct in sq ft and CFM is cubic feet per minute.
so like you said 400 cfm per ton (another industry standard) 2.5 tons = 1000 cfm.
the equation looks like this.....1000/900= A in sq feet
so 1000/900= 1.1 sq feet is the area of your trunk duct at the unit.
1.1 x 144 (144 to change to sq inches) = 160 sq inches so you can pick any multiple of sizes 20x8=160 16x10=160 etc.
start out with 20x8 pick up half your run outs and and do the cals again and pick a new size for the rest of the supply trunk. (500/900=.55 etc)
few tips when you pick the last size make it a little larger the air cools of as you get near the end of the run so you need more of it.
no run outs smaller than 6" dia., all with dampers
insulate everything in the attic
make the return the same size as the supply (the air don't know which way its going)
if you want make the sizes a little bit bigger you can't get hurt making them larger. but 25 x10 seems a little large for 2.5 tons
here's the table we used for years sizing run outs.
6" dia. 150 cfm or less
8" dia. 250 cfm or less
10" dia. 450 cfm or less
12" dia. 650 cfm or less