Big Loss in flex elbows vs. metal elbows?


Old 07-20-15, 03:47 PM
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Join Date: Feb 2015
Posts: 27
Big Loss in flex elbows vs. metal elbows?

I'm replacing one flex duct run - 8", ~ 15 linear ft.
Researching losses from bending flex in 90's vs. using metal elbows in a flex run.

* Do the adjustable elbows still have far less press. / CFM loss than bending flex at 90 deg?
Do AC supply houses still sell smooth, fixed 90 Els (didn't show me any)?

My run is "unusual". If there was another way to route it, I would.
* It must turn 90* off the plenum;
* has ~ a 60 - 70 deg turn at the wall boot;
* must slope upward - off the plenum, then back down to the wall boot.

Some technical sources show flex bent at 90* (even done "right"), adds pressure loss* equivalent to 20 extra ft of flex duct.
This & other published papers discuss pressure losses from bending flex duct, not actual CFMs.*

Wondering if for this this many bends, if I should
1. absolutely use metal elbows at all bends (seal & insulate).
2. for true 90 deg bend(s), if I should keep looking for "fixed" 90's - smoother inside than adjustable ones?
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Old 07-21-15, 12:54 AM
Join Date: Mar 2006
Location: Wet side of Washington state.
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Smooth metal duct will always have significantly less pressure drop than will flexible duct. As for the elbows, I don't think that adjustable will be much different than fixed, if you can even find fixed elbows. Using as much metal duct as possible will give you the least pressure drop versus flex.

must slope upward - off the plenum, then back down to the wall boot.
Why? Can you post a picture of this installation?
Old 07-21-15, 08:13 PM
Thread Starter
Join Date: Feb 2015
Posts: 27
Well, for now I used adjustable elbows at the start collar & supply boot.
I've not "completely" finished 100% sealing elbow joints, etc. Enough to get AC back on last nite, but was exhausted.

I could post pics (dozens, actually). Suffice to say, this run goes near the attic stairs. Must avoid that. The furnace flue also must be avoided - somewhat. And, if the run didn't arch upward a few ft, it'd cut off getting across the air handler, to other side (and to other end of attic). Too many obstacles.

Could it be routed another way? Sure, but would add lots of extra ft & even more 90's than it already has. Not a good option. I've thought about it for yrs & had pros look / comment on "there's really not a better route" even though the current one is far less than ideal.

Now, I'm a little concerned about how the flex liner was (or can / should be) connected to the elbows - esp. on the crimped end.

If trying to pull the liner so it & a zip tie are past the 1st "bead" (adjustable joint), so the tie / liner can't slip off. That presents problems on an elbow, as the liner must slide on the fitting MUCH farther on the outside radius vs. inside radius, to reach the raised bead.

And if that uneven stretching may cause the liner to press more against one side of the elbow - damaging liner.

Or, for flex connections to elbows, most pros don't worry about getting the liner / zip tie past a bead / roll.

Taping the liner to inside radius of elbow also gives problems. When the liner goes on as far as it can (inside radius), then the elbow makes an immediate 90. Very hard to tape effectively.

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