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  1. #1
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    Aug 2019
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    Bad condo vibrations

    I live in a 17th floor of an 18th floor building. Since March, I started experiencing mild to severe vibrations that woke me up multiple times during the night. Important detail: all HVAC/AHUs/RTUs, etc are mounted on the rooftop of the 18th floor. I cannot say with certainty what’s up there because I have no access. Management said everything is working well and hasn’t done anything about it. Two things became obvious pretty quickly: the patterns were highly dependent on the outside temperature (or perhaps the “delta” in temperature from one day to the next), and sometimes they get worse. There are times vibrations feel almost “seismic” in nature and other times they cause a very disturbing sensation in my ears.

    I recently hired and engineer to measure vibrations from “0” to 20 KHz. An illustrative segment (in seconds) of one files is attached, focusing on frequencies below 100Hz where most of the activity was found.
    The engineer concluded that the 29-58Hz signature is indicative of a misaligned or imbalanced motor running at 29Hz (1750 rpm actual, 1800 rpm nominal) and its first harmonic. Those 2 frequencies are present all the time, but as you can see these vibrations are low in amplitude compared to everything else in the plot. There are huge oscillations below 1.5Hz, unexplained peaks at around 10Hz (between 7.5-12.5Hz) and a pretty constant 15 Hz signature. All these are larger than the 29-58Hz frequencies and the fact some of them come and go, makes me think these underlie my subjective experience of worsening at times. The question I’m hoping the community will help answer is what the source(s) of these unexplained vibrations are. Thanks all for your advice!100Hz time course 5.pdf

  2. #2
    Join Date
    Apr 2003
    Location
    St. Louis
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    Sound engineer with a mechanical engineer has to get together to resolve this problem

  3. #3
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    Aug 2019
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    Thread Starter
    Without having access to the units in the rooftop, all I am looking for are good working hypotheses. The engineer who came did both acoustic and vibration measurements, but he doesn't know where those could be coming from. Any insight would be helpful!

  4. #4
    Join Date
    Jul 2018
    Location
    Wisconsin
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    Take a google earth screen shot of the roof and post it here.

    Sent from my SM-G960U using Tapatalk

  5. #5
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    Thread Starter
    Attached google earth views from rooftop
    Attached Images Attached Images   

  6. #6
    Join Date
    Sep 2007
    Location
    edmonds wa
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    Is your home owner association involved? Looks like a pile of stuff on the roof generating noise. I would also ask the people who live above you if they are feeling anything. If those are condensing units scattered around the roof, that means there are a lot of line sets going down a long way and thru common wall areas.
    UA Local 32 retired as of Jan 2020

  7. #7
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    Aug 2019
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    Thread Starter
    Good questions Buford. Association insists there is no problem, and I think this is proof there is. Also, they said "someone" looked at it and everything looks OK, so whatever it is, is not something in-your face obvious (unless whoever looked at it is an idiot). No one lives in the 18th floor (it's all common areas), but vibration up there is MUCH worse. My unit happens to be directly under one of the AHU/RTU , and the vertical duct from the rooftop runs adjacent to my unit.

  8. #8
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    Jul 2018
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    Wisconsin
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    Looks like quite the hodge podge on that roof. Could be lineset noise in your wall as Buford stated. Could be a exhaust duct could be that rooftop unit. It all depends where they routed the refrigerant lines and ductwork. You may have a pipe chase in or near your apartment.

    Sent from my SM-G960U using Tapatalk

  9. #9
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    Aug 2019
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    Thread Starter
    Thanks Spitz. There is definitely a pipe chase inside my apt, which makes noise but it has been doing noise from the beginning of times. The problem in question is more recent. Also, that pipe generates vibrations at much higher frequencies (higher than 100Hz for sure). What I have is a low frequency problem as displayed in my graph. Any other thoughts are appreciated.

  10. #10
    Join Date
    Apr 2003
    Location
    St. Louis
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    Roof top units run 24/7 ? Since march it started do you think thats when the compressors started it warmed up enough for the units to come out of economizer. Rtu on a spring curb? Compressors set on a pedestal or they sitting on the main curb? Has anyone made sure all shipping bolts out of the blower and compressors?

  11. #11
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    Sep 2007
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    edmonds wa
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    Acoustic guy needs to be on the roof to pinpoint it.
    UA Local 32 retired as of Jan 2020

  12. #12
    Join Date
    Aug 2006
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    My first suspect would be a Frequency Drive on a Return or Supply Fan. We had a drive that got so bad it would throw the belts off, also I've seen Fan drives where someone has programmed "Braking," into the drive rather than coast down. A Condenser Fan motor on the RTU's would be my next choice. Most modern units randomly choose which motors to run depending on head pressures.
    Retired, after 43 Years

  13. #13
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    Aug 2019
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    Thread Starter
    Thanks ServiceFitter. I had replied to your msg earlier and it didn't post! Anyway, what I said earlier is that it started at the beginning of March and in fact, it was reeeeeally cold (subzero), so I doubt it was related to coming out of economizer mode. I don't have access to the rooftop, but when I do, I'll take note on the configuration of the unit. I also doubt there is a bolt of anything in there, unless they left a wrench in there during the March service.

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