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Thread: Bubbles from main drain

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    Bubbles from main drain

    Hey guys,

    Just inherited an old pool and I have some issues I was wondering if I could get advice on. I'm not familiar with exactly how the main drains are constructed but this is a 1982 sylvan in ground gunite free form with 2 main drains connected together (proabably for safety). It has 2 hydrostatic relief screws on either side of the main drains and 1 in the shallow end as well. The problem is when I manipulate the valve to the main drain I can get air up into the pump basket or if I shut it down completely the main drain can bubble for a short time. Often the first couple of bubbles will be very large (maybe 3") and for a minute or so there will be finer bubbles. When I start the pump up, even with the main drain valve closed I get the same bubbles. If I wait for the bubbles to stop a minute and then turn on the main drain, I tend not to get any air or very little and the system runs fine. There's no evidence of water loss (i.e. level is staying precisely on the middle of hte tile line)... however we have had rain which could mean I am in fact losing water.

    First of all do bubbles mean leaking? It would seem to me to be a relative certainty. Secondly since I saw the bubbles coming out of both main drain covers does that suggest that the air is in the plumbing between them and thus the furthest could be capped (even if it creates a less safe pool - I could always turn off main drain when swimmers are present but sitll get the circulation benefits otherwise). Has anyone ever dealt with anything like this? If it's not leaking much I might just deal with it. I need to replaster next year anyway. It's not looking to good at all.

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    Mod Squad JohnT's Avatar
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    Re: Bubbles from main drain

    Quote Originally Posted by piku
    The problem is when I manipulate the valve to the main drain I can get air up into the pump basket
    That's not unusual. You are reducing the flow of water into the pump, and it can create bubbles in the basket. On my pool, even though my pump can run for days on either skimmer only or drain only, closing either valve at anything more than a snail's pace results in the pump chugging like a washing machine.
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    Yes but I actually get bubbling from the main drain up into the pool. It's just that if the valve is on those bubbles don't escape and instead come up through the plumbing.

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    Mod Squad JohnT's Avatar
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    Quote Originally Posted by piku
    Yes but I actually get bubbling from the main drain up into the pool. It's just that if the valve is on those bubbles don't escape and instead come up through the plumbing.
    When you shut the valve off, there is still quite a bit of pressure pushing water into the main drain. I had to cut my drain line last spring to reroute it, and I can vouch that even near the water level, the water is pushing hard to get out of that pipe While a leak could introduce air into the lines which would then go through the pump, I can't imagine any way for air to come out the drain like you are describing because of a leak. As a matter of fact, when closing a pool, one of the hardest things to do is to push air into the drain line to winterize. It takes a compressor to force a bubble through the drain. I'm puzzled, but I bet somebody will figure it out.
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    hrmmph. I lubed the pump basket gasket and now the system is holding prime even for many hours later with no air bubbles coming into the basket. For some reason this holding of pressure is also preventing the air bubbles from coming out of the main drain. This pool is dirt old so whatever works for now

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    Quote Originally Posted by JohnT
    As a matter of fact, when closing a pool, one of the hardest things to do is to push air into the drain line to winterize. It takes a compressor to force a bubble through the drain.

    I was told by an old pool builder to never force air into the main drain. It could cause twisting on the drain itself and even tho it is set in concrete, it could cause a problem. You were trying to mix the anti-freeze with the water. IMHO.
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    Nope... After letting it sit all night what happened was the pump basket was full of water but it sucked a bunch of air bubbles from the main drain. I let it run a little to see if the pump basket would fill - it wouldn't, so I gently closed the main drain valve. Now the pump basket filled competely with water and stabilized. At this point air started bubbling from the main drain. It kept bubbling for like 20 seconds and stopped. Once it stopped I was able to open the main drain valve and everything runs fine all day. So it's like a self-sealing leak.

    Something else of note, we had a lto of rain last night and it would seem that a ton of earthworms came UP through the main drain. Interesting. I wonder how common people who can scuba dive and know how to epoxy a main drain are

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    Mod Squad JohnT's Avatar
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    Well, this is weird as heck. I got home tonight and was tired as heck. My pump had been running all day after I brushed the pool this morning. It needs backwashing, but I was lazy, so I just shut it off tonight. Guess what I got! A burp out of the main drain. I don't understand it, but it did it. Maybe your problem is excessive filter backpressure, as I know mine is high.
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    I'm at 10 psi. I didn't brush yet this week, waiting until I decide to do about my possible ammonia contamination. My starting psi is 7 and it's fully clogged at 22 so I'm pretty good. How old is your pool? When was your last resurfacing? I'm 25 years on original plaster, that's why I suspect a leak. I also see evidence of previously repaired leaks here and there (this is my first year with this pool or any pool for that matter).

    Mine will bubble if I bump it with the vacuum. If I put the vacuum on top of the main drain it will start sucking air (from somewhere) and will actually cause the pump to lose prime.

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    Quote Originally Posted by piku
    I'm at 10 psi. I didn't brush yet this week, waiting until I decide to do about my possible ammonia contamination. My starting psi is 7 and it's fully clogged at 22 so I'm pretty good. How old is your pool? When was your last resurfacing? I'm 25 years on original plaster, that's why I suspect a leak. I also see evidence of previously repaired leaks here and there (this is my first year with this pool or any pool for that matter).

    Mine will bubble if I bump it with the vacuum. If I put the vacuum on top of the main drain it will start sucking air (from somewhere) and will actually cause the pump to lose prime.
    My pool is only 4 years old, and is vinyl.

    Getting bubbles when you put the vacuum head over the drain would be normal, and are produce from cavitation. This isn't air leaking in, but just the pump having inadequate water flow. Bubbles form from the water being at very low pressure on the inlet side of the pump. These bubbles aren't air, but are nearly a vacuum.
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    Yeah I know what you mean about cavitation but this isn't it. I can bump it and then move back quickly and it'll bubble all by itself up into the pool. There should be no suction there then. And shouldn't there be enough cavitation everywhere else in the pool not just over the main drain due to the sucton of the vacuum head? It doesn't add up for me.

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    Thought you guys might want some entertainment so I took some video of this phenomena. Sorry it's not voice narrated.

    http://www.youtube.com/watch?v=-xd5FGOp0gg

    Note that you have to watch the whole thing through to see what I mean abotu the main drain blowing bubbles. When I first start up, those bubbles are expected and are due to the hose bib leaking. The cawing noise in the background was my hilarious wife.

    Normally when starting with skimmers open adn main drain closed it will bubble immediately from the main drain. That didn't happen this time... also normally it bubbles a LOT more from the main drain. Also, once the bubbling stops, all is well and the main drain can be turned on and it won't suck air. So... unique.

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