I'm trying to clean up some sloppy HVAC work in my home prior to finishing my basement. I have 2 VERY long 6" flex duct runs coming off the supply master that feeds the great room in my house. Both runs make a hard 90 degree turn at one point as well, which i understand is a no-no with flex-duct. Both ducts also run parallel to each other for most of the run.
What I'd like to do is combine the flex duct runs into a single, rigid duct up until the point where they would split. Then use a T or Y to attach the flex for the last few feet of each.
I've been reading about calculating the CFM and it's making my head spin. From what I've read, it seems that flex does have more friction than rigid. THe area of 2×6" circles is 56.5486. The area of a single 8" circle is 50.2655. So, it's relatively close in terms of area.
Would I be OK using an 8" rigid duct prior to the split, or would I need to go up to a 10" duct. Space is a consideration, as I may not have room for the 10". THANKS!
Best Answer
Considering how much worse flexible duct is for airflow .vs. smoothwall, I think you'll be fine with 8"; better than it was, for certain.
Assuming real-world installation, this paper is quite enlightening (figure 2 also appears to show that 8" is slightly more than twice as good as 6" even under ideal conditions, where there's less difference between flex and smoothwall) but figure 3 is very enlightening, and that's for what's considered a "good, nominal" flex install.
Texas A&M airflow & pressure in flex ducts
Figures 4, 5 & 6 are downright scary.