Contractor wants to install 2nd air-return on single Central AC unit... please advise

awesome guys... thanks for all the input. I'll post back tomorrow after they get here and do what you recommended - or as much of it as I can get them to :)

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.

The contractor come by and fixed the crushed flex ducts to the problem rooms... but as we all suspected it helped a bit but did not fix the temp balancing issue completely.

They do not want to build a new trunk, plenum, or redo any of the existing ducts. - I think this is fine since they agreed to do the balancing dampers.

They were not prepared with the balancing dampers... so they will be coming back a week from this Friday to install 3 16" dampers on master branch & each branch for the 2 main areas.

Can you guys give me tips on how to use the balancing dampers to get rooms balanced?... since these guys are definitely not AC guys.