What's bad about flex duct?


I posted a question about furnace ducts that quickly turned into a flurry of emotion. Buried in the noise there appeared to be a significant bias against flexible ducts for return air. But I didn't find any clues as to why that is.
So, if flex duct is bad, what is the preferred material? And what symptoms would I experience with flex that are NOT experienced with the preferred material?
Emotion won't help me. Calling me or others names won't help me. What I need is actual information pertinent to the question.
What are the observable bad symptoms of flex ducting for gas furnace return air?
Thanks, mike
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spamme0 wrote:

To me, life span? difficulty for cleaning when needed, rigidity? Air likes to flow straight w/o friction.
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Exactly right, as well as friction/restriction = noise
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Tony Hwang wrote:

Thanks for the input. Can you be a little more descriptive or quantitative?
How much life span? My metal ducts are 37 years old and are expected to last the life of the structure. 20 years from now, I'll be living in an old-folks home...if I'm lucky.
I've never cleaned my ducts and they're pristine.
What's the advantage of rigidity? Less rigidity should couple less noise???
Friction and turbulence due to the non-smooth surface is an issue. Is there any way to quantify it?
I'm getting way too much furnace blower "rumble" (not air noise) through a straight 25' section of 16" steel return pipe.
I think what we're gonna try is to cut out the center section. Leave about 4' of steel on each end at the furnace and return vent and put in two right angles. Connect the right angles with flex with maybe some more loops in it. Because of the arrangement of the attic, flex will be a lot easier to manage than trying to install more steel. My theory is that the rigid right-angle will be more noise suppressive than just bending a chunk of flex with a bigger radius. Since I've never seen what flex looks like, I'm just guessing. I think the contractor will pressure me to leave out the steel right angles and just bend the flex. I don't want to push back if the right right angles won't make the system quieter. Recommendations?
The rigid segment lengths were a guess from reading this page: http://www.engineeringtoolbox.com/sound-attenuation-ducts-elbows-d_76.html Looks like I need three pipe diameters of straight section to be effective. If the numbers apply, two right angles should fix me right up.
Is this a dumb idea? Different rigid pipe segment lengths? Better idea? Changing contractors is not an option at this point. I gotta make what I got work.
Thanks, mike
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yes
still a dumb idea

Quit trying to put a bandaid on it, and get it done right. Replace the return air round pipe with correctly sized internally insulated square duct. But then what do I know..... I'm not an EE.

Just for grinns, is the new furnace the same btu output as the old one?? Its most certainly not the same efficiency.
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I am an EE. So have no valid real experience. But if I were trying to reduce noise, I would think about how mufflers work. So would try running the duct into one side of a box lined with fiberglass. And have the outlet on the other side of the box.
........................................ ----------------/ \\ \\ ----------------\\ \\ ---------------------------- \\ \\ --------------------------- ----------------------------------------/
Dan
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I am an EE. So have no valid real experience. But if I were trying to reduce noise, I would think about how mufflers work. So would try running the duct into one side of a box lined with fiberglass. And have the outlet on the other side of the box.
........................................ ----------------/ \\ \\ ----------------\\ \\ ---------------------------- \\ \\ --------------------------- ----------------------------------------/
Dan
Your still missing the point.... the RA ductwork is WAYYYYY too small, and with tin round pipe, there is no internal insulation to absorb sound.....the sounds bounces around in the pipe and can ever be amplified in some cases.
Small pipe = increased velocity = increased noise.
If you want the RA to be very quiet, for a 60K btu furnace, you need to have a *MINIMUM* of 20x20, internally insulated square duct, with a 90 degree angle at the correctly sized RA grill, and another 90 degree angle approximately 4 times the width of the duct from the furnace. Then set the blower speed to the middle of the specified temperature rise range.
You can do this and the noise will go away, as well as being comfortable in your home with lower utility bills, or you can continue whining about having to put a bandaid on it. Its your choice.... pay to get it done right, or do it cheap and live with the noise, and higher energy bills cause by the restriction in the RA duct.
While I am at it, do before and after static pressure tests, and post the results.
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Galvanized. Air resistance. Increased delta T. Stinky doo-doo head.
Yeah, I'm a bit short worded, tonight.
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The real problem here is that most everybody here knows *exactly* what the problem with you system is, and have told you not only what the problem is, but what you need to do to correct it. Now with you being an EE and all, you come in here and expect us to bow down and kiss your ass... it ain't gonna happen. Any animosity in this group was brought on by your attitude, and your attitude only. get over it.
Sure you can draw pretty pictures, and have all kinds of book learnin, but when it comes to practical applications...you don't have a clue. You refuse to accept the advice and the fix for your problem. I can't help but wonder if you didn't buy your furnace on Ebay, and tried to put it in yourself....and now your not happy with the results. You do realize that when you buy HVAC equipment off the interwebs, that the manufacturers warranty is null and void. Then there is the little clause in the warranty paperwork about being installed by a licensed professional (not an EE).
According to you, as an EE, you supposedly make a hell of a lot more money than we do... then you can afford to get it done right.
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.p.jm.@see_my_sig_for_address.com wrote:

http://nm7u.tripod.com/homepage/Noise.html
throwaway websites are not always reliable. mike
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Yup... air noise because of undersized RA ductwork because it wasn't done right to begin with. The blower noise is being transmitted through the duct and also the joists that the furnace is sitting on.
Correct the ductwork issues like I told you, isolate the furnace so its not sitting on the joists(either hang it, or use isolation pads), and in addition, add a vibration isolation boot between the new RA duct and the furnace.
No its not gonna be free. your can either correct it as I told you or you can live with it.... there is no bandaid.
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Steve wrote:

I wonder what the noise level is in db measured with sound(noise) level meter???? Is the reading in danger zone to damage hearing for long term exposure?
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Maximum permissive noise levels can be found in 29CFR
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Steve wrote:

To the ONE person in this newsgroup who replied with TWO, count 'em TWO lines of text pointing me to relevant technical information... THANK YOU VERY MUCH.
To those of you who insist that a noise problem is always an "undersized duct problem"...in spite of experiments demonstrating that it was NOT air noise...well...NO THANK YOU VERY MUCH!!
Contractor came out today. This dumb ol EE told him what to do. He disregarded his work order and did what I asked. The system is now as quiet as the proverbial church mouse.
To those of you who just like to bitch...have at each other...I'm outa here. mike
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So what was the problem and what was the fix? What was the technical info that led to the fix? What experiments told you it was not air noise?
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See page 2 of the following data sheet http://www.owenscorningcommercial.com/docs/datasheet/61261_QuietZone_Spiral_Duct_Liner.pdf which shows that flex duct is not much help in reducing low frequency noise.
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