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Thread: aaon question

  1. #1
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    Hey could I get a hand with this write up? I just did a call on an aaon unit and I'm not sure what to write up for repairs.

    1 st stage compresser was disabled by a previous tech, he pulled a wire off the contactor.

    2 nd stage had a failed contactor and timer relay. worked fine after replacement.

    ok 1 st stage.... written on the front panel was "replaced txv, drier, 8-04"

    I connected the wire to the contactor, i st fired and immidiately brought head to cutoff at about 375 psi, while suction dropped to arond 40. suction was still in desent when high press cutout killed monentum. right as suction began to hit 40 I could hear the h gas bypass start to open.

    At this point I held in the contactor. this time as the head hit about 400, suction was at 20 and the gas bypass opened full throttle. with bypass open, pressures regulated. having bypassed the condenser, pressure gradually rose untill pressure switch pulls the cord @ 375.

    I called the senior tech a few minutes ago for advise on the write up. he said the txv is bad, write up a replacement. he disagreed with my thoughts.

    my thoughts were....if the txv were once again bad, plugged even, the refrig would stack in the condenser untill the low suction added hot gas, not fire right up to 375 and die in seconds.

    he contends that the bypass is already open on startup, is dumping gas right into the compresser and causing an immidiate rise.

    problem is I think I'm right, and this is a costly f-up if I replace it just to have it do the same thing. then again I'm a rookie and with the way that thing is piped, if I try to redo the piping and it is a txv problem....and against the advise of the senior tech...im a dick.

    point is I really think there is a restriction between the bypass and the condenser.




    [Edited by greenears on 07-01-2005 at 06:08 PM]
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  2. #2
    Join Date
    Oct 2001
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    your suction is the problem not the head pressure,and where is the HGBP piped into the txv area or the suction back at the compressor?that shouldn't open till as you have the suction drops below a typical 50PSI and cranks at your now running pressue of 20 PSI.that head going up fast like that is a blockage(FANS on with contactor pushed in) check the temp drop across the dryer see if it chilled going >>>>>> to the evap,just out of the condenser!
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  3. #3
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    Originally posted by maxster
    check the temp drop across the dryer see if it chilled going >>>>>> to the evap,just out of the condenser!
    the filter is piped in right before the txv. plus it's brand new, so I think that even if it were clogges I'd see this behavior......if you put a soliniod in it's place it'd pump down.

    the hot gas pipes in right before the compresser to before the condenser. it runs a short loop.

    were talking running about three seconds before the hps breaks. the suction is on it's way down very quickly, but isn't deep enough yep to open the hgbp.

    once it opens my suction jumps to 70 and discharge goes to maybe 225.
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  4. #4
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    If your head is skyrocketing on startup the refrigerant doesn't have anywhere to go. Check the temps right down the discharge line. Are you measuring the high pressure on the discharge or the liquid and is there a liquid access? If there's a muffler that's the likely problem (clogged up).

  5. #5
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    head press is being read right after the compresser. if you look at these Aaon units, the discharge is only visable for about a foot run. the line after the hgbp was hot, so it's probably moving a little and I had my handometer, but I just couldn't get to it. I wouldn't be too surprised to find sweat at some point in the gas line. checking this line will be a big job though.

    thats why I really don't want to be wrong.
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  6. #6
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    Unless someone has just dumped a ton of extra gas into a system, a bad txv will not cause the head pressure to sky up on startup and trip fast. Only a blockage close to the compressor can do that.

  7. #7
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    seaboard, thats what I thought. it's gotta be between the hgbp and the condenser, right?
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  8. #8
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    Sounds like it. You can't take anything for granted. Once I was working on a Carrier with signs of a restriction and found the small condenser subcooling header had one tube coming out of the tube sheet shoved completely against the back wall of the header. You stare at it and tell yourself the temps you're measuring can't be right, there's no way, but where the temp changes is where your problem is.

  9. #9
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    ok thanks dude I'm just gonna keep the write up vague.
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  10. #10
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    Some times you have to remove some gas to keep them running long enough to find the restriction.

    If you want to prove it is, or isn't the txv, put a piercing vale on right before the txv.

    If the pressure right before the txv is low, its not the txv.
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  11. #11
    i think you need to give us subcooling and superheat.
    a bad txv will not make head pressure go high unless overcharged. make sure condenser coils are clean, all fans are right rotation, kill hgbp and check, check and see if driers are not restricted,look at txv and if its freezing up then i would replace txv, install new drier cores and i would also turn compressor off run the condenser fans then check and check to see if refrigerant is pure and not contaminated.

  12. #12
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    just curious as to the size of machine we have a similar situation on a aaon machine it is a 60 ton machine and cir 1 has a high head and lo suction symptoms we have had to replace compressor a few times and hvave never remedted the situation everything has been replaced even switched cond> between cir 1 and cir 2 to see if something was going on in the cond.interested we talked to aaon and they had retrofit cond. fan blades tried them no help very interested to see your end result>

  13. #13
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    Originally posted by airworx
    i think you need to give us subcooling and superheat.
    a bad txv will not make head pressure go high unless overcharged. make sure condenser coils are clean, all fans are right rotation, kill hgbp and check, check and see if driers are not restricted,look at txv and if its freezing up then i would replace txv, install new drier cores and i would also turn compressor off run the condenser fans then check and check to see if refrigerant is pure and not contaminated.
    well with the hgbp wide open and screamin, sh and sc are pretty much arbitrary numbers imo. I didn't bother with them. like I say, head flys up, suct flies down..in about 3 secs. mechanical type hgbp opens to the demand of the low suction, pressures equalize and then climb to cutout. the only reason I could see them climbing that fast after the hgbp has opened, is if weve bypassed the condensor.

    I found myself arguing with the first tech to go there this morning. he wrote up the hgbp a few weeks ago, but I say it's doing its job just fine. suction goes to 40 and it opens. plain and simple, it works.

    the more I think about this, the more I am absolutely convinced that the discharge line is restricted before the condensor.

    this turned into a discussion of about five guys and I not one of em agrees with me, and unfortunately the job was not approved. 2nd stage is keeping up fine.


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