HVAC Plenum and Duct Design
I'm making some improvements to my noisy and undersized ductwork, and seeing big improvements to the areas I have done so far.
The next step is to install a correct plenum and ducts to a family room that is over a crawl space, which has a very inadequate system. My plan is to used insulated flex duct, connected directly to the furnace plenum, starting at 10" diameter, and running it through this space, wyeing off 6" ducts for individual registers along the way, and reducing the diameter of the main plenum along the way. Similar to the attached picture. The rest of the house is supplied by a traditional rectangular main trunk off the opposite side of the furnace plenum.
I'm trying to follow good duct design practice as I've learned by reading, but something mystifies me. In a rectangular trunk installation, we are told never to install a branch duct off the very end of the trunk, but when I see diagrams of reducing trunk systems, there is a takeoff off the end.
What exactly is the reason for not taking a branch duct off the end of an extended plenum? And why does this rule not seem to apply to a reducing trunk layout? I want to do this right, assuring the best airflow and least noise.