It might improve air delivery performance to the furthermost rooms by reducing the main trunk after the closer branch runs.
In a Reducing Trunk System the main trunk run is reduced in size usually after each one or two branch takeoffs.
These multiple size reductions make it possible to maintain a constant ft/min velocity & static pressure in the trunk even though the total air volume is reduced as each branch is supplied.
This type of system takes a little more material to build and requires more labor to fabricate and install. Another major concern is that there are more joints to air-seal with mastic.
However, those trunk reductions ‘regain’ the velocity & static pressure needed for the remaining branch runs & supply air diffusers.
The same reduction principles apply to the round duct branch runs - where they are split into two outlet runs, etc.
IMO, genduct gave you good advice; Using a main trunk would be the better way to go, with shorter runs of flex duct, if you have to use flex, stretch it tight & use supports a max of 5 feet apart to keep it straight as possible.
Last edited by udarrell; 01-17-2012 at 06:55 PM.