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Thread: Main drain not sucking full volume of water

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    Main drain not sucking full volume of water

    Hello. When I have my skimmer valve and main drain valve open, the pump basket is full of water. When I try to run main drain only, water flows, but it struggles to maintain a full basket and air is introduced into the system. Obviously, my assumption is that air is getting in somewhere. But, why is air NOT getting it at all when the skimmer line is open too? Shouldn't air be getting in regardless if the main drain valve is involved? How does the issue go completely away when the skimmer valve is open? Secondly, I have checked all the unions above ground to see where air may be getting in and don't see any. Are there other causes of this issue? I.e. debris in and around the main drain (pool is still cloudy so I can't see if anything is down there) or air stuck in the line some how? I used a compressor to blow out the main drain for the winter. I use a little contraption to blow out the main drain until I see a good volume of air bubbles coming up. Could this have caused anything? Thank you very much for the help.
    29K Gallon, Concrete bottom/Fiberglass sides, SWG, Sand Filter, 1HP Pump, Gas Heater
    Kansas City, KS

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    Richard320's Avatar
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    Jan 2010
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    San Dimas, CA (LA County)
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    Re: Main drain not sucking full volume of water

    Before you go and start pulling your hair out, check that it is indeed sucking air and not just starving for water.

    How? Easy. Run things normally so that the pump basket is full and air-free. Open the filter bleeder and let any trapped air escape. Do that twice a few minutes apart. Then switch to drain only and wait a few minutes until you see the air and froth through the lid. Open the air bleed again. If air comes out, you're sucking air. If water squirts out immediately, it's just starving. That air you see in the pump basket isn't air; it's just emptimess. In that case it just means that the pump is capable of pulling more water than the underground plumbing to the main drain alone can supply. I get that sometimes when I'm vacuuming. The reduced diameter and the length of the ose and the drag from all those ribs combine to starve the pump.

    The cure is to crack the skimmer valve just enough to keep the pump basket full.

    If air comes out when you test, then you need to seek out a suction side leak. I'd start by coating the valve and connections with shaving cream and see if it gets suckedin. You'll see foam in the basket and maybe even out the returns.
    Last edited by Richard320; 05-14-2017 at 12:15 PM. Reason: rephrased terminology
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
    Troublefree does not mean Maintenancefree. It's like brushing your teeth: You can spend a couple minutes a day and pennies a week or go to the dentist once a year and spend several thousand dollars.
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    Re: Main drain not sucking full volume of water

    Thank you very much. Appreciate the thoroughness of your answer. I will give this all a go and see if I can get it corrected. Thank you.

    Quote Originally Posted by Richard320 View Post
    Before you go and start pulling your hair out, check that it is indeed sucking air and not just starving for water.

    How? Easy. Run things normally so that the pump basket is full and air-free. Open the filter bleeder and let any trapped air escape. Do that twice a few minutes apart. Then switch to drain only and wait a few minutes until you see the air and froth through the lid. Open the air bleed again. If air comes out, you're sucking air. If water squirts out immediately, it's just starving. That air you see in the pump basket isn't air; it's just emptimess. In that case it just means that the pump is capable of pulling more water than the underground plumbing to the main drain alone can supply. I get that sometimes when I'm vacuuming. The reduced diameter and the length of the ose and the drag from all those ribs combine to starve the pump.

    The cure is to crack the skimmer valve just enough to keep the pump basket full.

    If air comes out when you test, then you need to seek out a suction side leak. I'd start by coating the valve and connections with shaving cream and see if it gets suckedin. You'll see foam in the basket and maybe even out the returns.
    29K Gallon, Concrete bottom/Fiberglass sides, SWG, Sand Filter, 1HP Pump, Gas Heater
    Kansas City, KS

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    mas985's Avatar
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    May 2007
    Location
    Pleasanton, CA
    Posts
    11,983

    Re: Main drain not sucking full volume of water

    If you see air in a pump basket, it is sucking air. A pure vacuum in a pump basket is impossible.

    What is the filter pressure when you switch to MD only? The MD could be blocked or perhaps just have a lot of head loss which is causing an air leak.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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