resonating noise from new furnace
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  1. #1
    Join Date
    Oct 2005
    Posts
    5
    We have just had two new forced air gas furnaces installed. Both are two-stage variable speed units, with single stage t-stats. One furnace services the bottom two floors of the house and is 75,000 btus, and the other furnace services the top two floors and is 50,000 btus. The unit for the bottom two floors works perfectly. Unfortunately, the unit for the top two floors has been creating a loud resonating noise, which can be VERY annoying. The installer has been very good, and he has offered to come and replace the motor if necessary. But first he wanted to try some trouble shooting. Makes sense to me. He spoke with the tech support people at the manufacturer, and they suggested adjusting the some dip switches to change blower speeds, so I have been working with the installer to do just this. The factory default for the furnace had a Lo heat speed of about 625cfm, and a hi heat speed of about 825cfm (both at 0.1 of static pressure). This was the setting that was originally producing the loud resonating sound. Over a few days I have tried lower and lower blower speeds in an attempt to eliminate the noise. I have now gone as low as it will go, with a Lo heat speed of 500cfm and a hi heat speed of about 675cfm. At this point the noise at lo speed is completely gone, but instead there is some noise at hi speed. I should note that with the factory settings the noise occurred at low speed. This leaves me to believe that the "noisy" range is from about 675 to 825cfm. Unfortunately, it is not possible to adjust the lo heat and hi heat speeds independently, so it doesn't seem like I can come up with a totally quiet combination, although the way it is set now is considerably better. My question is - is a new motor likely to fix the problem? If we opt to leave it as it is now (ie. 500cfm/675cfm), will it function properly? It seems that at lo speed there is very little air movement out of the vents. Should I be concerned about this? Am I compromising efficiency at this low blower speed? Any input is much appreciated.

  2. #2
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,686
    may have a blower wheel out of balance or bad motor, i would replace both if noise is coming from heater and not duct work.

  3. #3
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    Did the tech try repositioning the blower wheel on the motor shaft? I have seen a few applications where this will solve the problem. I doubt that changing the motor will fix your problem if the noise is not coming from the motor itself. Changing the motor may make accomplish the same thing as repositioning the blower wheel and therefore solving the problem, but I would try adjusting the wheel on the shaft on the old motor first.
    "Go big or Go Home"

  4. #4
    Join Date
    Oct 2005
    Location
    Minnesota
    Posts
    12
    Personally, I would try to apply hand pressure on the duct work at all possible locations and see if the pressure eliminates the noise. If/when you find the problem area, then focus attention there.

  5. #5
    Join Date
    Oct 2005
    Posts
    5
    Thanks for all the info. The noise is definitely coming from the furnace - not the duct work. One question, that will make this decision easy. By adjusting the blower speeds am I really just masking the underlying problem? In other words, is there any scenario where a noise like this would be expected if the blower speed was not configured properly? Or should I be able to run at any speed with little or no noise?

  6. #6
    Join Date
    Nov 2000
    Location
    Waco, Texas, USA
    Posts
    6,153
    Is this a noise that only women can hear?
    "And remember my sentimental friend......that a heart is not judged by how much you love, but by how much you are loved by others" - Wizard of Oz.

  7. #7
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,686
    only noise you should hear at any speed is air moving.

  8. #8
    Join Date
    Oct 2005
    Posts
    5
    Thanks t527ed,

    So nice to get clear and concise answers! (Not to mention intelligent) Mr. Wiggins. I am not quite sure what you mean by "noise that only women can hear". I can only assume you are suggesting that this noise is so negligible, as to not be worth mentioning. This is not the case. At factory settings, the noise is very loud, and you can almost see the furnace shaking.

  9. #9
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    How does the return air enter the furnace?

    What brand/modle is the furnace?
    If more government is the answer, then it's a really stupid question.

  10. #10
    Join Date
    Oct 2005
    Location
    Minnesota
    Posts
    12
    Originally posted by rhombeus
    and you can almost see the furnace shaking.
    Is the furnace leveled? A furnace is a working machine, it's going to have some vibration to it. The one thing that I understand needs to be done is the furnace is leveled with an absorbant material (thin pieces of wood work pretty good) and the duct work needs to be isolated from the unit with a flexible link...pretty standard stuff from what I have read.

    Leveling is very important if it is anything like a washing machine?

  11. #11
    Join Date
    Apr 2004
    Posts
    7,680
    Originally posted by rhombeus

    At factory settings, the noise is very loud, and you can almost see the furnace shaking.
    The factory settings are not "factory settings". The factory connects the wires and the switctes are either on or off. It is up to the installer to configure both properly EVERYTIME. The factory has no clue what size a/c is going to be installed and the heating speed is dependant on temperature rise.

    As for your noise, I also suspect a clearance issue with the blower wheel to the housing or a wheel that is out of balance.

  12. #12
    Join Date
    Jul 2001
    Posts
    3,112
    Is the noise present all the time the furnace is operating or just when the inducer is running? Had a similiar problem with a 90% furnace. Problem was the gas line to the unit was jammed against the floor joist and when the inducer came on, the vibration would transfer to the joist. try to determine when the noise is present.

  13. #13
    Join Date
    Oct 2005
    Posts
    5
    The furnace is level, and there is no play between the furnace and the ground (ie. it can't be rocked back and forth), so I don't think leveling is the issue. Seems like the consensus is the blower wheel alignment, so I will follow up with this. Just for my curiosity. How do you go about configuring the proper blower speed? In this case there is no a/c hooked up to it yet, so I am just concerned about the heat settings. I know there are a lot of factors involved here, including static pressure etc. But my understanding is that the higher the cfm the faster the air will move over the heat exchanger and therefore the less chance it will have to be heated and therefore the lower the temperature rise (temp diff. between air going in and coming out). Is this in the ballpark? If this is the case, how do you go about setting the cfm? Is there a particular temperature rise you want to achieve?

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