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  1. #1
    Join Date
    Nov 2008
    Location
    DFW,TX
    Posts
    2,169

    a little diagnostic help with some numbers

    Ok have a unit that is under performing per customer.. went and checked it out just a Payne R22 straight electric /cool horizontal system about 6 yrs old with a scroll compressor and txv..

    Has a low indoor split delta t around 15 at plenums however i took numbers to verify theres an issue..

    The indoor temp was 77 and outdoor was 83 and in the shade the pressure were 87 suction with 4 degree super heat and 176 head with 4 sub cool.

    the plate states 10 degree target sc but if i try and add r22 the suction will go up as will the evap temp which wont solve my issues and the SH will drop so low it cause future issues with flooding condenser etc..

    My amps for compressor were 3.75 and plate stated rla 7.7..so a little low?

    so i didnt have a megger (dont own one yet) or my airflow instruments with me because this is a 6 yr old unit but my first guess is the compressor valves are weak or not opening closing properly etc..

    what do you guys think is an issue if any? the suction pressure is way high and sc and sh way low for these condition's IMO and im thinking compressor issues?

    any thoughts?

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,077
    TXV over feeding. If it was a Rheem I'd say blow the wax out of it. But since it isn't, may have to be replaced. Bulb on good and insulated? Equalizer properly hooked up?

  3. #3
    Join Date
    Sep 2007
    Location
    State College, PA
    Posts
    1,049

    You may be right

    If it was a recip, I would say valves. Not quite sure what can go wrong on a scroll internally.

    Does the compressor run hot?

    If so I would definitely say, new pump.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  4. #4
    Join Date
    Jul 2010
    Location
    nebraska
    Posts
    1,629
    Over feeding txv. A system contain x amount of refrigerant. Some in the condensor,some in the evap and some in the line set. When one area gets over fed the other doesn't have enough. A bad compressor is not going to give a low SH.

  5. #5
    Join Date
    Sep 2007
    Location
    State College, PA
    Posts
    1,049
    Quote Originally Posted by acguytx View Post

    so i didnt have a megger (dont own one yet) or my airflow instruments with me because this is a 6 yr old unit but my first guess is the compressor valves are weak or not opening closing properly etc..
    A megger won't tell you if the compressor is not pumping. It will only test the insulation on the windings.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  6. #6
    Join Date
    Sep 2002
    Posts
    2,728
    Block the condenser,build up head,get new readings.

  7. #7
    Join Date
    Jan 2001
    Location
    Nebraska
    Posts
    319
    First of all you need to give us some more numbers to work with.

    It appears your condenser is not getting alot of heat to reject, your TD is only about 9.9°F (What is the seer rating onthe unit?).

    Unless you have a restriction with flashing present the subcooling must always be less than TD. With little rise in the TD you can’t get normal amounts of subcooling.

    On the Evaporator there is not too much to work with here you really need some wb readings here. I guess if you are saying you had 77° with a 15 degree split I guess you are indicating the EAT db =77° and LAT db of about 62° with a SST of 51.8° and a superheat of about 4° Where was that SH measured?. You don’t appearto have much of a load here.

    Did you try the normal of TXV checks, check the bulb attachment and position, check the equalizer, do the warming the bulb and cooling the bulb?

    You really don’t have enough metering dP to operate a TXV anyway but if you have service valves you might try cranking down on the LL service valve to turn it in to a metering device and see if the pressures will change, if they do watch the amps to see if they rise.

    PS: I was not sure what the ODA temp of the air on the CU was, you gave the temp. in the shade was that the same as at the CU?

  8. #8
    Join Date
    Apr 2009
    Location
    Pan Handle, Fl
    Posts
    599
    If the evap coil has a txv you check and charge using subcool. You need to try and bring it up to 10 like name plate states. The tvx will open and close as needed so your suction pressure will change. The low amp draw and TD (which you need wb and return db to determine target temp split) could be from low charge.
    Insanity: Doing the same thing over and over again expecting a different outcome!

  9. #9
    Join Date
    Nov 2007
    Location
    Sherman, TX
    Posts
    9,441
    Sounds like TXV problem....If it was a piston, I'd say it was too big....
    Technical incompetence is NOT a sales tool....

  10. #10
    Join Date
    Feb 2004
    Location
    michigan
    Posts
    677
    Clean the evap coil then recheck!
    I am the "Wally". All others are meer imitations of the original.

  11. #11
    Join Date
    Sep 2011
    Location
    Vancouver, WA
    Posts
    254
    You stated a 4deg subcool and a 77 inside temp with a back pressure of 87#. You added r-22 and the suction went up. To charge by subcooling you add refrigerant to lower subcooling temp, you remove refrigerant to raise subcooling temp. Your subcooling temp is low which indicates you have an over charge. Also your superheat is low, 4deg, 20deg is a general target. With out knowing if the unit has a correct charge, adding or removing refrigerant is unproductive. I would would recover and weigh back in the rated charge on the unit. Adjust to the proper 10deg subcool. Add refrigerant to lower, remove to raise.

  12. #12
    Join Date
    Nov 2007
    Location
    Sherman, TX
    Posts
    9,441
    Quote Originally Posted by Williamsma View Post
    You stated a 4deg subcool and a 77 inside temp with a back pressure of 87#. You added r-22 and the suction went up. To charge by subcooling you add refrigerant to lower subcooling temp, you remove refrigerant to raise subcooling temp. Your subcooling temp is low which indicates you have an over charge. Also your superheat is low, 4deg, 20deg is a general target. With out knowing if the unit has a correct charge, adding or removing refrigerant is unproductive. I would would recover and weigh back in the rated charge on the unit. Adjust to the proper 10deg subcool. Add refrigerant to lower, remove to raise.
    4 degrees of subcooling does not indicate an overcharge....

    High suction, low head, with low superheat and subcooling (as well as low compressor current) leads to a metering device that is gushing refrigerant, or perhaps bad compressor valves, which is easy enough to check.
    Technical incompetence is NOT a sales tool....

  13. #13
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,935
    Quote Originally Posted by Williamsma View Post
    Also your superheat is low, 4deg, 20deg is a general target.
    How many TXV's are set at 20° SH? maybe on a Lennox.
    __________________________________________________ _______________________
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