Noise suppression in ductwork? Minimum segment length?

Posted by spamme0 on October 23, 2009, 1:55 am

I had a new Coleman 99.5% gas furnace installed.

The return duct is a 25' 16" diameter metal straight shot from the
hallway to the furnace in the attic. Air flow is about 900 linear
feet/minute. That's over twice the airflow for my 37 year-old Lennox.

I get lots of furnace noise in the hallway. There's "wind" noise plus
what sounds like rumble from the blower. Noise level is 74dBC/60dBA
at ear level under the duct. The difference in the numbers suggests that
the problem is in the low frequency area. A spectrogram of the noise
confirms that the peak is at 100-200Hz. area.

I've got 9 source vents in the floor. Noise from them is tolerable.

Googling suggests that there should be at least three
right angle bends in the return duct to abate the noise.
I couldn't find any more detailed recommendations.

There will be straight segments between each right angle bend.
What's the optimal/practical length for each segment?

As the length of the segment approaches zero, it's contribution
to noise abatement will be reduced toward zero.
As the length of a segment increases, the marginal effect on noise
abatement decreases. Somewhere in the middle should be a sweet spot
for segment length. What should that segment length be???
How does that relate to the noise frequencies involved?
Would different lengths for each segment be better to avoid resonances
or spread the
losses over the frequency range of interest?

The contractor wants to insert 25' of flex duct and "serpentine"
it between the ends after cutting out a section of the straight pipe.
His idea of a right angle is to cut another hole in the side of
the return plenum. That's not an effective right angle at all.

How does inserting curves of flex duct compare with right-angle joints?
Radius recommendations?
How would one circular loop compare? That would take minimal adjustments
to the existing pipe.

Discussion suggests that one of us (me and the contractor) doesn't
understand the optimum duct situation.

The contractor is not willing to discuss noise level goals.
He wants to modify the ducts for a fee, without making any
promises about solving the noise problem.

Recommendations?

Thanks, mike

Posted by Steve on October 23, 2009, 8:19 am

>I had a new Coleman 99.5% gas furnace installed.
> The return duct is a 25' 16" diameter metal straight shot from the hallway
> to the furnace in the attic. Air flow is about 900 linear feet/minute.
> That's over twice the airflow for my 37 year-old Lennox.
> I get lots of furnace noise in the hallway. There's "wind" noise plus
> what sounds like rumble from the blower. Noise level is 74dBC/60dBA
> at ear level under the duct. The difference in the numbers suggests that
> the problem is in the low frequency area. A spectrogram of the noise
> confirms that the peak is at 100-200Hz. area.
> I've got 9 source vents in the floor. Noise from them is tolerable.
> Googling suggests that there should be at least three
> right angle bends in the return duct to abate the noise.
> I couldn't find any more detailed recommendations.
> There will be straight segments between each right angle bend.
> What's the optimal/practical length for each segment?
> As the length of the segment approaches zero, it's contribution
> to noise abatement will be reduced toward zero.
> As the length of a segment increases, the marginal effect on noise
> abatement decreases. Somewhere in the middle should be a sweet spot
> for segment length. What should that segment length be???
> How does that relate to the noise frequencies involved?
> Would different lengths for each segment be better to avoid resonances or
> spread the
> losses over the frequency range of interest?
> The contractor wants to insert 25' of flex duct and "serpentine"
> it between the ends after cutting out a section of the straight pipe.
> His idea of a right angle is to cut another hole in the side of
> the return plenum. That's not an effective right angle at all.
> How does inserting curves of flex duct compare with right-angle joints?
> Radius recommendations?
> How would one circular loop compare? That would take minimal adjustments
> to the existing pipe.
> Discussion suggests that one of us (me and the contractor) doesn't
> understand the optimum duct situation.
> The contractor is not willing to discuss noise level goals.
> He wants to modify the ducts for a fee, without making any
> promises about solving the noise problem.
> Recommendations?
> Thanks, mike

You sound like an EE that hired the lowest bidder. sorry bout your luck.

If you want to correct the noise issues, then you need to insure that the
ducts are correctly sized (Manual D duct calculation) and installed. 900fpm
is a bit much for RA. Correct the ductwork issues, and the noise will go
away.
Flex duct is *NOT* the way to go. find a contractor that is going to do it
right, not just cheap.

