Compressor Sluggin-Best Place For P-Trap?
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  1. #1
    Join Date
    Jun 2011
    Location
    Seattle WA
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    1,225

    Compressor Sluggin-Best Place For P-Trap?

    Hi Everyone, we just installed a system with a fairly long (75' or so) 1 1/8 lineset. The evaporator is around 15-20' higher than the compressor. I started the system up and dialed the charge in. After the system was turned off for about 45 min I made a call for cooling and went out to the heat pump. The compressor was trying to slug liquid. I'm guessing the oil migrated over the 45 min. I'm fairly certain the fix for this is to install a P-trap. Where is the best location to put the trap???
    America; first we fight for our freedom,
    then we make laws to take it away.

    -Alfred E Newman

  2. #2
    Join Date
    Apr 2012
    Location
    new york
    Posts
    134
    at the bottom of the riser

  3. #3
    Join Date
    Jul 2005
    Posts
    5,304
    If oil migrates to the compressor, then that's always a good thing because that's where it's needed. Oil traps are designed to help oil return to the compressor, not to prevent it from getting there. It's more likely that you're talking about flooded starts, which are due to migration of liquid refrigerant into the compressor during the off cycle. With the evaporator above the compressor an inverted trap is required at the evaporator outlet. It involves running the suction line up higher than the top of the evaporator coil before running it out and down to the compressor. The inverted trap prevents gravity flow of liquid refrigerant out of the evaporator. You don't need an oil trap with your configration, since it would be counter-productive to oil return.

  4. #4
    Join Date
    Sep 2007
    Location
    State College, PA
    Posts
    1,014
    Just thinking out loud, but would a liquid line solenoid help? Like just after the condenser?
    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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  5. #5
    Join Date
    Apr 2012
    Location
    new york
    Posts
    134
    good catch medic

  6. #6
    Join Date
    Aug 2012
    Location
    Hot Springs, Arkansas, United States
    Posts
    55
    crankcase heater working?

  7. #7
    Join Date
    Nov 2010
    Location
    cali
    Posts
    94
    i believe every 25 ft. vertical a p-trap must be added i may be wrong but for some reason in my head i remember learning this. please feel free to correct me... you can always contact the manufacturer and ask there recommendations they should help you with that.

  8. #8
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    With that long of a line set, you need 3 things to prevent liquid migration.
    1) An inverted trap at the evaporator, like Hvacrmedic said.
    2) Either a hard shutoff TXV at the indoor coil, or a LL solenoid.
    3) A crankcase heater.

    Any time the evaporator is higher than the condenser, even a little, the suction line should exit the coil and go up above the level of the evaporator before going back down to the condenser.

    With the evaporator above the condenser, no p-trap is ever required, or useful. All it would be is a useless pressure drop.
    Even in applications where the condenser is significantly higher than the evaporator, p-traps are going out of style.
    If more government is the answer, then it's a really stupid question.

  9. #9
    Join Date
    Aug 2008
    Location
    San Clemente, CA
    Posts
    1,090
    OP, you did not state the metering device type piston, TXV?

  10. #10
    Join Date
    Jun 2011
    Location
    Seattle WA
    Posts
    1,225
    Thanks for the replies fellas. Isnt the suction side supposed to be pulling in vapor not liquid? I have always thought that liquid in the suction line was hard on the compressor, which is why you cant dump a bunch of liquid refrigerant in on the suction side of the system while charging. The lineset does go up about 3" before going horizontally to the ODU location but the compressor is still getting inundated. Could it be because of the long length of the large 1 1/8 lineset holding a few pounds of refrigerant? Maybe there's enough liquid in their to give the compressor a hard start?
    America; first we fight for our freedom,
    then we make laws to take it away.

    -Alfred E Newman

  11. #11
    Join Date
    Jun 2011
    Location
    Seattle WA
    Posts
    1,225
    The unit has a factory crankcase heater and I have an inverted trap already. How does the liquid line solenoid work? Closes up the LL when the unit is not running I would assume. How would this prevent liquid migrating through the suction line? Thanks
    America; first we fight for our freedom,
    then we make laws to take it away.

    -Alfred E Newman

  12. #12
    Join Date
    Mar 2007
    Location
    Chicago
    Posts
    28
    The liquid line solenoid would pump down the unit and trap all of the liquid refrigerant before it hits the low side of the system.


    The Day You Think You Know Everything Is The Day You Stop Learning.

  13. #13
    Join Date
    Mar 2007
    Location
    South Texas
    Posts
    213
    You said this was a heat pump outdoor, would not use a solenoid with a heat pump with out some specical consideration for the heating cycle. Is there a suction acculator installed between the reversing valve and the compressor? May be an option

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