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Thread: Main drain low flow??

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    Main drain low flow??

    I have a new pool installed in Aug 2012 and I have been noticing that when you switch the 3 way valve to full main drain, the water flow is roughly cut in half and there are lot of bubbles in the pump strainer basket and it sounds like cavitation. When I switch back to the middle or the skimmers, it's fine. Is this normal or should they both flow the same? I have always noticed the sound difference, but when I backwash, I can see the difference. I wonder if the line has some rocks in it or some other obstruction? I plan on re-plumbing the whole pump pad because the PB did a terrible job and would like to correct this, if possible.
    20' x 38' x 40' Lagoon, 38K Gal, Vinyl, Aqua Plus control, SuperFlow 2 hp pump, 3.7 sqft sand filter, T-15 SWG
    2 ColorLogic 4.0 LED lights, Polaris 280, TF-100 test kit, Finished in September 2012
    Master Spas Twilight 8.2

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    JasonLion's Avatar
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    Re: Main drain low flow??

    Those symptoms strongly suggest that the main drain is blocked. The blockage is most likely in the pipe directly below the drain fitting at the bottom of the pool, so it is unlikely that re-plumbing the equipment pad will have any effect. One thing you can try is to us a drain king or appropriately wrapped rag to try and blow out the blockage backwards, ie into the bottom of the pool. That doesn't always work, and when it does work it makes something of a mess, but that is just about the only approach likely to work without digging out the plumbing at the bottom of the pool.

    Because the pool is so new there are some other possibilities, major plumbing errors on the part of the builder for example, or the valve that selects between skimmer(s) and drain being broken or mis-installed is some strange way, but they are fairly unlikely.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    Re: Main drain low flow??

    So, you are saying I shouldn't see a difference in flow. That's what I thought. I was going to re-plumb the pad simply because it looks bad and they put pipes where the wall goes for the shed.
    20' x 38' x 40' Lagoon, 38K Gal, Vinyl, Aqua Plus control, SuperFlow 2 hp pump, 3.7 sqft sand filter, T-15 SWG
    2 ColorLogic 4.0 LED lights, Polaris 280, TF-100 test kit, Finished in September 2012
    Master Spas Twilight 8.2

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    duraleigh's Avatar
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    Re: Main drain low flow??

    So, you are saying I shouldn't see a difference in flow
    What you shouldn't see is air in the system. That's the indicator that the main drain is at least partially blocked.

    As Jason said, it could be a defective valve but the blockage seems the most likely.

    You may see some small to moderate change in flow when you switch from main drain to skimmer but you shouldn't see ANY air in either case.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Main drain low flow??

    I just took apart the valve and it was clean. There must be a rock in the pipes, as the returns had rocks in them and would plug up the eyeballs. I'll have to wait until the pool warms up and then try reversing the flow with the drain screens off and see if I can blow it out. I was there when the lines and drains were put in and didn't notice any issues then. I'm going to call the PB and see if I can get him to check it out. He has the scuba gear to go down and take the screens off.

    When I re-plumb the pad, I'm going back with a VS pump and some flow meters. That way I can see exactly how much flow I have at various places and from the main drains. Can I install a flow meter on the suction side of the pump?

    Thanks!
    20' x 38' x 40' Lagoon, 38K Gal, Vinyl, Aqua Plus control, SuperFlow 2 hp pump, 3.7 sqft sand filter, T-15 SWG
    2 ColorLogic 4.0 LED lights, Polaris 280, TF-100 test kit, Finished in September 2012
    Master Spas Twilight 8.2

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    JasonLion's Avatar
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    Re: Main drain low flow??

    Yes, you can install a flow meter on the suction side of the pump. You just need to have enough room to fit it in. Also, some flow meters require a certain length of straight pipe before and after, which is often difficult to find on the suction side, though others do not have that restriction.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    Re: Main drain low flow??

    Ok, I thought it would work. I will have plenty of room as I am re-plumbing from the ground up with new valves and all.
    20' x 38' x 40' Lagoon, 38K Gal, Vinyl, Aqua Plus control, SuperFlow 2 hp pump, 3.7 sqft sand filter, T-15 SWG
    2 ColorLogic 4.0 LED lights, Polaris 280, TF-100 test kit, Finished in September 2012
    Master Spas Twilight 8.2

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    Re: Main drain low flow??

    I just talked to the PB and he agreed that something was wrong. He's going to come over when the water warms up and work on it. I told him I was going to replace the pump with the VS pump that I wanted, but he talked me out of. He said that Hayward wanted him to start pushing the sales of these pumps now and he would like me to buy it from him, lol. I said I would if he could match $983 and free shipping. That was the end of that.
    20' x 38' x 40' Lagoon, 38K Gal, Vinyl, Aqua Plus control, SuperFlow 2 hp pump, 3.7 sqft sand filter, T-15 SWG
    2 ColorLogic 4.0 LED lights, Polaris 280, TF-100 test kit, Finished in September 2012
    Master Spas Twilight 8.2

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