I am trying to correct poorly installed ductwork. Everything is duct-board and flex duct. None of the flex duct is pulled tight and the 90s are pulled around truss webs so those have kinks inside the curves.

I think it would be an improvement to use metal 90s where the duct path makes a turn and pull all the flex duct tight and straight between the metal 90s and take-off plenums. I would tie the 90s back so that they stay where they are at and not be pulled as I pull the flex duct tight.

I have read other suggest this; but, in reality I can not find metal 90s that connect flex duct on both ends. The code states that if using flex to metal connection the metal will have a bead at that connection. Every 90 I have looked at is made to connect flex on one end and metal duct on the other, again, one end of the 90 will have a bead and the other end will not. I need 90s that have a bead on both ends; but, the marketing industry for these materials is making it very hard to find.

Has anyone have experience with this type of installation that will point me in the right direction?

** Update **

Nice response from everybody here! So, from the top down in the response on this page I am trying to give more depth to my thought process on this post. @freeman, this is the same result I came to. I ended up fabricating a collar with one bead, but the bead I placed within 1/2" toward one side so that I may end-up with more meat on the pipe to pipe connection hopes that it has more bite and maybe stays more stabile because the longer length of friction metal in contact between those two pipes and length.

What happened was!, "A hard lesson", hah. The 90 did not have much meat to grab to-start-with so those efforts spent on that endeavor were lost but the best part would have been having the bead so close to the edge with enough meat to grip the flex and allows you to sleep at night. It was way easy! The big box wanted $20.00 for the collar; I thought I was gonna make out budget wise but "time and gas is money", I learned more than it was worth, "you know what I mean?"

About @RMDman's response, there is less volume supplied by the current duct than the true demand on that duct. I am looking to make optimise the current Duct just cut it shorter by pulling it as straight as I can. That area of the house just does not have enough throw to allow the mixing to comfort. Before I did anything, the velocity was less than 5.5 fps, now the velocity is at 10.6 fps. That was from just pulling all the flex as tight as I could.

On the 10" branch take-off there feeds one 4", two 5", one seven", it can not provide the amount of air for the environment. It will be cheaper to add a crucial 90 where it will give significant payback than to replace the system with a 12" trunk-to-branch and possible damper at a cost that includes changing both 10" duct board connections to 12". The parts list and retrofit are more expensive than if I can just trim down this one line that feeds two small rooms, a laundry, a standard bath. So that is way this question is so important to me.

What I want to tell you is that I have taken an eight inch and 10 inch duct out of the box and wrapped it around a bucket and then a trash can to look and see if where there is a problem with the air movement and each time I can visualize that there is more turbulence on the inside of any bent flex than what I have to give. This system is all ready undersized, I do not want to change part of the system out and then adapt the rest, you know? I just want to make it work.

  • In relation to RMDman's answer, you might want to edit your question to show us a couple of pics of your bends.
    – FreeMan
    Mar 3 at 14:26

2 Answers 2


As RMDman noted you probably don't need to do this at all.

If, however, you are bound and determined to replace the flex bends with metal ones, each metal corner will take two pieces:

  1. An appropriate metal elbow with the bead for attaching to flex duct
  2. A short piece of straight metal duct with a bead at one end for attaching to the flex duct.

Once you have the necessary pieces:

  • Attach your flex to the beaded side of the elbow.
  • Attach the other end of the flex to the beaded side of the straight pipe
  • Attach the non-beaded ends of the elbow & straight pipe to each other with a couple of sheet metal screws, just like normal metal duct is attached.
    • After screwing it together, you may want to wrap it with proper duct tape (the expensive aluminum foil backed stuff found in the HVAC aisle, not the bright colorful (or just grey) "duck" tape at the local craft store).
    • You may want to wrap your elbow and straight piece in insulation so that you're not losing heat/cold out of all the uninsulated sections you've just added to your duct work.

Or, you could just relax the bends in the flex a bit and call it a day.


I have seen hundreds of homes here in FLa. with the duct-board and flex ducts you describe. None have ever had metal 90 degree parts for bends.
They are not pulled tight so as to allow adjustment and soften tight turns. The kink you see is just in the insulated jacket. I would concentrate on moving the bends slightly to increase the radius.

That may be your best bet if kinks are you concern.

Remember the metal elbows are not insulated and if you don't add insulation they will absorb or transfer heat and sweat...another reason they are not used.

  • Good answer. Though if the OP is desperate to make male ends on 90s, all they need is an hvac crimping tool, eg Wiss WCS5SN Mar 3 at 15:00
  • The tool won't work for him. He doesn't need crimps he needs a bead on both ends. He can't find that elbow with beads on both ends because it doesn't exist...because it is not used for the purpose he intends. But thanks...I didn't know there was a crimping tool.
    – RMDman
    Mar 3 at 15:18
  • Aaah, sorry — wasn’t paying enough attention to the “beads” part. Will leave my comment to point to the very useful crimping tool for other purposes. Mar 3 at 18:49

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