What's bad about flex duct?

Posted by dcaster@krl.org on October 26, 2009, 5:54 pm

> seen what flex looks like, I'm just guessing. =A0I think the contractor
> will pressure me to leave
> out the steel right angles and just bend the flex. =A0I don't want to
> push back if the right right angles won't make the system quieter.
> Recommendations?
> Thanks, =A0mike

Posted by Steve on October 26, 2009, 6:27 pm

> 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?
> Thanks, mike

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.

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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.

Posted by Stormin Mormon on October 25, 2009, 10:38 pm

Galvanized.
Air resistance.
Increased delta T.
Stinky doo-doo head.

Yeah, I'm a bit short worded, tonight.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
.

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

Posted by Steve on October 27, 2009, 9:08 am

> Pride has nothing to do with it. People who read, thought about and
> understood
> the problem statement would probably understand that the A-curve
> significantly suppresses the low frequencies where the problem lies.
> If I had A-weighted ears, I wouldn't have a problem.
> But, just for you, I put up a webpage:
> http://spamme0.netii.net/Noise.html
> There are spectral plots of the noise and even an mp3 of the sound.
> You can analyze it with any weighting you choose.
> Hope the link works. Given the amount of animosity in the group,
> I didn't want to expose my real isp web address.

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.

Posted by on October 27, 2009, 9:55 am

wrote:

>>>> And the total noise is reduced by about 1dBC.
>>> Oh ! dbC, now, is it ? Aren't we the fine ones ? Too proud
>>> to use the old dbA weight, are we ? :-)

"You see this page, because the system administrator of 000webhost.com
is currently checking this website for malicious content. This
redirect will be removed once we will finish manually checking all
files on this account."

Who the fuck hosts your website, the UN's HRC ? Perhaps
Canada's version of same ?

>There are spectral plots of the noise and even an mp3 of the sound.
>You can analyze it with any weighting you choose.
>Hope the link works. Given the amount of animosity in the group,
>I didn't want to expose my real isp web address.

I see you missed the signifigance of the Monty Python HTML
tags ..... why am I not suprised ?

>> Hmmmm, didn't I suggest almost exactly the same thing, in the original
>> thread?
>> Angle iron, eh??????
>> Didn't I say that a "duct" thick enough and heavy enough would guarantee
>> quiet?

If yuo did, then you are a fool, and wrong again.

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What's bad about flex duct? - Page 2