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  1. #1
    Join Date
    Mar 2008
    Posts
    14

    How to reduce low frequency noise in heat ducting?

    How to reduce low frequency noise in heat ducting?

    I had a gas furnace installed to replace the 28 year old gas furnace that’s been providing heat to my house since it was built. The old one was silent in operation. The new one is really noisy at startup. The sounds consist of a vacuum cleaner like sound, followed by a timpani like rumble, followed by the main fan engaging. My chief complaint is the rumble. The company that did the install says the sound is attributed to the burners lighting, they also claim the sound as normal. I had another company check it out and they agree that the sounds are typical of most 80% efficient furnaces.

    The furnace is located under the house. The house is a 2 floor structure and you can only hear the noises downstairs. The noise comes through the air vents. I'm faced with throwing this noisy furnace away or finding a way to dampen the startup sounds. Given the cost of the furnace, I’d rather fix the problem, if possible, than start over.

    Back to the question at the top of the thread. What can I put in line to reduce the low frequency rumble? I've heard of ducting lining, but given that the ducting in place is only about 8” across, there is scant room for lining inside the current ducting. I'm not against replacing the ducting, if it will work. Using soft fiberglass insulated ducting instead of rigid ducting is a possibility, but someone warned that rodents can easily gain access to the house by chewing through this ducting. Since I can’t hear the startup related noises upstairs, I'm thinking I may get to my goal by doubling length of the ducting to the downstairs rooms.

    Any suggestions?

  2. #2
    Join Date
    Jun 2008
    Location
    San Jose, CA
    Posts
    662
    I know that in commercial they use lined hard duct a lot. The lining insulates (not a lot) and reduces transmitted noise. Because of the space the lining takes, the duct has to be larger to offset the loss.

    There's also an acoustic version of the flex duct.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,317
    Might be able to have the first section of duct replaced with a lined piece.
    May or may not stop the noise.
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    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    That's what I was gonna suggest as well to have the first couple of feet linned or replaced with duct bd. It may be caused by some forgein objects rubbing against the duct somewhere on the top side of the ducting. Maybe having those flex connectors made with canvas or neoprene coming off of the plenum to the trunk lines may help to reduce vibration.

  5. #5
    Join Date
    Jun 2008
    Location
    Surrey, BC, Canada
    Posts
    35
    I wonder if the noise is comming when the burners are on or fan only. With the burners going we have the draft inducer running and the in shot burner noise. Circulation fan for air only could be a ducting problem.

  6. #6
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,677
    unfortunately every new furnace I have seen has inshot burners now. The only good way to get rid of the noise is to go to a sealed combustion 90+ furnace, I am guessing the furnace is in an unconditioned crawlspace, so this is not a good option for you do to the condensation produced by a 90+. If it is a conditioned space, then I would suggest a 90+ may get the installing company to let you trade up since you are unhappy with the one you purchased, might work, might not. All you can do is ask.
    You can't fix stupid

  7. #7
    Join Date
    Mar 2008
    Posts
    14
    Thanks for all the feedback!!!

    The installing company claims no responsibility. They failed to disclose the noise, which, other companies have stated is a common complaint. The installing company claimed that they did nothing wrong, and refuse to do any more than to sell me another new furnace. No discount. No refund. They are, IMO dishonest for failing to disclose the noise issue in the first place.

    That said, thanks for the recommendation for an upgrade. The noise is definitely coming from the burners. Replacing the furnace with a 90+ is one option, and due to the cost, it is not my first choice.

    Since you can’t hear this noise upstairs but can hear it downstairs, I'm thinking the least expensive way may be to extend the length of the ducting. Could I get the same goal of noise reduction by increasing the size of the ducting?

    Another possibility is to create a muffler of some sort, or to use ducting which will absorb low frequency noise. The furnace provides plenty of airflow. It heats the house in about half the time that the former furnace did.

    Where can I see an example of “hard lined" ducting?

  8. #8
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,677
    here is a link, they have a pic of what lined duct looks like

    http://www.insulation.org/mtl/pdfs/010325_384.pdf

    As far as upgrading to a 90+, you could call a different contractor, see if they are willing to buy your old unit, and install a new one. Or call a different contractor, KEEP your old furnace and Evil-bay it
    You can't fix stupid

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