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Thread: Trying to understand the complex chemistry of a pool

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    Trying to understand the complex chemistry of a pool

    Hi all. I'm a new pool owner that is trying to understand the complex chemistry of a pool.
    I read that CYA protects FC from the UV.
    But also CYA can be a problem if goes out of values lowering FC to critical values.
    I have a SWG so FC is produces all the time that the pump is on, in sunlight hours (10 hours in summer), so I know UV destroy FC but is also produced by the SWG.
    To have CYA in correct values is another task that needs, mesure it, take corrective actions, and sometimes to change big amounts of water: more work.
    The question is, can I have zero CYA ? or CYA is mandatory ?

    Thanks, Roberto
    9.400Gal, SWG, Sand Filter, IG, Fiber Glass, 1HP Pump 1 Velocity

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    JasonLion's Avatar
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    Re: Trying to understand the complex chemistry of a pool

    Welcome to TFP!

    While it is possible to maintain an outdoor pool without any CYA, it is rather difficult and expensive compared to using CYA, so much so that it would be silly to not use CYA. If used as we recommend, CYA will either never be an issue, or at least hardly ever.

    Adding CYA, without changing the FC level, lowers the amount of active chlorine in the water. However, all you need to do is raise the FC level enough to compensate and the problem goes away.

    Running the pool without CYA would mean using up your SWG lifetime at about four times the rate you would using CYA, or more. You would also experience far more PH drift without CYA. It probably also means running the pump more than you otherwise would need to, since it is extremely unusual to need to run the pump all day, yet that is exactly what you need to do when running without CYA.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Re: Trying to understand the complex chemistry of a pool

    See the table in this post that shows that if you needed a 24% ontime at 80 ppm CYA, then you would need 140% ontime to have 1 ppm FC with no CYA. Though technically you could use only 0.1 ppm FC with no CYA for only a 14% ontime, you cannot realistically maintain such a low FC everywhere in the pool. So think of the CYA as not only protecting chlorine from loss from sunlight, but also acting as a reserve or reservoir of chlorine to make sure it doesn't run out locally. It's like having a mini-SWG able to release chlorine as needed anywhere in the pool.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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    Re: Trying to understand the complex chemistry of a pool

    Ok, perfect explanation.
    Thanks !
    9.400Gal, SWG, Sand Filter, IG, Fiber Glass, 1HP Pump 1 Velocity

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