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Split system Ac liquid line sweat in attic

Central Ac ,r22. Air handler in attic and condenser 20ft or so below on ground. No Ac issues. When Ac shuts off and then liquid line then gets cold when off so it sweats in hot attic and makes little water marks. I think this is because refregerant is equalizing after shut down. I guess? I have never seen this in 37 years of Ac servicing. Maybe it has been in places and I have not seen it. Thx


  • njtommy
    njtommy Member Posts: 1,105
    edited June 2016
    Check the liquid line drier for temperature drop a crossed it. It could be partially clogged. It's also not a bad idea for Any liquid line that runs through an attic should also be insulated to keep good subcooling numbers.
  • hvacfreak2
    hvacfreak2 Member Posts: 500
    Refrigerant migration and liquid line solenoid valve.

    Mechanical Enthusiast

    Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

    Easyio FG20 Controller

    JUGHNE Member Posts: 8,962
    I have had LLSV not completely seating do the cold sweat.
    LLFD almost plugged could do the same??
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,217
    Check the head valves of the compressor. If you have leaking valves the refrigerant will equalize from both ends causing the liquid line to rapidly decrease in temp.
    Do a pump down on the condenser and see if the compressor holds refrigerant on the high side.
  • aircooled81
    aircooled81 Member Posts: 197
    20' of pressure drop when liquid backs out of attic and into condenser could drop the pressure in the liquid line. I could beleive this pressure drop causes the refrigerant to boil, then rapidly cooling below the dew point of surrounding air.
    Never seen this with my own eyes, but pretty cool find.
    What might make for less of this could be a solenoid valve at the ahu/evap to help keep the migration less and stop the equalizing, or to insulate the liquid line.
    But now i'm really curious, what kind of pressure drop is slow enough to let the refer coil-off in the liq line long enough to swet that much?
  • Techman
    Techman Member Posts: 2,144
    Paul, where do you live? Thats for temp/humidity #'s. So, can one of you #'s Guys do the figuring on figuring out at what attic temp/RH% will condensation will occur?

    Does the lineset rise above the AHU before going out/down to the CU?
  • Paul_69
    Paul_69 Member Posts: 251
    Mass. Yes the line is higher
  • Paul_69
    Paul_69 Member Posts: 251
    Another point for all,is this system runs gets cold when unit shuts off. ll is warm when running and gets cold when shuts off only and in hot attic it sweats and leaves small water stains.
  • ChrisJ
    ChrisJ Member Posts: 11,978
    Silly question about TXVs.

    Do all TXVs equalize pressure when the system shuts down? I seem to recall some of them, or all of them, not.

    Second question, wouldn't insulating the liquid line solve the issue assuming the system is working properly?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Paul_69
    Paul_69 Member Posts: 251
    Yes it will Chris,just like to learn.I have suggested insulating to customer as a fix,just like to know why something is different.

    JUGHNE Member Posts: 8,962
    With this being R-22, it must have been in service for some time.
    If the line dripped previously it would have been noticed.
    Something changed.....does the LLFD get cold after shut down?

    Harvey's compressor valve test is a good idea, easy to do (not in attic anyway).

    Is this a fixed orifice or txv?
  • John Mills_5
    John Mills_5 Member Posts: 935
    Most TXVs are non bleed so they don't equalize. We just did a job with long line across attic. Tech rep said mfr. calls for armaflex on the liquid line across the attic so we did. Probably a piston meter and cold refrigerant is leaving the evap into the liquid line. Should loop the lines up above the coil then down as a trap.