Author Topic: Flex Duct on Return Air Diffusers - Yes or No?  (Read 20495 times)

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Offline TFullerTopic starter

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Flex Duct on Return Air Diffusers - Yes or No?
« on: Oct 26, 2011, 21:30:52 PM »
Does anyone have any credible, logical, half-way scientific opinion on whether or not flex duct should be used on the connection to the lay-in diffuser? (We're talking about the last 3'-4')

My engineer says no, don't use flex on RA.  

I say yes, go ahead, type 3 from FlexMaster is rated up to -10" WC.

Opinions?
Thomas N Fuller II

Offline Phred

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Flex Duct on Return Air Diffusers - Yes or No?
« Reply #1 on: Oct 26, 2011, 22:35:57 PM »
As an engineer with 27 years experience, I have always used flex duct to RA and low pressure exhaust without ANY problems.  This guy must be a real dinosaur, or does not own a Vacuum cleaner.  I wish someone could convince these idiots, as it is a huge waste of time, money, and material

Thanks - Phred
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Offline TFullerTopic starter

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Flex Duct on Return Air Diffusers - Yes or No?
« Reply #2 on: Oct 26, 2011, 23:00:15 PM »
In his defense, he was going on the advice of the senior engineer he worked under as a designer. I imagine that the senior engineer was used to seeing product submittals with -1"WC ratings.
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Offline cadbyken

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Flex Duct on Return Air Diffusers - Yes or No?
« Reply #3 on: Oct 27, 2011, 00:49:29 AM »
I never used it on the design side but have seen other engineer's do it while I am on the construction side.  Projects include schools and hospitals.  It does make drawing a whole lot quicker.
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Offline tnbndr

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Flex Duct on Return Air Diffusers - Yes or No?
« Reply #4 on: Oct 27, 2011, 11:24:18 AM »
Quote
I wish someone could convince these idiots, as it is a huge waste of time, money, and material
I agree 100%. Some engineers worry about the pressure class rating but others don't care they just don't like flex. Probably from bad experiences with installs being wrong or some low bidder using 25' to cut corners.
Most specs around here state 5' max length, as a Design build company our spec states 8' and we have had no issues. It not only makes for a cleaner, quicker install which saves the contractor and owner money it also helps to cut out any sound that may be in the system from traveling through the grille.
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Offline Phred

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Flex Duct on Return Air Diffusers - Yes or No?
« Reply #5 on: Oct 27, 2011, 13:27:02 PM »
scientific reason -  atmospheric dust (abrasive) traveling at 600 FPM will erode the heels of turns (elbows) rendering the flexible ductwork totally useless, after only 2000 years of continuous service.

reality based reason - Sheet metal vendor has deeper pockets when it comes to treating engineers to a steak dinner, as opposed the the flex duct vendor.

Solution - offer to pay to have the flex duct vendors buy the engineers a steak dinner/ with video presentation showing flex duct in action under negative pressure conditions.
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Offline tombon

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Re: Flex Duct on Return Air Diffusers - Yes or No?
« Reply #6 on: Aug 27, 2013, 21:04:53 PM »
Yes…but…

Flex duct effectively creates a  tight radius elbow where it turns into the top of a diffuser.  The high pressure loss is further increased because the flex fabric gathers at the inside of the throat and reduces the effective diameter. 

But Thermaflex markets a simple cost effective device called a FlexFlow Elbow which mitigates this problem.   http://www.flexflowelbow.com/     It works no matter which way the air flows.
-Tom

Offline VirtualPilot

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Re: Flex Duct on Return Air Diffusers - Yes or No?
« Reply #7 on: Aug 28, 2013, 14:12:31 PM »
If a tight flexed elbow right on the diffuser is your concern, then add a metal elbow on top of the diffuser and use only straight runs of flex, we do this ALL the time...
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Offline Phred

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Re: Flex Duct on Return Air Diffusers - Yes or No?
« Reply #8 on: Aug 29, 2013, 13:45:12 PM »
Yes…but…

Flex duct effectively creates a  tight radius elbow where it turns into the top of a diffuser.  The high pressure loss is further increased because the flex fabric gathers at the inside of the throat and reduces the effective diameter. 

But Thermaflex markets a simple cost effective device called a FlexFlow Elbow which mitigates this problem.   http://www.flexflowelbow.com/     It works no matter which way the air flows.


Air has no brain cells, it does not know whether its coming or going except for a couple of feet from a fan intake or discharge (same thing with water), I agree with the flex support systems which help.  I think the real culprit that bugs engineers (like me) is that the test & balance reports almost always come up short on return or exhaust.  This can be traced to the use of flow hoods such as Alnor which do a piss poor job of reading "negative" flow (probably positive flow also)-do a "proper"duct traverse and get totally different results.  In this area TAB contractors are a bad joke and not worth a cent. They show up, take a few readings, claim its all F***-up, walk off the job, let alone BALANCE, collect their money and do nothing.

got to educate the engineers about this and also the fact that you cannot distribute air with exhaust or return (try sucking out a match)

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Offline Zorg

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Re: Flex Duct on Return Air Diffusers - Yes or No?
« Reply #9 on: Nov 13, 2013, 01:52:17 AM »
Often the flexible connection (for low pressure) is only disputed aesthetically by the architect when there is no ceiling and the services are exposed.
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