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Thread: Sound Dampening

  1. #1
    Join Date
    Mar 2008
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    14

    Sound Dampening

    Hi All,

    I have a question about sound dampening.

    We just got a new gas furnace. It replaced a 30 year old gas furnace.

    The new furnace makes two fairly obnoxious noises. One is a small motor, I guess it’s a fan, that comes on about 70 seconds before the primary distribution fan comes on. It sounds a lot like a vacuum cleaner. The second noise is what I call “The dragon clearing its throat.” This noise lasts about 40 seconds and occurs in the time immediately before the main fan comes on. Once the main fan comes on, I can no longer hear this “throat clearing” noise.

    I spoke with the installers. They say the noise is normal for the class of furnace (80% efficient).

    My goal is to reduce this noise. It appears I have 2 choices. 1) is to go with a 90% efficient furnace, which they say is much quieter. 2) is my idea. And this idea is to decouple the furnace from the ducting. This process would replace 1 to 2 feet of rigid ducting with flexible ducting. I asked the installers for a quote on doing this work.

    Other than for the noise, the new furnace is great. I don’t want to mention the brand, but it is a name brand furnace installed by a local gas furnace only company that has been around for decades.

    So my question to any who will reply is: if I replace the ducting as described above, do you think it will it make any difference in the throat clearing noise?

    Thanks in advance!

  2. #2
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,754
    doubt ducting will make any difference in the noise you are hearing. sounds like draft inducer and endshot burners, no duct mods will quiet them.


    90% furnace would.......

  3. #3
    Join Date
    Dec 2007
    Location
    Ohio
    Posts
    2,876
    Where do you live and what kind of furnace is it? Did the installing company actually come back out and listen to the noises that concern you? Adding flexible ducting will not do much for sound but could help with vibration. I love it when they tell you on the phone that it is "normal" If a customer of mine said said something was too loud right after I installed it....I would be back out there before the day was over!! Where is your furnace located....basement, closet.....etc..???
    I need a new signature.....

  4. #4
    Join Date
    Mar 2008
    Posts
    14
    Thanks for the replies. The install company has been great. They’ve been out several times and on very short notice. They adjusted something to attempt to quiet the throat clearing but the difference was negligible.

     Where do you live and what kind of furnace is it?

    Seattle. The furnace runs on natural gas. It is installed under the house. It is mounted horizontally. It is connected by long screws to the ceiling of the crawl space. I'm not going to disclose the brand as I don’t want branding preferences to enter the discussion.

    Someone else suggested doing a test to simply disconnect the air ducting from where it interfaces with the furnace and see if the sound can still be heard. Is this time consuming to do? Note that I’ll have the install company do the work.

    > 90% furnace would.......

    How does the construction of the 90% efficient furnace differ from the 80% efficient furnace WRT sound dampening?

  5. #5
    Join Date
    Dec 2007
    Location
    Ohio
    Posts
    2,876
    The reason I asked what type of unit it was.....was not for bashing purposes.... the kind of unit I intsall comes with a low nox emitions kit installed in the burner area....we have (on occasion) taken those out of the heat exchanger tubes because low nox is not required here in Ohio. This helps with some of the sound in the 80% furnaces....especially when furnaces are in a closet or just off a finished room in the basement.

    As far as the 80% vs. 90% the burner noise is almost completely gone because the burners are sealed in their own chamber....a 90% furnace brings in outside air for combustion instead of using the air around the furnace.
    I need a new signature.....

  6. #6
    Join Date
    Mar 2008
    Posts
    14
     ...low nox emitions kit installed in the burner area...

    Ah a potential tweak! Hypothetically, if this were part of the furnace, and if it could be removed, what would the reduction in burner related noise be?

     As far as the 80% vs. 90% the burner noise is almost completely gone because the burners are sealed in their own chamber....a 90% furnace brings in outside air for combustion instead of using the air around the furnace.

    By “noise is almost completely gone” what would I hear out of vents that are mere 5 or 10 feet from the furnace?

  7. #7
    Join Date
    Jan 2006
    Location
    Powell River, BC, Canada
    Posts
    763
    With a 90% furnace installed and setup correctly, if the house is TOTALLY silent and there is no traffic noise etc. from outside, and if you listen at the register closest to the furnace, you MIGHT be able to hear the burners. An if it's a 2 stage on low fire, maybe not even then!

    The flex duct connectors are a good idea regardless of furnace. As well as isolators between the furnace and joists it is mounted to. There will allways be some vibration from the inducer or blower. Sometimes the ductwork can transmit and amplify small vibrations.
    Where are you? Are you done yet? I got ONE more call for you.....

  8. #8
    Join Date
    Mar 2008
    Posts
    14
    This is interesting and kind of sad. Our recently replaced furnace was almost 30 years old. It was nearly silent in operation. You are saying that now-a days, one has to pay a large premium to get what was standard not so long ago. It seems that our contemporary sense of value puts a looter around nearly every corner.

    Evidently i should have had the furnace rebuilt. Sigh.

