What's bad about flex duct?

Posted by spamme0 on October 25, 2009, 6:47 pm

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 Tony Hwang on October 25, 2009, 7:08 pm

spamme0 wrote:

> 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

Hi,
To me, life span? difficulty for cleaning when needed, rigidity? Air
likes to flow straight w/o friction.

Posted by Steve on October 25, 2009, 7:45 pm

> spamme0 wrote:

>> 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

> Hi,
> To me, life span? difficulty for cleaning when needed, rigidity? Air
> likes to flow straight w/o friction.

Exactly right, as well as friction/restriction = noise

Posted by spamme0 on October 26, 2009, 4:47 am

Tony Hwang wrote:

> spamme0 wrote:

> Hi,
> To me, life span? difficulty for cleaning when needed, rigidity? Air
> likes to flow straight w/o friction.

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

Posted by Steve on October 26, 2009, 8:35 am

> Tony Hwang wrote:

>> spamme0 wrote:

>>> 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

>> Hi,
>> To me, life span? difficulty for cleaning when needed, rigidity? Air
>> likes to flow straight w/o friction.

> 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?

yes

> Different rigid pipe segment lengths?

still a dumb idea

> Better 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.

> Changing contractors is not an option at this point.
> I gotta make what I got work.
> Thanks, mike

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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