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  1. #1
    Join Date
    Jun 2005
    Location
    PA
    Posts
    16

    Question

    Following readings were taken today on a RUUD 4-ton R-22 system:

    Liquid Line
    200psig >> ~101 deg F
    80 deg F (reading taken near service valve)
    78 deg F (reading taken about 6" from entry to evap)
    I calculate 21 deg F SC (should not exceed 15 deg F)

    Suction Line
    50 psig >> ~30 deg F
    77 deg F (reading taken near service valve)
    I calculate 47 deg F SH (should be btwen 20-30 deg F)

    Ambient Air = 79 deg F
    Air Exit Condensor = 86 deg F
    Air Temp Entering Evap at return air register near blower = 80 deg F
    Air Temp Leaving Evap = 66 deg F

    Condensor Coil and Evap Coil were cleaned two weeks ago. Have not taken amperage reading at compressor.

    From Troubleshooting with Superheat at:
    http://hvac-talk.com/vbb/showthread.php?threadid=49522

    I read:

    "A low side superheat greater than 30 degrees indicates too little refrigerant is in the low side.
    · A condenser subcooling exceeding 15 degrees indicates too much liquid refrigerant is in the high side."

    Further it states an example:

    "High superheat with high condenser subcooling indicates a
    restriction. Too much liquid is in the high side and too little in the low side."

    Would I expect a restriction based upon the info above or could it be low on refrigerant?


  2. #2
    Join Date
    Nov 2001
    Posts
    1,874
    After I checked the blower wheel and filter.
    And they both were clean.

    I'd suspect the charge.
    If you try to fail, and succeed.
    Which have you done ?



  3. #3
    Join Date
    Mar 2002
    Location
    Richmond, Virginia
    Posts
    4,264
    I'd say undersized piston or a TXV which isn't opening properly.
    There is nothing more frightful than ignorance in action....Mark Twain

  4. #4
    Join Date
    Jun 2005
    Location
    PA
    Posts
    16
    Originally posted by HVAC Pro
    I'd say undersized piston or a TXV which isn't opening properly.
    Evaporator has a sticker which reads "Unit equipped with piston #84." I'm not clear how to determine undersized piston when this came factory installed.

    Could additional R-22 be added in limited quantity on the high side until the suction line temp begins to drop and then let the system stabilize and take new readings? Desire is for SH and SC readings to then be within acceptable limits or at least trending towards acceptable limits.



    [Edited by pehanlon on 06-21-2005 at 11:12 PM]

  5. #5
    Join Date
    Mar 2002
    Location
    Richmond, Virginia
    Posts
    4,264
    I would pump down or recover and look inside to see if the piston or strainer is partially blocked. Then I'd pull the piston and look on the side to see if it is the proper size. (Number will be stamped on the piston.) I had a similar problem with a system last year. Turned out the piston was much too small. The sticker was right but the piston was the wrong size. I've also seen shavings stuck in the orifice partially blocking the opening, which would of course effectively reduce it's size.
    There is nothing more frightful than ignorance in action....Mark Twain

  6. #6
    Join Date
    May 2003
    Location
    Sarasota. Fl
    Posts
    1,251
    Look for frost.

    Frost is your point of restriction.

    Start at the metering device.

    If your a Homeowner.....

    Look for the Phone Book.
    It is the job of thinking people not to be on the side of the executioners.

    ~Albert Camus

  7. #7
    Join Date
    Jun 2005
    Location
    PA
    Posts
    16
    Originally posted by miami mike
    Look for frost.

    Frost is your point of restriction.

    Start at the metering device.

    No frost evident. Liquid Line just as it enters evaporator is cold to the touch but no frost. I would not expect a restriction and suspect undercharge but was hopeful Superheat and Subcooling info in the original post would provide basis for some sound diagnostics advice from this forum.

    Any further advice is greatly appreciated.


