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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!

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  • Depending on the rest of the calculations, maybe the builder didn't need the extra 30 CFM and had the flex on the truck. How is the system working now? You can add a few supports to fix any sagging flex ducts. Any real reason to change it? Personally, I prefer the individual feeds off a main trunk. I've had trouble with WYE splits in the past.
    – JACK
    Commented Mar 17 at 15:28
  • @JACK the system is very functional but heating/cooling is uneven. Changes made by previous owner led to a dead supply and 2 dead returns. I want to get the ducts up between the joists so I can put up a ceiling. I can fit 2 rounds in a joist bay but not 3. Commented Mar 17 at 17:35

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Just remember that you've got a fixed amount of heating and/or cooling per unit time. The relative flow rate between different branches is how the system allocates this fixed amount of heating and/or cooling to the different branches. Less pressure loss in the altered branches will increase their budget and reduce the budget of all of the other branches.

If you like the current distribution of heating and/or cooling across your system's branches, then match the pressure loss characteristics of the old system as best as possible.

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  • The airflow distribution is off, so I have to adjust the dampers and balance anyway. Would it be better to undersize? Commented Mar 17 at 18:23
  • @Ellis, I'm no expert. The one worry that I know to have with constricting flow is that it can stress your blower motor and thereby reduce its life expectancy. You haven't specified how the current flow is mal-distributed, so I can't answer your undersizing question. If the flexible duct is currently delivering too much heating or cooling relative to the other ducts, then I would prefer to size by-the-book and install an adjustable damper.
    – popham
    Commented Mar 17 at 19:04

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