I've recently realized that my return is under sized. It's a 24" x 18" filter grill for a 3 1/2 ton unit. It's not possible to expand the existing return to the 24" x 24" required without some major changes. Therefore, I've decided to add a remote return. I think this approach will be more beneficial anyways since it's a split design house with a single return.
I wanted to connect the remote return using 14" insulated flex duct but the A/C equipment supplier freaked when I told him the length of this run will be 60'. He stated I couldn't get enough CFM to make it a viable option. I disagree since my figures show I only need an additional 600 CFM through the 14" flex. My calculator shows this can be accomplished and end up with a duct velocity of 562 FPM.
What do you guys say?