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Thread: Reversed pressure when turning pump off

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    Reversed pressure when turning pump off

    I've had my pool for 11 years and this season my pool is doing something new.

    Filter/pump runs fine all day long but when it's time to turn it off at the end of the day and the switch is flipped, immediately the water reverses direction and blows out of the skimmer and also the main.

    I have no clue why this is happening nor what I can do about it.

    Anybody have any ideas?


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    Mod Squad JohnT's Avatar
    Join Date
    Apr 2007
    SW Indiana

    Re: Reversed pressure when turning pump off

    Welcome to TFP!

    You almost certainly have an air leak. Maybe the pump basket lid or the basket drain plug has a bad O-ring or needs lubed. The air gets compressed inside the filter and then expands when you shut the pump off and pushes water out the skimmer.
    TFP Moderator
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    Re: Reversed pressure when turning pump off

    Thanks for the quick response!

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    Re: Reversed pressure when turning pump off

    I checked all my connections which a couple needed some tightening. Ran the pool again and still blew back after everything was tightened.

    Next, I ran the filter set to recirculate and then the water did not blow back when I shut off.

    I continued to look for something and I found that there is a very small crack where the pressure gauge screws into the valve and it's also slightly leaking in that spot.

    Could that hairline crack be my problem?
    Does the lack of blow back when set to recirculate help narrow the problem down at all?


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    Richard320's Avatar
    Join Date
    Jan 2010
    San Dimas, CA (LA County)

    Re: Reversed pressure when turning pump off

    You have air trapped in the filter tank. It compresses when the pump is running, and expands again when the pressure is off, blowing it out the pipes. Running on recirculate isolates the tank from the system, so now blowback. Bleed it out.
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
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    Mod Squad jblizzle's Avatar
    Join Date
    May 2010
    Tucson, AZ

    Re: Reversed pressure when turning pump off

    Air would only enter the crack at the gauge when the pump turned off. Then the weight of the water in the filter would pull air in as it drained back tot he pool. The next time you started the pump, that air would be compressed and when the pump goes off, you could see the blow-back.

    Put the valve on filter, open the air vent on the filter and turn on the pump, eventually all the air will be out of the filter and water will come out. Close the air vent and you should be good running. Turn off the pump and I would expect you do not see the blow-back. Wait awhile and theoretically the air may enter the filter from the crack. Then turn the pump back on for a bit and turn it off ... do you get blow back? Does air come out again if you open the air valve?

    If you never see air in the pump basket, that crack could very well be the problem.
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