Changing *all* the pool water without lowering the water level

I need you guys to check my logic on this. It seems so clear to me, but I ran it past the guys at the pool supply store, and got nothing but blank stares.

A friend has a fiberglass pool. It's seven years old, and he's never changed the water, not even a little. And since he uses stabilized cholorine, there's no telling what the stabilizer has built up to. He lost about six weeks of swimming time last year trying to get the algae under control.

But he has been warned never to drain the pool more than a foot or so because of the water table level - it will pop up out of the ground just like a boat, which would be a disaster.

I think it should be relatively easy to change out essentially all the old water with new, and to do so without dropping the water level in the pool at all, and without wasting any water. All you need is a membrane that separates the old water from the new.

So you go to Home Depot and buy a roll of 0.35 mil plastic sheeting (12'x400') for $24, and some shipping tape, and you tape together what looks like it might be a liner for the pool, or at least that big - doesn't have to be exact, just not too small in any dimension. Then you turn off the pump, anchor the edges of this liner around the edge of the pool deck with a few bricks, and pile all the excess out onto the pool surface. You want to do this on a calm day.

You set up maybe four or five garden hoses as siphons that suck water out from under the liner, and start them draining the pool. You put maybe two garden hoses as supply hoses on top of the liner, and over time adjust one spiggot so the inflow rate is about the same as the outflow, so the water level stays constant.

Over a day or two, the liner will slowly expand as the new water comes in and the old water goes out, and will fill out to conform more or less to the bottom and sides of the pool. It doesn't have to have any strength at all - all the forces on it will be applied very very slowly and gently, and it will never have to actually "hold" water. It's just a diaphanous membrane that always stays at the boundary between the new water and the old.

When it's done, turn everything off, gently remove the sheeting liner, turn on the pump, superchlorinate, and you're done.

If you want to get fancy, you can run one of the fill hoses through an auto-fill gizmo. Then you don't even have to check on the fill rate - the water level will be automatically maintained.

I think this will work. Is there a problem with it, or something I haven't considered? Whadayathink?

I would appreciate any comments.

Reply to
Peabody
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I think your friend should get a new friend.

Reply to
Limp Arbor

The physics seems sound, but is there a lower elevation available to make those "draining siphons" practical?

Jeff

Reply to
Jeff Wisnia

I think it's a grand idea. How about doing it backwards?

Put the liner on the bottom, following all the contours of the pool. Put the garden hose under the liner and turn it on.

Then just let all the old water pour off the top.

When the plastic is sitting on top, you're done.

Reply to
The Reverend Natural Light

There is no reason to change the water. What are you thinking? Just keep the chemicals right. The guys at the pool supply store told you a bunch without saying a word...

Reply to
Shanghai McCoy

on 7/17/2008 1:44 PM Peabody said the following:

To me, it sounds like it might work. The reason I think that, is that I have to siphon off all the rain water and snow melt off my winter cover in the spring, which water may be 4 feet deep in the center above the cover , and that's another suggestion, how about an oversized winter cover instead of the plastic sheeting? Maybe you can borrow one from a neighbor. It will be more durable and there won't be a taped seam that may come apart. Where I live, a rural area, you don't want to run your well pump constantly to fill a pool of my size at 26,000 gallons. I have a lot of dairy cattle farms around and there is a water/milk transport company near me that will truck the water to your site in semi-tractor tankers. All the new pools in my area use this service. The last time I used them, it was $.01 cents per gallon, plus the transportation costs (IIRC $65 per truckload?). It took two trucks to fill the pool, plus some help from my well. Have them pump the water under the pool cover which will push the cover up, and the old water above it will flow over the sides of the pool. Have a number of friends around for any adjustments that may be needed. You may not have to have the cover all the way at the bottom of the pool to get rid of the all the old water and stabilizer, since a chlorine pool needs some stabilizer.

Reply to
willshak

The theory sounds reasonable. If you do it take a lot of pictures and put them online somewhere.

Reply to
Pete C.

Why pour Liquid nitrogen into the pool and freeze all the water.... Then drill a hole to the bottom and pump water into it. This would lift the huge frozen cube up and out as the pool fills. No need for the membrane.

Reply to
grodenhiATgmailDOTcom

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This is the weak point in your proposed method. I seriously doubt your going to find shipping tape or any other kind that will not lose it's bond under water. If you used an above ground pool liner, then I could see it working. But it would also work better to use a submersible sump pump to remove the water rather siphoning unless you have a convenient low area to siphon to. You couldn't run it constantly because the incoming water from a garden hose won't go in that fast but you could run it periodically.

Reply to
SRN

................................................................................................................................... .> If you used an above ground pool liner, then I could see it working.

Re-thinking, it would probably need to be a flexible IN-ground liner to be the right dimensions. But they do sell blue poly tarps that are 40' x 60' and 50' x 50' that would probably work since you wouldn't be putting a lot of stress on it. EBay has them or possibly rent one.

Reply to
SRN

Yes, but the siphon outlets don't have to be below the bottom level of the pool. They just have to be below the surface level. And the surface level is unchanging. So I think a drop of one foot from surface level going out to the curb would be enough.

Reply to
Peabody

I think it would hold. I'm talking about the thin, clear tape they use on roller dispensers for shipping boxes. The adhesive they use is not water based. But I'll do a test on a sample for several days first to see what happens to the tape.

It may also be possible to find the plastic sheeting in a 40 or 50-foot width, which would also eliminate the taping requirement.

Reply to
Peabody

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Yes, a poly tarp should be adequate in this non-pressure isolation barrier application.

Reply to
Pete C.

A roll of 3M double sided tape between sections would also work, as would heat sealing them.

Reply to
Pete C.

What is the water table depth, and how long before the pool pops out from pressure?... Drain it fast and fill it.

There are ways to relieve the water table pressure (a hole made for a future drain. Allows the ground water in and plugged for filling the pool.

Many pools sit empty during construction without damage. Think about portions of Florida, below sea level.

Fiberglass is different I'm, sure.

Best I can tell is my water table is about 385 feet below.

Reply to
Oren

If this is true, how did they dig the hole for the pool and then sink the pool into it in the first place?

Reply to
Harlan Messinger

This is a common issue, Harlan. He's not making it up.

Reply to
salty

He needs to get a cyanuric acid tester to see the level of his stabilizer. Stabilizer will reduce the amount of chlorine that evaporates. Sounds like the algae problem is related to times when the chlorine gasses off, and then the algae grows. Algae can't grow in properly chlorinated water. It's been proven. I would start there. On my pool, I check the stabilizer every year, and remarkably, it is about the same. I don't back flush a lot, but do have incredible buildup of minerals from hard water. At times, I will put an inflow and outflow and "flush" some of the water.

I'd give you a blank stare, too, if you came into my shop and put out your idea. You obviously are well intentioned but not knowledgeable about how it works.

Quit overthinking this. Go get a stabilizer kit and test kit. Get the water right by changing some if you have to or adding additives. Then get the stabilizer level right. Then go swimming. It isn't rocket surgery.

Steve

Reply to
SteveB

Then I'm all the more curious. *Since* this is true, how did they dig the hole for the pool and then sink the pool into it in the first place?

Reply to
Harlan Messinger

On Thu, 17 Jul 2008 12:44:54 -0500, Peabody wrote Re Changing *all* the pool water without lowering the water level:

Seems like a good plan. It should work well.

Reply to
Caesar Romano

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