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Thread: Opening Main Drain Valve Causes Pump to lose Prime - Drain N

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    Opening Main Drain Valve Causes Pump to lose Prime - Drain N

    I purchased a 4 year old home in FL last year with an inground gunite pool. The pool has 3 ball valves, one for each of the two skimmers, and the third one is for the 2 bottom main drains. If I open the main drain valve (partially or full), the water in the basket section before the pool pump drains out and the pump loses prime. This happens as soon as I start to open the main drain valve. I cannot get prime back unless I close the main drain valve and have one or both of the skimmers on.

    It does this whether the pump is on or off, and if the skimmer valves are open or closed (either one of both of them) and with the pump on or off.

    I had a pool guy for a year since I bought the house, and he never said anything about the main drain not working. He has always had the main drain valve turned off. I found the problem when opening/closing the valves to better understand how the pump operates.

    Hoping someone can help me solve the problem, and that it will not be a major repair cost. Thanks in advance.

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    Mod Squad Bama Rambler's Avatar
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    Re: Opening Main Drain Valve Causes Pump to lose Prime - Dra

    Post a picture of the pad where the valves are.
    I would think that if the MD (main drain) plumbing was leaking that bad you wouldn't be able to keep water in the pool. If it is leaking that bad then my bet is that the MD's are plugged off in the bottom of the pool.

    How long are you letting the pump run trying to prime the MD lines?
    I would shut the skimmers off and open the MD valve and let the pump run several seconds to a minute to see if it'll prime.
    Dave J. TFP Moderator
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    Melt In The Sun's Avatar
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    Re: Opening Main Drain Valve Causes Pump to lose Prime - Dra

    I agree w/ Bama; give it a minute to see if it will pull up water. It's also conceivable that you could have a huge air leak between the ball valve and the drains; perhaps something like a cracked pipe.

    Worst case scenario: you leave it as is and don't use the drains. That won't hurt anything.
    11,200 gal, Pebble-Tec; Tristar 2-speed 1hp - Swimclear 325 ft2 cart - SWG - A & A in-floor cleaner - Heat pump. For the poolside cooking, a Yoder Wichita and a Big Steel Keg!
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    Re: Opening Main Drain Valve Causes Pump to lose Prime - Dra

    I'm not in Florida for a couple of months so I can't post a picture. The pool has been running since I bought the house last year with the MD valve turned off. When I was last there, I shut off both skimmers, opened the main drain and turned the pump back on. As I mentioned the basket /reservoir going into the pump drained out almost immediately. I let the pump run for about 3-4 mins but it never re-primed.

    I thought there might be a crack in the MD pipe causing a suction/vacuum loss, but since this is my first pool I've had no experience to compare it to. I guess for now I'll just leave the MD closed. It just irks me that a part of the pool does not work.

    Thanks to Bama Rambler and Melt In The Sun for your responses.

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    Mod Squad Bama Rambler's Avatar
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    Re: Opening Main Drain Valve Causes Pump to lose Prime - Dra

    Sounds like it's probably been plugged off inside the pool. I'd leave it alone.
    Dave J. TFP Moderator
    24' x 52" Round AGP. 2hp/¼hp SPL Power-Flo 2-speed pump. 200sqft Waterway Cartridge Filter. 45MHP2(3GPD) Stenner Peristaltic Pump
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