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  1. #79
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    Quote Originally Posted by Bworth74 View Post
    I disagree, superheat will decrease with a pump that cannot handle the load. Even though the txv is opening and doing its job the compressor does not have the capacity to remove the heat. That is why compressors are sized the way they are. If you put an undersized compressor in a system then you will have a hard time handling the heat load and otherwise have high superheat.


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    If you put an undersized compressor in a system, the evap temp will run warmer, it will not be able to pull down. That would not cause the superheat to be higher, the txv will still modulate according to its setting.
    Worry is a really gross misuse of one's imagination. -- PHM

  2. #80
    Join Date
    Aug 2018
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    Fort Worth, Tx.
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    Thread Starter
    Quote Originally Posted by Chuck View Post
    If you put an undersized compressor in a system, the evap temp will run warmer, it will not be able to pull down. That would not cause the superheat to be higher, the txv will still modulate according to its setting.
    If the valves on a compressor are weak then it loses capacity. The txv will not get the right flow rate causing normal to high superheat it will not drop. If a system is not getting the right flow rate through the valve due to a weak compressor then you will not get enough refrigerant through the coil even though the txv is wide open calling for more refrigerant. Which will result in normal to high superheat across the evap coil.


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  3. #81
    Join Date
    May 2014
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    I have never seen a system with a weak compressor have a lower superheat. Lower meaning getting closer to 0*F superheat.

    In fact, before I learned how to use performance charts, high superheat on a system that otherwise seemed fine became my first clue that I was looking at a weak compressor.


    Quote Originally Posted by Chuck View Post
    Incorrect, try throttling the suction valve on a working system to simulate a weak pump and report the result.

    So if a box is at 40 degrees and the evaporating temp of 25 the superheat cannot be over 15 on the suction line exiting the box because the air in the box cannot heat the suction gas any higher than 40.

    Now let's say that compressor is getting weak and not pumping as well. This will raise the evap temp to say 30, now the max sh would be 10 because the air in the box cannot heat the refrigerant above 40 (until the box temp rises due to the warmer evap which would also raise the evap temp keeping the decreased SH about the same).

    The only way you would see increased sh is if you had a long run of uninsulated suction line that could gain more heat when the pump gets weak due to lower refrigerant mass flow picking up more heat on its way back to the compressor.

    It is completely impossible for the SH at the evap to increase if the compressor gets weak all other things being equal.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

  4. #82
    Join Date
    Jun 2011
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    Quote Originally Posted by BBeerme View Post
    I have never seen a system with a weak compressor have a lower superheat. Lower meaning getting closer to 0*F superheat.

    In fact, before I learned how to use performance charts, high superheat on a system that otherwise seemed fine became my first clue that I was looking at a weak compressor.
    looks as if the manufacturer of the equipment is calling for an evaporator outlet superheat of 3 - 5 degrees ...

    I think his numbers might point to a restriction ...

    highside seems normal and has adequate to high subcooling, lowside being low seems to eliminate bad compressor valves with existing load, low lowside seems to rule out too high of an evaporator fan speed to give high superheat....

    a 0 degree coil is 32 psi
    Last edited by hvacskills; 08-12-2018 at 08:45 PM.

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