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Thread: Both suction & discharge pressure drops into vacuum when unit runs.

  1. #1
    DIY Junior Member Grayfeathers's Avatar
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    Question Both suction & discharge pressure drops into vacuum when unit runs.

    Unit is a rooftop 5 ton Goodman about 10 years old. Unit was not running and compressor was grounded when I first arrived. Unit was also void of refrigerant because of two leaks, one in suction service line and the other in the middle of liquid line dryer.

    I repaired leaks, replaced compressor and dryer, pulled it into a deep vacuum and charged unit by weight to factory charge of 5.125 lbs. Unit starts after three minute delay and within seconds both the suction and discharge pressure drops at the same rate. I don't know how far it would drop because it's evident to me that it is not going back up and I quickly switch the unit off at the disconnect. I have had this done before and it's always been the dryer stopped up but this is a new dryer that I have installed. And before you ask, yes the dryer is in the system the correct direction. The only other option that I can see that it could be would be the metering device has trash in it. Am I missing something else? This metering device is hard to get to and will require a lot of man hours to check.

    Brief history. I have been licensed by the State of Alabama sense 1996 but I don't work on rooftop units much and I wanted other opinions before I began tearing this unit apart.

    Thanks for your opinions.

    Tim

  2. #2
    Jack of all trades DonL's Avatar
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    Hello Tim,

    Did you flush out the system before you filled it ?
    What is the model number ?

    Have a great day.


    DonL
    Theory only works perfect in a vacuum.

    Cyber Security Protection for Windows C:\ > WWW.WinForce.Net

  3. #3
    DIY Junior Member Grayfeathers's Avatar
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    Hey Don, thanks for your reply. Now when you say flush; I didn't use any chemicals used to flush out a system but I did run Nitrogen through the system while sweating in the dryer and compressor. I didn't notice any blockage while while sweating the connections. I don't have the model # now but can get it tomorrow.

    Tim

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    Nuclear Engineer nukeman's Avatar
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    Might seem silly, but are both of our king valves open? If high side was closed and suction open, that could pull both into vacuum. Depending on how the service valves are setup, you are usually looking at line pressure and not the pressure on the other side of the valve. If that one valve was closed (or blocked), both would show the same pressure and would show a vacuum if the compressor can pull from the system, but not return.

    Is this a split system or packaged unit? Heat pump or A/C only?

  5. #5
    DIY Junior Member Grayfeathers's Avatar
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    Default No king valves

    Unit is a package unit with gas heat. Everything is on the roof. There is no king valve just a straight shot from the service ports.



    Quote Originally Posted by nukeman View Post
    Might seem silly, but are both of our king valves open? If high side was closed and suction open, that could pull both into vacuum. Depending on how the service valves are setup, you are usually looking at line pressure and not the pressure on the other side of the valve. If that one valve was closed (or blocked), both would show the same pressure and would show a vacuum if the compressor can pull from the system, but not return.

    Is this a split system or packaged unit? Heat pump or A/C only?

  6. #6
    DIY Junior Member Grayfeathers's Avatar
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    Model number is PGB060100
    Serial # is 9810627804

  7. #7
    Nuclear Engineer nukeman's Avatar
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    What type of metering device? TXV?

    For both to show vaccum, it seems like you would have to have the blockage on the liquid side, before the high pressure gauge connection.

    Any other driers in the system? Any acid in the system from it being flat or the dead compressor?

  8. #8
    Jack of all trades DonL's Avatar
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    Normally when a compressor burns up, It puts many contaminants in the rest of the system.

    The only way to properly repair, after compressor replacement is to uses a liquid flush solution, or it may not last long.

    That insures that all contaminates and the contaminated excess oil is removed from the system.

    It is good that you shut it down when you noticed the problem, as not to contaminate the new compressor.

    That Model has the capillary built into the evaporator coil, and most likely is clogged from the compressor burning up.

    You may have to remove the compressor to flush it out, so you do not put contaminates into the new compressor.
    You may also want to replace the dryer, if there was any chance that it got contaminated.

    Also the coolant will have a funny color, and is visible most of the time, but not always, but will smell of burnt motor windings.


    Good Luck on your project.


    Have a Great Day.


    DonL


    P.S. I am sure you know this, Cutting corners don't pay off.
    Last edited by DonL; 05-20-2011 at 04:27 PM. Reason: P.S.
    Theory only works perfect in a vacuum.

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    DIY Junior Member Grayfeathers's Avatar
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    NukeMan, It has a piston style metering device.

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