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Thread: No pressure when running on main drain

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    Join Date
    Jun 2014
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    Question No pressure when running on main drain

    Hi All,

    This is my second summer with this pool. Most of the time I have the valves set so its running 50/50 skimmer/drain. While I was working with it last weekend, I noticed that if I turned the valves to favor the main drain, pressure bottomed out and the filter skimmer would fail to fill with water. although it looked like there was good water flow coming in. If I turned the valves back to 50/50 or 100% skimmer, things would return to normal. I did the shaving cream test on the exposed pipework and didn't detect any air being sucked in at the joints. I took a video of whats happening for better clarification.


    I frequently ran 100% on main drain last summer and while the pressure would drop a psi or two over the skimmer, it did nothing close to what it is doing now. Does anybody have an idea of what might be wrong or how I can further troubleshoot?

    Thanks and kind regards,
    Brian

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    JasonLion's Avatar
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    Re: No pressure when running on main drain

    Welcome to TFP!

    That does look like an air leak somewhere in the plumbing between the main drain and the valve you are turning in the video. There are some suggestions on fixing suction side air leaks in Pool School. Finding a leak can be tedious, especially if it turns out to be underground.
    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: No pressure when running on main drain

    That's what I was afraid of. Not only does it run under ground, but under concrete. I'm not about to get involved in tearing that up. I guess I'll have to look around for for those suggestions on fixing a leak.

    Thanks!

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    Richard320's Avatar
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    Re: No pressure when running on main drain

    Stop and think a second... If you're drawing air, you should have air in the tank. Run that same test and as soon as you switch back to skimmer, open the air bleed on the filter and see what comes out. Do it a couple times.

    You could have a collapsed or clogged line to the drain and what you're seeing isn't air in the pump basket, but nothingness. A vacuum. I get the same thing when the vacuum head hits the hump where my pool floor slopes and sucks itself flat against the concrete, starving the pump.

    Yes, a vacuum is possible. It's just much lower pressure than atmosphere. You know the pump is capable of that, because the lower pressure is what allows atmospheric pressure to push water into the pump. The pump doesn't suck water; it just lowers the pressure enough that atmospheric pressure can shove the water uphill against gravity to fill it. You suck on a straw, pressure drops, atmospheric pressure pushes the liquid uphill. Cap the straw with your finger and suck. There will be a point where the straw is under a vacuum, and then it collapses. You've never sucked all the air out of a coke bottle and had it hang from your lip? What's in the bottle? Maybe some liquid, and a lot of nothing.
    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.
    A pool is like a pet - you have to feed it every day, even the days you don't want to play with it!

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    Re: No pressure when running on main drain

    I actually have been looking at that. Not much air comes out of the bleed. OK assuming I have a clog, is there a way to clear that?

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    Richard320's Avatar
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    Re: No pressure when running on main drain

    Quote Originally Posted by marmaladejackson View Post
    I actually have been looking at that. Not much air comes out of the bleed. OK assuming I have a clog, is there a way to clear that?
    Shut everything down. Move the valve to full main drain. Open the pump strainer, remove the basket, shove a garden hose down the inlet pipe from inside and seal it up with a rag (or use a drain king bladder) and have someone turn on the hose full blast. If it's a bunch of debris down in the drain area, you should see it start coming up into the pool.

    There's no chance your drain cover is easily removed, or has big openings? I recall someone a year or two ago that discovered a matchbox car caught at the inlet to the 3-way valve. But theirs came through the skimmer.
    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.
    A pool is like a pet - you have to feed it every day, even the days you don't want to play with it!

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    Re: No pressure when running on main drain

    Ah ha! yes! I will try that right now!

    Thanks!

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    Re: No pressure when running on main drain

    Thanks Richard! It was clogged! I had to pull the covers off the drains and scooped a couple of pounds of silt and leaf debris out of the drain. Works like a champ now!

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