Replacing multiple 6″ flexible ducts with rigid

by | Mar 17, 2024

Questions › Category: Questions › Replacing multiple 6″ flexible ducts with rigid
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Handyman Staff asked 4 months ago

I’m cleaning up ductwork in my basement and want to replace the runs of flexible duct with rigid.

Current

There is a group of 3 duct runs (6” uninsulated flexible round) that all have very close takeoffs from the trunk and travel to the same sides of the house. The flex ducts aren't compressed but have a little sag and hang under joists in a couple spots — so they hang a little lower than I'd like since we'd like to someday finish the basement.

current design

Plan

I'd like to replace the round flex with a rigid rectangular branch that runs between two joists, then use 6" flex or rigid round for the takeoffs. According to this chart each 6" flex duct has a design airflow of 75 CFM – for a total of 225 CFM. If the builder had spent a little more and used 6" rigid the total would be 255 CFM. Looking at the same chart, a 10"x8" rectangular duct would have 310 CFM.

plan

Questions

Does my plan make sense? Any issues with the larger CFM?

Any feedback or suggestions are greatly appreciated!