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  1. #1
    Join Date
    Jun 2012
    Posts
    35

    Causes for low delta T over evap?

    Hi Guys,

    Can't wrap my head around something for some reason today, any ideas.

    With good subcooling (14) and superheat (13) and good airflow (350 per ton verified) what could be the cause of a low split? (13)

    entering wet bulb 65
    return air 75
    supply 62
    all taken at unit.

    I considered a restriction in the evap coil/metering dervice (TXV) but superheat should have been lower (coil) or higher (TXV) and pressure were close to normal 345/120.

    Cond. delta t was 10, ambient 95.

    I can see there is a problem but what I'm just not seeing.

  2. #2
    Join Date
    Dec 2011
    Location
    somewhere between here and there
    Posts
    478
    try putting probes in dif locations...maybe pocket of swirling air?


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  3. #3
    Join Date
    Sep 2006
    Posts
    95
    Check electric heat. If it has some.

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  4. #4
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,004
    Quote Originally Posted by 0351 View Post
    With good subcooling (14) and superheat (13) and good airflow (350 per ton verified) what could be the cause of a low split? (13).
    My first guess would be leak in the return somewhere, if you measured your WB at the return air grille, it would not effect the reading, if your leak is between the return grille and the AH/Furnace.
    __________________________________________________ _______________________
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  5. #5
    Join Date
    Dec 2011
    Location
    Asheville, N.C.
    Posts
    103
    410A?

  6. #6
    I have always heard 10 degrees is a good rule of thumb subcooling and you are getting more than a 3/1 ratio out of your compressor. Great on efficiancy but could be a sign of a slight charge offset?

  7. #7
    Join Date
    Jul 2001
    Location
    Anderson,SC
    Posts
    997
    Check duct work.

    STUD

  8. #8
    Join Date
    Dec 2009
    Location
    Upstate, South Carolina
    Posts
    103
    could be humidity in the air of the house

  9. #9
    Join Date
    Jul 2009
    Location
    South Carolina
    Posts
    3,230
    What size unit is it?
    It sounds like you're picking up heat in the system.
    If its a 4 or 5 ton I'd say you've got 5 kw strips running.
    If its a 2 ton I'd say you've got hot air leaking into the return.
    Gary
    -----------
    http://www.oceanhvac.com
    An engineer designs what he would never work on.
    A technician works on what he would never design.

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,123
    R410A at 345PSIG is 107F, with 14F SC, that brings your liquid line temp down to 95F. Your outdoor temp was also 95F. That should you give you an idea of where to look for a restriction.
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  11. #11
    Join Date
    Jun 2012
    Posts
    35
    Quote Originally Posted by beenthere View Post
    R410A at 345PSIG is 107F, with 14F SC, that brings your liquid line temp down to 95F. Your outdoor temp was also 95F. That should you give you an idea of where to look for a restriction.
    Yuppers liquid at ambient or below=liquid line restriction was my first thought, I was just excepting higher high side.

    Refrigerant cycle dxing is definitely a weak point for me as its so nebulous.

    I guess its one of those thing that you may or may not see high high side?

  12. #12
    Join Date
    Jun 2012
    Posts
    35
    It is a 5 ton A/C + furnace R410A, just had the ducts sealed so they "should" be good.

    Never thought about the electric heat issue, I'll keep that in my toolbox if I see the same issue on a fan coil.

    It was wet in house for the area but my airflow calcs said it should be an 18 split.

    Thanks for the help guys, sometimes being new to the trade I'm hesitant to make a call, glad to know I wasn't completely off in left field.

  13. #13
    Join Date
    Apr 2008
    Location
    Maryland
    Posts
    18
    Non-condensables will cause low split. Have to monitor pressures, SH, and SC for a while and see if you get fluctuation. I use min/max feature on digital guages. Can also use temp. clamp meter.

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