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  1. #1

    Unhappy Thump at condenser startup

    I own 4 brand new Carrier units whose condensers sit on a platform on my 2nd floor roof. They replaced 4 Trane units. When one of the Carrier condensers starts up, it generates a low frequency thump. The sound is very abrupt, lasting less than a second, and occurs just at startup. With four units starting up every few minutes, this is like popcorn popping at low frequency. My dealer/installer diagnosed it as a power surge at startup as the unit(s) draw power. He then installed a delay circuit on each unit and added rubber insulation below the units. The thumps are not as loud now, but they still happen and are very annoying. A neighbor's HVAC technician thought they might be caused by air in the freon lines due to incomplete flush at installation. Please help...

  2. #2
    Join Date
    Jan 2008
    Location
    Texas
    Posts
    1,953
    You might ask the tech if a start kit might help. I had a trane unit making a similar sound at start up and this solved it.

  3. #3
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,111
    Quote Originally Posted by thump View Post

    1. When one of the Carrier condensers starts up, it generates a low frequency thump.

    2. A neighbor's HVAC technician thought they might be caused by air in the freon lines due to incomplete flush at installation. Please help...
    1. Could just be the Scroll compressor starting up, some just sound like that at start up, 9 times out of 10 compressor noise is a result of the refrigerant system not being correct.
    2. Your neighbors a/c dude sure does a lot of assuming.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

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  4. #4
    Join Date
    Sep 2008
    Location
    Western PA
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    25,862

  5. #5

    thump

    Starter kits were what my seller installed to address the problem. It helped, but the thumps are still there.

  6. #6
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,111
    Quote Originally Posted by thump View Post
    Starter kits were what my seller installed to address the problem. It helped, but the thumps are still there.
    Personally speaking, I think start assist's kits should be the very last thing to address issues like this, until all other possible issue's are exhausted. The thumps still there, it's just not as noticeable because the start assist "defibrillator" energizes it with more power at start up.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  7. #7
    Join Date
    Mar 2003
    Posts
    1,234
    When one of the Carrier condensers starts up, it generates a low frequency thump. The sound is very abrupt, lasting less than a second, and occurs just at startup. With four units starting up every few minutes, this is like popcorn popping at low frequency.

    Is one unit making the noise or all four?

  8. #8

    thump

    all four -- each one does. two 4 ton units and two 2.5 ton units. The 4 ton thump is louder than the 2.5 ton thump.

  9. #9
    Join Date
    Mar 2003
    Posts
    1,234
    Have you had anyone, other than your neighbors hvac guy, actually check the charge? Adding a start assist may bandaid the issue but a more indepth checkout couldn't hurt. Some compressors/contactors make noises when they start up and the sound may be different from the older reciprocating compressors you had before.

  10. #10

    thump

    I'm pretty sure my seller/installer checked the charge (freon pressure/level?). They were here for several hours testing voltage (OK), installing the start kits, and adding rubber insulation under the units.

  11. #11
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,602
    How much insulation do you have in your attic?

  12. #12
    Join Date
    Feb 2010
    Location
    NE Alabama
    Posts
    301
    I'm pretty sure my seller/installer checked the charge (freon pressure/level?). They were here for several hours testing voltage (OK), installing the start kits, and adding rubber insulation under the units.


    I;m pretty sure is just like saying "I have no idea and don't want to ask". Sorry for the abruptness of my comment, but REALLY.

  13. #13
    Don't get me wrong, they had all kinds of equipment over, but I will ask about the charge anyway.

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