Just need to check to see if I'm right regarding my thoughts for a furnance install in my shop. Here goes.
Shop is 30 x 56 18 ft walls pole barn, metal skin,lightly insulated
Furnace 80%+ 96K btu, 4 ton blower. factory fan set med high (approx 1700 cfm no AC, Im installing horizontally midway long wall 8 feet off the floor.
I have same size insulated ducts Im using as return air and supply plenum, both are 20 X16 Hey they were free.
Return Ill have two 12 inch flex from the "plenum" running along trusses down opposite wall no grills. one run about 40 feet the other 50.
Supply will have two "mains" off supply using flex. 10inch 90elbow to 4ft flex to 10inch tee reduced to 8inch to 8 ft of flex to 8inch tee reduced to 6inch then 8ft of flex to 6inch tee capped. next "main" is 8inch elbow 2ft of flex to 8inch tee reduced to 6inch to 6 feet of flex to tee capped.