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  1. #1
    Join Date
    Jun 2009
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    Albuquerque NM
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    Best way to reduce noise from a big air return box (plenum)

    I'm looking for opinions on this one. I have an upflow furnace in my garage that sits on top of a big air return "plenum" box constructed from 2x4s and wood sheathing. This is a common set up around here. The box has been sealed to keep garage air from getting sucked into it. This plenum is fed by two returns, one of which is 20x10 duct (about 10 feet long) near the front entryway, and the other is a very short 20x14 duct in the living room. My airflow is only 1100/1300 on heating and 1000 on cooling, so I believe there is plenty of return for that.

    The 20x10 does not produce much noise, but the 20x14 "short return" does. It's more of a low frequency nosie like maybe the blower noise (and echos inside the hollow box), rather than the noise you get from high velocity airflow. The grilles are the ones with the larger spacing and are not noisy at all. I think the problem is that the 20x14 return is very short and feeds directly into the box.

    I'm looking for the best way to reduce the noise from the 20x14 return. The big air return box is 18H x 29W x 96L. The 20x10 duct feeds one end through about 10ft of duct, and the 20x14 feeds the other end almost directly.

    My regular HVAC guy said he could add some turns inside the box to lengthen the return air path. I don't think there is enough room inside there to create separate air pathways, so what he would be doing is adding some partial "walls" inside there to make the air make a couple turns. I'm concerned that adding those inside the box to lengthen the air path might actually create more air noise because the air has to go around them. I have another HVAC guy who says he would line the box with duct board and that would work best. So who is right?

    Thoughts and opinions?

    ETA: Oh, and I forgot to mention I have a concern about using the duct board as it is made of fiberglass, right?
    Last edited by garya505; 11-13-2012 at 02:55 PM. Reason: Added concern about fiberglass.

  2. #2
    Join Date
    Nov 2001
    Location
    East Texas
    Posts
    949
    L - shaped baffle to turn the air under the unit.

  3. #3
    Join Date
    Jun 2009
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    Albuquerque NM
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    Quote Originally Posted by Timber View Post
    L - shaped baffle to turn the air under the unit.
    So, you've fixed this problem before using that? I think what my HVAC guy was going to put in was straight baffles.

  4. #4
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    2,111
    Ductboard is fiberglass which is impregnated with resin to make it a rigid, non-eroding material. It is also a good sound dampener. Some people have a bias against it, but in my 20+years of using it I have not encountered any problems with erosion.

  5. #5
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,327
    I'm with Dean, although GaryL also has a good idea.
    with ductboard, the cuts are sealed with paint on mastic.
    so that no fibers escape. new and old ductboard are different creatures.
    old was not encapsulated, new is. face black side into return. mastic
    seal joints. you would want the ductboard with black coating...older
    ductboard didn't have this visible coating.

    with Gary's solution..I've seen sheetmetal with the same type of
    insulation that is inside plenums, and hard ducting.

    both worked. but still sealing edges/cuts of insulation is important.

    also, the hvac company caulk 2x's to floor of return, and all sides of
    r/a. the opening thru the wall into the house should also be sealed.
    open stud bays suck all kinds of stuff into return. this stuff collects
    on the coil & reduces performance.

    bet of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  6. #6
    Join Date
    Jun 2009
    Location
    Albuquerque NM
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    2,485
    So if go with the ductboard lining solution I should specifically ask for the ductboard with the black coating? The guy who suggested this had a piece of it I think he said was "OC 703", whatever that means, but it looked like it had like a silver backing on one side and was yellow on the other side. I didn't ask if it was coated because I didn't know anything about it at the time. And I need to ask him how he is going to seal the joints in the ductboard?

    Edit: Does duct board have good sound deadening properties?
    Last edited by garya505; 11-14-2012 at 02:21 PM. Reason: Added question about sound deadening.

  7. #7
    Join Date
    May 2004
    Location
    south louisiana
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    3,327
    ductboard is silver on one side and fiberglass on the other.
    you would want the antibicorbal black coating.

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  8. #8
    Join Date
    Jun 2009
    Location
    Albuquerque NM
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    Quote Originally Posted by energy_rater_La View Post
    ductboard is silver on one side and fiberglass on the other.
    you would want the antibicorbal black coating.

    best of luck.
    I see, the black coating is antimicrobial.

  9. #9
    Join Date
    Jun 2010
    Location
    Jacksonville, Florida
    Posts
    54
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

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    Last edited by jpsmith1cm; 11-15-2012 at 10:56 PM. Reason: non AOP member

  10. #10
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,946
    myott


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