  8. #8
    Join Date
    Sep 2004
    Posts
    172
    Can you get the CFM and Static pressure for the supply duct also check the return duct for proper sizing and restriction. Low airflow (blower set to incorrect speed/dragging blower/restricted supply or return duct/etc.) can make it appear to be undercharged. 4 ton should be around 1600 CFM. Once proper airflow has been verified those posted readings might actually be useful. For better info list the following information; Unit design CFM and static pressure / Supply duct actual total CFM / Supply duct actual static pressure / Return duct size and return filter size with efficiency rating / Evaporator inlet WET BULB temp / outside ambient DRY BULB temp / low side pressure / low side line temp leaving evaporator / high side pressure / high side line temp at exit on condensor / temp at bottom of compressor housing / Dry Bulb temp at inlet of evap / Dry Bulb temp at outlet of evap / amp draw of compressor / condition of the klackenbocker (note: a klackenbocker can only be properly checked between the hours of 1:00 a.m. and 3:00 a.m.)

  9. #9
    Join Date
    Feb 2004
    Location
    Maine
    Posts
    483
    Originally posted by pehanlon
    Originally posted by miami mike
    Look for frost.

    Frost is your point of restriction.

    Start at the metering device.

    No frost evident. Liquid Line just as it enters evaporator is cold to the touch but no frost. I would not expect a restriction and suspect undercharge but was hopeful Superheat and Subcooling info in the original post would provide basis for some sound diagnostics advice from this forum.

    Any further advice is greatly appreciated.

    Liquid line should not be cold, should be around room temp. Is there a filter/drier on the liquid line? If so I bet your restriction is there. As others stated pull the piston and check for debris, and right size.

  10. #10
    Join Date
    Jun 2005
    Location
    PA
    Posts
    16
    Originally posted by dnt
    Can you get the CFM and Static pressure for the supply duct also check the return duct for proper sizing and restriction. Low airflow (blower set to incorrect speed/dragging blower/restricted supply or return duct/etc.) can make it appear to be undercharged. 4 ton should be around 1600 CFM. Once proper airflow has been verified those posted readings might actually be useful.
    Interesting comments as postings in the For Your Interest Fourm that speak to system efficiency discuss SH and SC but do not directly speak to CFM and Static Pressure.

    I could see where that may be important in low air flow situations but the condensor, evap and blower fan are clean. Very good airflow from what I can tell although it has not been exactly measured. System is about eight years old and has been working fine until this cooling season.

    I still suspect undercharge but am seeking advice from those well versed in SH and SC diagnostics.

  11. #11
    Join Date
    Mar 2002
    Location
    Richmond, Virginia
    Posts
    4,264
    So add some refrigerant! What are you waiting for? Use your digital scales and measure what you put in. If you don't get the results you like then go to the truck, grab the recovery cylinder and take it back out. Don't be skeeered.
    There is nothing more frightful than ignorance in action....Mark Twain

  12. #12
    Join Date
    Jun 2005
    Location
    PA
    Posts
    16
    Added R-22 today and took the following readings:

    Liquid Line
    275psig >> ~123 deg F
    80 deg F (reading taken near service valve)
    ---------
    43 deg F SC (should not exceed 15 deg F)

    Suction Line
    69 psig >> ~40 deg F
    50 deg F (reading taken near service valve)
    --------
    10 deg F SH (should be btwen 20-30 deg F)

    Ambient Air = 84 deg F
    Air Exit Condensor = 99 deg F >> 15 deg F delta

    Air Temp Entering Evap = 78 deg F
    Air Temp Leaving Evap = 66 deg F >> 12 deg F delta

    At this point I see low SH (10* vs 20*-30) and high SC (43* vs 12-15*F). This now indicates an overcharge from what I know. Agree? Any advice on how to tweak to get just right?

    Thanks!

  13. #13
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579


    self edit

    Norm

    [Edited by NormChris on 06-23-2005 at 10:11 PM]

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