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  1. #1
    Join Date
    May 2018
    Location
    Central Florida
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    196
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    Piston system.... hunting?

    I’m fairly familiar with seeing a TXV hunt on startup or when failing.... but today I watched a piston metered system appear to “hunt”.

    4 ton straight cool split with maybe 10 ft lineset.

    3 phase compressor took a dump from running hot with little to no refrigerant.

    We found and repaired a rub in an evaporator distributor tube and verified the system tight with nitrogen and deep vacuum.

    I thoroughly cleaned the evaporator (pulled and cleaned). We also isolated the refrigerant lines and verified no leaks.

    We ended up replacing the condenser with an older (still newer than the air handler) R-22 unit that was pumped down several years ago.

    When we got it online and running the first thing I noticed was very low superheat ... 3 to 4 degrees. With inside temps near 90 I advised overcharged and proceeded to reclaim some refrigerant to get somewhere in the ballpark of 25 superheat before I really dialed it in. ( This unit was pumped down off of a much longer line set, so this made total sense).

    While I started doing this I stopped after a lb or two as the superheat seemed to be coming into a more reasonable range.... but noticed it was fluctuating somewhat quickly (2 minute cycles?) from around 4 degrees to 25 or so. Subcool never really fluctuated much.

    We have stopped there for the day as there were more important things to attend to.

    My first thought was airflow, but in my mind an airflow issue would still have a consistent result on superheat.

    My second thought is contaminated refrigerant or too much oil in the lines (there was quite a bit of oil coming out of every joint we touched).

    Any thoughts on what would make a piston metered system look like it’s hunting?

  2. #2
    Join Date
    Mar 2015
    Location
    Medford, N.Y.
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    5,407
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    Low of refrigerant.Bad comp.

  3. #3
    Join Date
    Dec 2014
    Posts
    2,561
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    How much oil was in the compressor that you took out. And have there been other compressor changes on that system? Any inadvertent traps in the suction line?

    Sent from my SM-G950U using Tapatalk

  4. #4
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    8,758
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    What was the SC. I have seen fixed systems that when gas bubbles would go through the piston it would look like it was hunting.

  5. Likes Fender60, gravity liked this post
  6. #5
    Join Date
    Jul 2000
    Location
    Dallas,Texas
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    5,145
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    Quote Originally Posted by icy78 View Post
    How much oil was in the compressor that you took out. And have there been other compressor changes on that system? Any inadvertent traps in the suction line?

    Sent from my SM-G950U using Tapatalk
    Ditto to that. You added another compressor system and it is probably oil logging.

  7. #6
    Join Date
    Nov 2011
    Location
    East Side
    Posts
    6,179
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    Wrong sized orifice.

    Research the correct piston size for the condenser you installed.

  8. #7
    Join Date
    Jul 2009
    Location
    Columbia, MD
    Posts
    5,681
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    Oil logged evap would be low superheat but normalish suction pressure.

    Sounds like it was flashing

    What was the OAT, RAT WB, piston size, pressures sh, sc, llt, slt...


    Sent from my iPhone using Tapatalk

  9. #8
    Join Date
    Dec 2012
    Location
    columbus, OH
    Posts
    6,782
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    Ide have to agree about the oil. If the evap is lower its filling up then it gets pushed through.


    Sent from my iPhone using Tapatalk

  10. #9
    Join Date
    Nov 2011
    Location
    East Side
    Posts
    6,179
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    Are we going to get a follow up?

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