location of main drain pipe

[PoolDoc]

The hydrostatic valve, if there is one, is in the bottom of your main drain:
http://www.a1poolparts.com/-strse-83...lve/Detail.bok

[sks24]

I think there may be one in the MD next to the intake. I think I recall seeing two elements: the drain intake, and something else, which could be the hydro valve. Tomorrow I will dive down there and take a look at it. I think that there could be an "O" ring failure such that it would allow water through until the water was shallow, after which there wouldn't be enough pressure to breach the O ring. So that could account for the very specific level to which the water descends. But the inability to draw water exclusively from the MD? It seems like that air would have to come from a hole in the MD pipe.

Thanks.

[PoolDoc]

When hydrostatic valves fail, often it's because they've opened, and allowed ground water to flow into the pool -- what they are supposed to do -- but also allowed a bit of grit or organic matter to enter, and wedge the valve open a bit.

[sks24]

I didn't look at that drain today to verify whether there's a hydrostatic valve there. But I'm pretty sure there is, not least because when I drained the pool completely last month water would seep back into the pool via the MD, and very quickly. The water table is quite high here. I did try to remove the MD grate when the pool was empty, but there was no obvious way to do so.

Bottom line at this point is that it appears that the first order of business is to control for this valve. Since I don't have scuba gear, it looks like I need to drain the pool again, remove the BD grate, and just go ahead and replace the valve using the valve tool they sell at that site you linked to above. I guess I'll buy a new grate, too, since I might have to destroy the one which is on there now to get it off.

But my inability to get the pump to keep its prime if I try to pull more than about 25% of the water from the bottom drain concerns me. It seems like the only circumstance which could account for this would be a hole in that MD pipe.

A somewhat related question. I noticed a new problem yesterday on my equipment pad. When I run the pump with the selector which is on top of the sand filter set to either "Filter" or "Recirculate" some water leaks out of my exhaust drain. It seems like the handle is pretty hard to turn on that selector. What might be causing this? This just started yesterday. Here's a pic of the flow with the selector set to "Filter" : https://drive.google.com/file/d/0B4q...it?usp=sharing

[PoolDoc]

Bottom line at this point is that it appears that the first order of business is to control for this valve. Since I don't have scuba gear, it looks like I need to drain the pool again, remove the BD grate, and just go ahead and replace the valve using the valve tool they sell at that site you linked to above.

Do NOT drain the pool when the water is high; even concrete pools WILL float out of the ground. If you must drain, you MUST lower the water level around the pool.

[sks24]

Thank you for your reply PoolDoc,
Whoever had this house before us - we purchased it in 1997 - drained the pool when the water table was high, and the pool did lift out of the ground about an inch on the west side of the deep end. I've drained it perhaps three times over the past 15 years when it was dry, and it's been stable. About 18 inches of water will seep into the pool when the water table is low. Anyway, I will be careful to wait until we haven't had rain for awhile. There's a creek about 100yds south of the pool which all but runs dry occasionally, and that's when I empty the pool.

So is waiting until the rain subsides, draining the pool, and replacing the valve a reasonable way to proceed at this point? I realize that you would want to be very careful about specifically recommending any particular course of action. Please don't concern yourself over the consequences of anything I might or might do at your suggestion. I'm not going to fault you in any way if things go wrong. I'm sure I've agreed somewhere not to sue anybody over anything that might happen here, and of course that would be my preference, as I want for people to feel free to offer their best advice.

Thanks so much for your help,
Scott

[PoolDoc]

Actually, I don't have enough money to make me a target for any attorney with an IG above the double digits!

My caution is more ignorance based: the sort of situation you're in tends to include surprises, even for very experienced service guys.

I would recommend checking the hydrostatic valve, simply because it's (a) a likely cause, and (b) one that can be repaired. Some of the other possibilities involve MAJOR expenses and demolition. In those cases, pool owners often end up abandoning their main drains.

On a positive note, the mere fact that you've repeatedly had water enter the pool via the hydrostatic valve greatly increases the chance that that valve IS the problem.