    Thanks all for the excellent feed back!

  9. #9
    Join Date
    Jul 2000
    Location
    Northern Wisconsin
    Posts
    2,032
    I'm not familiar with Seattle or the amount of heating that your system would be doing per year. But..... like others have said a 90% sealed combustion furnace will eliminate the throat clearing noise you're hearing because of the burners being enclosed verses the one you have now. The sealed combustion furnace will also reduce the amount of noise you hear from the venter fan when it comes on.

    Comparing what was 30 years ago to what is today and the cost differences....... well you can look at anything you own and think back to what it replaced I don't think you'll find anything that hasn't gone up in price by large amounts. Noise wise is a different story. The older furnace was quieter because it used burners and most times blowers that were inefficient by design or by how they were set up to operate. The burners in todays furnaces are designed to reduce polution and to increase the amount of time that they'll burn cleanly without maintenance in less than ideal conditions.

    If you're ready to spend the money that will give you the best of both worlds I'd suggest having a 90%+ sealed combustion furnace with two stage heat and ECM blower technology. Most of these furnaces come with or can be covered by a 10 year parts and labor warranty. This will give you ultimate comfort, maximum efficiency and reduce the electrical usage of the furnace by more than half.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  10. #10
    Join Date
    Mar 2008
    Posts
    14
    > … This will give you ultimate comfort, maximum efficiency and reduce the electrical usage of the furnace by more than half.

    Very fair observations. I have to admit that my former furnace had only 2 service visits in the nearly 15 years I've owned my house. I only replaced the filter once every couple of years. The fuel (natural gas) and electric bills were always trivial, even given that the house is nearly 3,000 sq feet in size. There was little special about the former furnace. It simply did it’s job. That a new furnace would not be built to the same spec of quiet operation speaks to opportunism on the part of the manufacturers. Not just the one who made my new furnace but the industry which conspired. Of course, that is nothing new or unique to any industry.

    The only cost differences I know of is the almost double cost for “quiet” operation. Just because it is an industry standard – meaning that evidently all vendors do this, doesn’t make it less of a scam. Oh well, it’s only money. If the difference in cost means I can sleep at night that is well worth it to me. I’ll politely mention to the installation company that they failed to disclose that the term “quiet operation” as stated in their sales manual didn’t equate to quiet operation in my house. They have been highly compliant to this point, and offered a completely reasonable value and excellent service. I suspect they will continue to do so.

    Oh, BTW the installer said their 90% efficient furnace and their 80% efficient furnace both have a 20 year p&l warranty, so from what you said, that puts their warranty at above average.

    One thing I don’t get is the new trend for continual running fans, even at variable speeds. I understand that by circulating air more frequently there will be a more uniform temperature. But you don’t get something for nothing, and there has to be a higher electrical cost compared to a fan that runs long enough to fulfill the needs of heating the house and then shuts off. Can anyone explain this?

  11. #11
    Join Date
    Jan 2006
    Location
    Powell River, BC, Canada
    Posts
    763
    Your old furnace was probably well built to last for years. The problem is pretty soon you won't be able to afford to run it! Government mandates and consumer demand for higher efficiencies have led to all new designs and operations. The efficiency available now just simply wasn't possible then. They are MUCH smaller to allow the HO to cram it into the tiniest space possible so they don't have to give up room for their prized collection of beanie babies.

    And have you seen the price of metals these days!?! Even IF someone were to build a furnace as heavy as your old one, your kids would still be paying for it! They try to make them as quiet as they can, but efficiency and cost are the deciding factors.

    You're quite right. TANSTAAFL. It's up to YOU to decide if the gains are worth the cost. You do pay a little nore in electrical, but with a DC variable speed blower, running all day on low uses pennies. Not only will you get more uniform temps, but you are allways filtering your air keeping your house cleaner.
    Where are you? Are you done yet? I got ONE more call for you.....

  12. #12
    Join Date
    Feb 2008
    Location
    Everett, Wa.
    Posts
    88
    The fist sound you hear is the inducer, the dragon clearing his throat is the burners. You don't hear that after the blower kicks on is because the blower drowns out the burner sound. Going to a 90% furnace will only fix the dragon clearing his throat sound. I would have a second company come out and check the install and make sure it up to par. I'm not to sure I would trust a company that just sold me a furnace and immediately try to sell you a new one.
    Where in Seattle are you? How close is your neighbor? With the 90% all that noise comes out the venting and if your neighbor is only 10 feet away it will piss them off and most 90% furnaces are over the decibel level at 10ft that is allowed in Seattle.

  13. #13
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    i have to wonder if there isnt a problem here.

    most of the 80 percent furnaces today are not all that loud. If this were in a closet or utility room off the living space i could understand it being a little noisey. Being this unit is under the house and the noise is still offensive leads me to think something is in need of addressing. Also being it is hung from the beams this may be a transfer noise and isolation pads might solve the problem. The main blower noise can only be resolved by duct changes

    A90 percent will solve part of the noise issue and the blower noise will remain

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