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  1. #1

    High Superheat & High Subcooling

    Hi,

    I'm a newbie tech and I am stumped over a unit that I was trying to service. The unit a rudd model #uka-060 JAS, a 5 ton unit manufactured 1/93 hooked up to matching 5 ton capillary tube metered coil. According to the homeowner they had a tech fill it up with refridgerant last year when they first started having problems with the unit and it is still not working properly.

    The following readings were taken on a 87 degree day aproximately 50% humidity with a indoor temperature of 82 degrees. Low pressure 62psi @ 65 degrees superheat=30 degrees, High pressure 330psi @ 90 degrees sub cooling = 40 degees. inside temperature differential aprox 14 degrees.

    At first I suspected a dirty evaporator coil so I took it apart to check it and that wasn't the case while I was in there I took a few temperature measurements after the metering device it was 42 the middle was 47 and near the suction line it was 62 degrees. I had also cleaned the condensing coils as well but not improvement either. I was tempted to add more refridgerant but the high side pressure seems high already. There was no charging table so I am not really sure. I would like to charge to about 15-20 degrees superheat and about the same amount of subcooling. I am suspecting there may be some restriction but I cannot be certain.

    Thanks in advance,

    Ed

  2. #2
    Join Date
    Mar 2005
    Location
    Houston, Texas
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    11,847
    Sounds like your heading in the right direction.

  3. #3
    Join Date
    Jun 2006
    Posts
    294
    Adding more refrigerant is not indicated. You may have more than enough. You have a faulty metering device or a restriction as you surmised and that problem must be adddressed first.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,778
    I could be wrong. But I don't think it has a cap tube metering system. More like an orfice system with distributor tubes.

    Since your reading 330 head pressure, you kow the refrigerant has a good path up to the point of the liquid line access port.

    Now you just have to think how many points there are that could have a restriction between the access fitting, and the coil its self. And check them.
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  5. #5
    Join Date
    Jan 2008
    Location
    California
    Posts
    108
    Refrigeration effect seems really poor, I'd look for a clogged screen upstream of the metering device. Although I've always questioned the diagnosis in the past - I've had guys change out the evap coil due to oil blockage (really) and the result was the system worked normally. I'm clueless how that would end up restricting the system - but it was only on cap tube coils.

  6. #6
    Join Date
    Jun 2008
    Location
    Phoenix, AZ
    Posts
    70

    Cool

    It's 100% a restriction, I can't find your exact unit online, but those units have screens inside of the lines... look for a spot where the copper has been rolled... If you open the indoor coil and have a copper fitting screwed on to the distribution tube assembly there's actually a screen in there, if you pull the R-22 into the unit itself and open that fitting you can shine a flashlight up there and see the screen... On heat pump units there's one on the outside unit also near that metering device.. that's normally where I find the restriction on a Rudd unit... their TXV's have a decent amount of problems too, but if there's not one then there you go GOODLUCK young padawan

  7. #7
    Join Date
    Feb 2006
    Posts
    89
    amp out condenser fan motor. acid wash cond. coil. then check and adjust charge to proper superheat using a superheat charge sliding chart or the ruud recommendations (if available). 15 to 20sh isn't a magic number. if it is extremely warm in the house, you may see by the chart, you will require a higher superheat. or when hot as heck outside, but cool and dry in the home. you may see under 10 superheat is required for a proper charge.

    lower efficiency equipment runs higher head. on 90 degree days you are likely to run into units with over 300 heads, but ensure proper heat transfer of your condenser coils by cleaning them, and checking for proper airflow, before assuming a restriction. (head pressure may return to normal after cleaning.)

    when the evap is dirty or there is a lack of airflow, you will see low suction pressures and low superheat.

    if after ensuring the condensor is ok

    then move to phase 2: possible restriction

  8. #8
    Join Date
    Jun 2008
    Location
    Phoenix, AZ
    Posts
    70

    Cool

    If there was a problem with the airflow on the condenser coil, the suction should be a lot higher I would think because the heat isn't being released...???

  9. #9
    Join Date
    Mar 2003
    Posts
    7,321
    any chance you got some air in there ladd?

  10. #10
    Join Date
    Jun 2008
    Location
    west point, ga.
    Posts
    328
    u-RUUD
    K-10 seer
    a-Strait air

    Should not have TXV.
    Does the furnace have a 5 ton blower in it?
    How is the line set ran?Is it posible some idiot put a filter dyer where you can't see it?(under the house or somewhere in the attic)
    10 seer so it should not have a doubled cond. coil.
    Non condenseables.
    I also say that the evap. coil could have oil in it as well.
    Start from a and work to z and I promise once you solve the puzzle you won't forget it.

  11. #11
    Join Date
    Jun 2008
    Location
    South,Tx
    Posts
    331

    there it is the problem

    I took a few temperature measurements after the metering device it was 42
    the middle was 47
    and near the suction line it was 62 degrees.
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  12. #12
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,907
    Quote Originally Posted by elum28 View Post
    Hi,

    I'm a newbie tech and I am stumped over a unit that I was trying to service. The unit a Rudd model #UKA-060 JAS, a 5 ton unit manufactured 1/93 hooked up to matching 5 ton capillary tube metered coil.(?) According to the homeowner they had a tech fill it up with refrigerant last year when they first started having problems with the unit and it is still not working properly.

    The following readings were taken on a 87 degree day approximately 50% humidity with a indoor temperature of 82 degrees. Low pressure 62psi @ 65 degrees superheat=30 degrees.
    With the above temps & 50% RH, the IWB depression should be close to 13 degrees, or RA 69-IWB with a SH target of approx 18 or 19-SH, not 30. Ultra High SC & High SH indicate restriction, probably at metering device or back toward condenser. If LLT was measured near metering device, then restriction should be there.

    High pressure 330psi @ 90 degrees sub cooling = 40 degrees. inside temperature differential approx 14 degrees.
    What was the LLT & where did you take it? i must be dense tonight, I am not sure how you got 40 SC. 330-psig is about 138 SCT minus LLT 90? = 48p SC?
    Unit is probably also way over charged!

    Indoor split depends on CFM per/ton humidity sources, & whether there is any hot air source entering the Return Air. Ball-park split with perfect load situation should be around 17-F.

    At first I suspected a dirty evaporator coil so I took it apart to check it and that wasn't the case (dirty E-coil produces low pressures & low SH with high SC) while I was in there I took a few temperature measurements after the metering device it was 42 the middle was 47 and near the suction line it was 62 degrees. I had also cleaned the condensing coils as well but not improvement either. I was tempted to add more refrigerant but the high side pressure seems high already. There was no charging table so I am not really sure. I would like to charge to about 15-20 degrees superheat and about the same amount of subcooling. I am suspecting there may be some restriction but I cannot be certain. Thanks in advance, Ed

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