hm. right outta manual d and on the back of the ductulator :recommended velocty for residential main ducts 700-900 fpm.,...maybe we've all been doing it wrong and didnt know it.
You're right, the old velocity guidelines of 900 FPM max for supply and 700 FPM for return were mostly for noise concerns with bare sheet metal. The latest edition removes those guidelines because if you were to use ductboard or trunk with flex branches then noise is not as much of a concern.
BUT it has always been the intent of MAN D to CALCULATE how much static pressure is left after you deduct the coil, filter (sometimes) and registers and grill so you can get the ASP ( available static pressure) which you then divide by the measured length and fitting equivalent lengths to CALCULATE A FRICTION RATE. the friction rate you use is a calculation NOT a Guess!
Oh if you go above the 900/700 then you need to adjust the fitting length values in the appendix.
You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!