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Thread: Lowering CYA by letting algae bloom over the winter?

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    Lowering CYA by letting algae bloom over the winter?

    My CYA is on the high side around 100-120. With winter just around the corner, was thinking of letting the pool stay green from november through april next year. I remember reading threads about decreased CYA over the winter months, and how algae may actually consume it over this period of time? Is that a viable option, or was it just guesswork on the posts? I'm in FL so the winters aren't harsh, so i'm sure algae will have no problem staying around.
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    JasonLion's Avatar
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    You have fairly good odds of having CYA go down, or away completely, over the winter, particuarly if you don't do anything to prevent algae. However, no one has any solid idea of what is really going on when the CYA goes away and thus we can only guess at things that can imporve the odds. The leading theory at the moment is CYA getting broken down by some particular kinds of anaerobic bacteria that sometimes grow when chlorine levels go to zero for an extended period. Algae insures that chlorine will go to zero.
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    My only concern is having more serious forms of algae develop (mustard, black) as a result of this intentional process. I have no problem letting "good ole fashioned" green algae flourish, easy to kill anytime. But it's another story with the other two.

    Is this too big a risk to take? I'm not too familiar with mustard or black algae and the details of how/when/where/climate they grow in.
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    ivyleager's Avatar
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    Food for thought, why not allow it to decrease on its own?

    I had CYA of 80 last year at closing (down from 100 the year before), and opened to a CYA of 30-35 this spring. I did not have any algae. Just miraculously degraded somehow. However, I do find that at low CYA levels, I'm adding large amounts of bleach daily, whereas with high CYA levels (80ppm), I was only adding bleach every other day or so. My pool's about as big as yours, so the chlorine costs were greater this year for me. I'm upping my CYA next year.

    CaryB
    CaryB
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    mbar's Avatar
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    Another thing to consider - This year I opened to algae - I let the water in the pool fill too high, and with a mesh cover, and hot spring weather, I had a bad case of green algae. My cya was too low to register, and my ph was 8 . While I did clear the water very quickly with lots of bleach raising it to 15 very chance I got, it took about 2 weeks to hold chlorine overnight. So while it may lower your cya, it may also cause you to have a lot of problems next year with being able to keep the chlorine holdig overnight, which may lead to a summer of problems if you don't keep up with the bleach bombardment It would probably be cheaper to do a partial drain and refill next season
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    You should be able to use winter rains to readily overflow and dilute your pool water to lower the CYA level (and everything else as well). If your pool has an average pool depth of 4.5 feet, then half of that, or 27" of rain, would dilute by 40% (so everything will be 60% of their original value) due to continuous drain/refill from the overflow. You have to eliminate evaporation or else you'll not dilute the water -- you can use a solid winter cover to prevent evaporation (I have an electric opaque safety cover). That's what I have done in the past, though not recently since I wanted to build up the salt level to see how that felt in the pool.

    Richard
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    I'm trying to avoid draining because I have no easy option to backwash,vacuum to waste, etc. My house/pool were built in the late 70's, so the pool/pump/filter is pretty much a closed system. Only way to drain is the old fashioned way.... drag a garden hose out to the street and hope all the water gets absorbed or evaporate fast enough that it won't flood neigbors' lawns. Summer time is best of course

    So which is it? Does CYA decrease because of prolonged algae presence, or because of cold temperatures? If my CYA now is at 100+, seems a bit extreme to expect it to go down to almost zero by next spring. Or is it?
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    mbar's Avatar
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    Hmmmm, mine went from 70 to basically nothing. We do get a lot of precipitation over the winter, so I would say my pool probably gets about 1/2 new water. So I would say it depends on how much new water goes into the pool, as welll as how much is used up with the bio beasties.
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    JasonLion's Avatar
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    We know that CYA can disappear without visible algae and that cold temperatures alone are not sufficient. The best guess I have is that it won't go away if the chlorine level is maintained, but will only sometimes go away if the chlorine level goes to zero. But even that is a little speculative. Significant numbers of people lose CYA over the winter and significant numbers of people don't. Maybe in the spring we can do another survey and see if we can pin it down better than that.
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