Posted by zero on October 23, 2009, 11:17 am

> Mebbe if you offer to let Steve SYD, he'll come over on a bus and give you
> a hand for free.
> --
> EA

Enough already! ...."SMD" and "SYD"!!! Nobody wants to hear about Steve
or anyone else sodomizing anyone's poor dog!

-zero

Posted by The King on October 23, 2009, 5:51 pm

On Fri, 23 Oct 2009 11:21:16 -0400, "Existential Angst"

>The right blower/air handler (read: air velocity) might could solve all,

>>> altho typical multi-speed furnace blowers (vs a real air handler) are
>>> just fast and faster.

>> Only the cheap ones, not the good ones.

>Such as....?
>And what makes them good?

Its all the same shit. There's only about seven different furnace
manufactures and most of them all use the same hodge podge of parts.
There are a lot of label makers. Steve likes and sells Rheem. I
don't think Rheem ia any greater than anything else and I think Rheems
commercial stuff sucks. I don't endorse any one brand because they
are all junk. That's why we have jobs.

Posted by spamme0 on October 23, 2009, 6:18 pm

Steve wrote:

>> I had a new Coleman 99.5% gas furnace installed.
>> The return duct is a 25' 16" diameter metal straight shot from the hallway
>> to the furnace in the attic. Air flow is about 900 linear feet/minute.
>> That's over twice the airflow for my 37 year-old Lennox.
>> I get lots of furnace noise in the hallway. There's "wind" noise plus
>> what sounds like rumble from the blower. Noise level is 74dBC/60dBA
>> at ear level under the duct. The difference in the numbers suggests that
>> the problem is in the low frequency area. A spectrogram of the noise
>> confirms that the peak is at 100-200Hz. area.
>> I've got 9 source vents in the floor. Noise from them is tolerable.
>> Googling suggests that there should be at least three
>> right angle bends in the return duct to abate the noise.
>> I couldn't find any more detailed recommendations.
>> There will be straight segments between each right angle bend.
>> What's the optimal/practical length for each segment?
>> As the length of the segment approaches zero, it's contribution
>> to noise abatement will be reduced toward zero.
>> As the length of a segment increases, the marginal effect on noise
>> abatement decreases. Somewhere in the middle should be a sweet spot
>> for segment length. What should that segment length be???
>> How does that relate to the noise frequencies involved?
>> Would different lengths for each segment be better to avoid resonances or
>> spread the
>> losses over the frequency range of interest?
>> The contractor wants to insert 25' of flex duct and "serpentine"
>> it between the ends after cutting out a section of the straight pipe.
>> His idea of a right angle is to cut another hole in the side of
>> the return plenum. That's not an effective right angle at all.
>> How does inserting curves of flex duct compare with right-angle joints?
>> Radius recommendations?
>> How would one circular loop compare? That would take minimal adjustments
>> to the existing pipe.
>> Discussion suggests that one of us (me and the contractor) doesn't
>> understand the optimum duct situation.
>> The contractor is not willing to discuss noise level goals.
>> He wants to modify the ducts for a fee, without making any
>> promises about solving the noise problem.
>> Recommendations?
>> Thanks, mike

>
> You sound like an EE that hired the lowest bidder. sorry bout your luck.

Yep, I'm an EE. But I've probably had more formal education in
thermodynamics and physics than most people.

It's a long story, but I didn't have any choice of contractor.
And the cost of the upgrade was ZERO, so I can't complain about getting
what I paid for.

All I wanted was some engineering advice from someone in the know
so I could make knowledgeable input to the process.

Bitching about each other is not helpful to me.

I need somebody smart and experienced to give me USEFUL input
on the question I asked about minimum duct segment lengths for optimum
noise abatement. Making the duct segments too short may have the same
negative impact on the airflow, but possibly less positive impact on the
noise abatement.

Thanks, mike

>
> If you want to correct the noise issues, then you need to insure that the
> ducts are correctly sized (Manual D duct calculation) and installed. 900fpm
> is a bit much for RA. Correct the ductwork issues, and the noise will go
> away.
> Flex duct is *NOT* the way to go. find a contractor that is going to do it
> right, not just cheap.
>
>

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