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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 2x6" 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!

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    You're probably OK with reduced area because of less friction, especially if the sharp bends are eliminated. I'd still rather be safe and go bigger. If 10" round doesn't fit you could always go with rectangular ducts. – bcworkz Feb 25 '14 at 21:29
  • The ACCA Manual D equates 6" flexible to 5" rigid. The areas of these circles: 5" dia -> 19.6 sq inches, 7" dia -> 38.5 sq in; 8" dia -> 50.3 sq in; 10" dia -> 78.5. Based on area alone, I think 8" would be sufficient for you. If you do the calculations for friction and CFM, you might get by with 7". I think 10" would be too large. – RobertL Jul 7 '16 at 20:32
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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.

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You should get the specs for the plenum and see if they tell you how far and how many turns the system can tolerate. It is not that turns are a "no-no", they just reduce the air flow and create back pressure on the system. Every vent system I have worked with, including the that built into my microwave and the downdraft fan in my cooktop, told me what size ducting to run, and how various duct adapters affected the performance.

  • So, you're saying that 90 degree turns with flex-duct w/o using a rigid elbow is OK? Even w/o googling it, I can see that the flex-duct is already collapsing in on itself. – SBerg413 Feb 26 '14 at 20:34
  • It has been done with 8 and 12 inch ducting in my current and previous houses. It's all about the total distance and turns; and coefficient of drag. – sborsher Dec 17 '14 at 17:55

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