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

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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.
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I think your friend should get a new friend.
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Peabody wrote:

The physics seems sound, but is there a lower elevation available to make those "draining siphons" practical?
Jeff
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Jeffry Wisnia
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Jeff Wisnia says...
> The physics seems sound, but is there a lower elevation > available to make those "draining siphons" practical?
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.
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Peabody wrote:

Good call!
Jeff
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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
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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.
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That makes more sense. If the pool had a drain at bottom, you could use the bottom drain to pump water out, using the backwash feature.
Fill from the bottom does make a lot more sense. Unless you are trying to figure out how to get the liner on the bottom, in the first place. Then things get a bit confused.
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Christopher A. Young
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Peabody wrote:

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

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.
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Bill
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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.

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willshak wrote:

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Peabody wrote:

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

Yes, a poly tarp should be adequate in this non-pressure isolation barrier application.
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SRN says...
>> 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...................
> 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.
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.
> If you used an above ground pool liner, then I could see > it working.
It may also be possible to find the plastic sheeting in a 40 or 50-foot width, which would also eliminate the taping requirement.
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Peabody wrote:

A roll of 3M double sided tape between sections would also work, as would heat sealing them.
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Heat sealing (use a household iron) yes, but tape, none will stick very well to poly.
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Edwin Pawlowski wrote:

You haven't played with the 3M high bond stuff much have you?
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wrote in message

Only played with it once. Great stuff for sticking windows to the outside of skyscrapers, but I think they use poly between layers as a release agent. . I still have my doubts about sticking well to poly though.
It has very low adhesion to polyethylene, Teflon, etc. http://solutions.3m.com/wps/portal/3M/en_US/VHB/Tapes/Product-Information/How-to-use/Design-Tape-Selection/
I'll see if I can find some in work and try it. We had a roll some months ago for a metal application.
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