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  1. #1
    Join Date
    Jun 2005
    Posts
    42
    I have a question about the PSC motor versus the variable speed motor as it relates to noise. My furnace has always been located in a closet in my kitchen and is mounted on a raised platform. The return air plenum consists of the space between the concrete slab and the raised platform. The return air grills are in the room opposite the furnace. There has been some sound deadening material and baffling in the return air plenum since the old system was installed.

    I just had my furnace and air conditioner replaced a couple of months ago. My old system was a Lennox 5-ton 2-speed system with a furnace that had a PSC motor that had 2 blower speeds. My new system is a Lennox 5-ton 2-stage system with a 2-stage furnace with a variable speed motor.

    I understand that the variable speed motor has a number of advantages such as lower electrical costs, motor feedback to achieve a given cfm, dehumidification options, and the flexibility to change ramping and cfm with switches,. I was also told the VS motor was quieter than the PSC motor. I have found that the variable speed motor has a lot more noises than my old PSC. The only noise I remember on my old PSC motor was on start up. The stopping seemed to be quiet. On the VS motor, the stopping seems to have a lot of noises toward the end of the stopping. I have been told that this is due to braking and that the noises are normal for a VS motor. I find these noises very noticeable. I presume that if the furnace was in the attic, I would not hear these noises. I realize that you cannot hear the noises since you are not at my house

    1. For those of you that install these VS motors, do you notice more noises during the stopping phase than the PSC motors?

    2. Are the PSC motors the same as 15 years ago or do they also have new braking that would make them noisy on stopping?

    3. Do you find some of your other customers have noticed what I am noticing?

    Considering the additional noises, a part of me wishes I had stayed with the PSC motor. I really had very few complaints about it.

  2. #2

    Comparison of noises on VS and PSC motors

    Sounds like excessive banding on the VS feedback circuit. Check the drive current.

  3. #3
    Join Date
    Apr 2004
    Posts
    7,680
    Part of the problem is the lack of return duct, this gives you direct noise from the motor itself and yes, you are right, the ECM motors do have different sounds, it's unfortunate your lack of return brings attention to it. I generally hear a similar complaint when the return is short like that. I'm not sure there is a good solution short of ducting a return elsewhere.

    I understand this is at shutdown? If you are getting a huffing noise durring operation, you are probably running a high static but at shutdown, I'm not sure what it is you might hear.


  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    If there's just one return grille,in the space under the furnace, Quietturn Vanes can help,with noise at the grille.

    http://www.durodyne.com

    I'd have them sheck the static pressure ,as doc mentioned high static will make more noises.It may also be trying to deliver more air then needed if it's not setup correctly.

    Does the air flow seem greater than the old furnace??

  5. #5
    Join Date
    Dec 2002
    Location
    in a house, Appomattox, Va.
    Posts
    3,281
    Any new motor would seem noisy, since the sound is different from old unit.

    They do have a unique sound at startup as they ramp up, also
    Col 3:23


    questions asked, answers received, ignorance abated

  6. #6
    Join Date
    May 2005
    Posts
    996
    i think it sounds kinda like a jet engine on start up but i find that they sorta make a 'grind-whine' noise on coast down

  7. #7
    Join Date
    May 2000
    Location
    Urbandale IA. USA
    Posts
    4,913
    Have you considered letting it run all the time?
    I just installed a VS 2 stage unit in my house and I keep the fan on all the time.
    I have not noticed any unusual noises on shutdown in any of the VS units I have worked on or installed.
    I have noticed a whining/squeeking noise on startup.
    It is like the motor starts in reverse and then imediately goes in the forward direction.
    My RA plenum is longer than yours though...


    Those who dance, appear insane to those who do not hear the music.
    Those who believe, appear ignorant to those who do not know God.

  8. #8
    Join Date
    May 2005
    Posts
    996
    i have noticed they tend to want to start in reverse but dont quite make it. they switch directions and seems like the motor pulses up to speed.

  9. #9
    Join Date
    Jun 2005
    Posts
    42
    Sorry it took so long to reply to some of your comments.

    cehs

    Have you considered letting it run all the time?
    I live in a very humid climate and have been told that running the fan all of the time could lead to mold problems.

    docholiday

    If you are getting a huffing noise durring operation, you are probably running a high static but at shutdown, I'm not sure what it is you might hear.

    The noise is not while it is running and the static pressure has been checked and is ok.

    mr hvac41

    i think it sounds kinda like a jet engine on start up but i find that they sorta make a 'grind-whine' noise on coast down
    The 'grind-whine' noise on coast down is a good description of what I am hearing.

    Does anyone know if the current PSC-motor furnaces have noise on shutdown or are they like the ones from the 1980's?


  10. #10
    Join Date
    May 2005
    Posts
    996
    why would you have mold problems it your constantly moving the air in your house? besides, if your keeping the ac on all summer, the house would never get humid enough to even cause mold. correct?

  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,271

    R.H. Control

    Originally posted by mr hvac41
    if you keep the ac on all summer, the house would never get humid enough to even cause mold. Correct?
    No.
    Use a ceiling fan if you wish to move air.

    Some older (or relatively new leaky) systems may not be maintained to continuously achieve < 60% R.H.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  12. #12
    Join Date
    May 2005
    Posts
    996
    well, a celieng fan moves air. so why would it cause mold issues if the blower on the furnace runs all the time? besides, most houses i've seen have dehumidifiers in the basements and thye are running

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