Duct system design varies alot from circumstance to circumstance. Stick with all metal duct and hard pipe. If someone tries to sell you on flex duct or duct board, tell them thanks but no thanks. With fuel prices these days who can afford the extra costs of flex duct created by the leakage and/or the loss of velocity. And duct board is just a bad idea. It can't be really cleaned and when it deterirates and starts spewing fiberglass into your system, it must be replaces. This doesn't even mentions how easily it molds up. The the whole thing must be torn out and replaced. Stick with the metal, have it cleaned every couple years and be happy.
The only thing I found incorrect about the "Best Practice" video is the steps of designing a duct system. The video stated that the heat gain and loss loads should be calculated first, then the equipment blower sizing, and then the duct design based on the equipment blower sizing. THIS IS INCORRECT! The blower sizing should be decided after the duct design has been completed. The blower size must be able to overcome the pressure caused by the duct system design and equipment. The only way to know the total pressure created by the duct system design (per ACCA manual D) is to calculate all the external static pressures (ie. electonic air cleaners, evap. coil, registers, etc.) and the total effective length (ie. duct system). Once the total pressure has been calculated you then have to select a blower that is capable of overcoming it. Most contractors probably don't do this but this is the correct steps to duct sizing.
