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  1. #1
    Join Date
    Nov 2011
    Posts
    17

    Unhappy New A/C system sounds like space ship landing

    Just had installed a complete Lennox system with variable speed furnace(SL280V). The furnace is indeed much quieter than the dinosaur it replaced, but it seems that when it is running on the slow heat speed, there is a constant whine or "singing" sound from the motor. It seems to run most all the time, and when it does shut off, it sounds like a space ship engine winding down. This can be heard through the register in the master bedroom and I find it aggravating me when trying to sleep. When the motor is spinning down, I can also feel the frequency of the whine in the walls near the unit.

    The old furnace had a kind of spongy board underneath it, but my installers said they don't usually use such a board. I am thinking I need some type of vibration absorbent material that the unit can rest on. Any thoughts?

  2. #2
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    sounds like harmonic noise from a return grille caused by higher air flo
    spread the fins on the grille so as to change the angle they are on and may help

    could also be a bearing in the motor that wasnt oiled at the factory and the motor needs replacing

    call your installer back and have it looked at
    tell him all your concerns about the unit and see what he comes up with

  3. #3
    Join Date
    Nov 2011
    Posts
    17
    Quote Originally Posted by tinknocker service tech View Post
    sounds like harmonic noise from a return grille caused by higher air flo
    spread the fins on the grille so as to change the angle they are on and may help

    could also be a bearing in the motor that wasnt oiled at the factory and the motor needs replacing

    call your installer back and have it looked at
    tell him all your concerns about the unit and see what he comes up with
    Thanks, yes two different techs from the installer have come by and listened to the furnace fan. They say that is how the new fan motors operate. The whine is definitely coming from the fan motor.

  4. #4
    Join Date
    Nov 2011
    Posts
    17
    Quote Originally Posted by DGIO-Not View Post
    I do understand where tinknocker is coming from.

    Mind if I ask some other questions? Is the harmonic noise coming from a supply or return (sorry, if my next question is simplistic, but sometimes one has to ask: does the air blow out or is it pulled in?) Is your ductwork all metal? Is the unit sitting on a wooden floor?

    I noticed you said it is vibrating the walls near the unit although you have harmonics at one area only. It is hard to diagnose without seeing, and sometimes, it is the design of the original system (sorry, all components do need to work well together and that includes original ductwork, registers and grilles) that creates that very annoying "pitch." And of course, I have other questions, but this should help get the conversation going in the right direction for everyone.
    Thanks, the whine is coming from the fan motor, but I can hear it through the register in the master bedroom. The furnace/coil unit is sitting on a wood base in a closet in the middle of the house. What I'd like to do is isolate the furnace from the wood base it sits on, as well as from the nearby walls.

  5. #5
    Join Date
    Nov 2011
    Posts
    17
    Quote Originally Posted by DGIO-Not View Post
    The real easy answer would to ask them to install iso pads, and they should not have objection to this.

    Is this over a crawlspace or basement?

    Keeping mum on other questions for now, sorry. But not seeing it in person, I know these are very quiet units.
    Pics of the install:





    As they say, a picture is worth a thousand words. It sits on a wood shelf in this closet. And yes, Lennox claims it is the quietest operating furnace on the market. It is not the air induction noise that is the issue. It is that the motor has a constant high pitch whine when not on the fast speed. Unfortunately, it spends 90% of the time on the lower fan speed, at which the whine occurs.

    What are these isolator pads you mentioned? Wouldn't they need to be such that they supported the entire perimeter of the base, so that no closet air could enter the furnace? As you can see, the filter base sits directly on a plywood board. Thanks again.

  6. #6
    Join Date
    Apr 2001
    Location
    Over Here
    Posts
    1,105
    The noise you mentioned is coming out of the supply grille in the master BR? How close is it in relation to the furnace closet. Looks like you have plenty of filtered return opening for 5 tons, so I don't think it is return velocity noise. You have second'ed that.

    The whine you hear is the motor. I have a variable speed (ECM) motor in an A/H next to my office. It is also two speed, and when it is in low speed, it also has a whine. It's caused by the 'cogging' of a permanent magnet motor and is in its nature.

    You might be able to 'tune' the harmonic you are experiencing out by making a slight adjustment in speed to the motor. Call your installing company, and see if they could make a small adjustment in speed to the motor. There is a small window to work with as it is also important that the motor is @ the right RPM as correct airflow is important for your system's efficiency and performance. But a small % either side of your harmonic just might remedy the situation.

  7. #7
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,140

    need to register

    DGIO-Not, This is the Ask Our Pro's forum. In order to post responses here, you must have verified qualifications and have been approved by the AOP Committee. Please see my signature line for links to register and review the AOP Forum Rules. Thank you. Your post will now be deleted. Further infractions may result in loss of posting privileges.

    You've made 5 posts so far in the AOP today. Please refrain from futher posting until you get your * Thank you.

  8. #8
    Join Date
    Oct 2004
    Location
    So Cal
    Posts
    3,465
    I see several things wrong w/ install but it has nothing to do with the noise you hear.
    On the install- the gas line where it passes through platform must be sealed. Base of filter box must be sealed to platform. Furnace must be sealed to filter box or EZ base.
    I don't see a trap on that condensate drain from coil.

    On the noise- may be a bad motor. Did you have ducts replaced?

  9. #9
    Join Date
    Apr 2001
    Location
    Over Here
    Posts
    1,105
    Quote Originally Posted by precision hvac View Post
    I don't see a trap on that condensate drain from coil.
    The drain typically runs in a chase under the slab, then exits above grade creating the required trap. Typical install.

  10. #10
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,007
    Remove filter in base of unit and see if it goes away.

  11. #11
    Join Date
    Nov 2011
    Posts
    17
    Quote Originally Posted by thermojohn View Post
    The noise you mentioned is coming out of the supply grille in the master BR? How close is it in relation to the furnace closet. Looks like you have plenty of filtered return opening for 5 tons, so I don't think it is return velocity noise. You have second'ed that.

    The whine you hear is the motor. I have a variable speed (ECM) motor in an A/H next to my office. It is also two speed, and when it is in low speed, it also has a whine. It's caused by the 'cogging' of a permanent magnet motor and is in its nature.

    You might be able to 'tune' the harmonic you are experiencing out by making a slight adjustment in speed to the motor. Call your installing company, and see if they could make a small adjustment in speed to the motor. There is a small window to work with as it is also important that the motor is @ the right RPM as correct airflow is important for your system's efficiency and performance. But a small % either side of your harmonic just might remedy the situation.
    Thanks so much. That is very helpful info. I will get them to vary the speed a bit and try to mitigate the whine.

    Yes, I can hear the motor whine through the register in the master bedroom. Sounds just like it does elsewhere in the house.

  12. #12
    Join Date
    Nov 2011
    Posts
    17
    Quote Originally Posted by SBKold View Post
    Remove filter in base of unit and see if it goes away.
    Thanks, I already played around a bit with the filter. What happens is that if I pull the filter out about 5 or 6 inches, the fan will speed up dramatically. If I remove the filter completely the fan will speed up for a while, and then it slows down significantly. I am assuming it has some kind of air flow sensor that realizes that without the filter it can achieve the same air flow with less fan power.

    I also replaced the pleated filter with a standard fiberglass type filter, which seemed to reduce the motor whine a bit, presumably again due to freer air flow.

  13. #13
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,007
    I mentioned the filter because it should be a significant restriction causing your unit to struggle to move the proper amount of air.

    Increasing or decreasing fan speed away from what it is optimal to run is not helping the system be as efficient as its SEER, AFUE, ratings.

    You may have better, quieter results by redesigning that part of it.

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