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Thread: Overnight chlorine test: should I lose chlorine w/ clean water and no sun?

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    Join Date
    Jul 2014
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    Madison, WI
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    Overnight chlorine test: should I lose chlorine w/ clean water and no sun?

    From what I've gathered on this site, it seems the chlorine should only be decreased by sunlight and attacking organic materials in the water.

    So, if I have clean water and no sunlight, should my chlorine levels be stable (such that I should not have to add chlorine)?
    This question is stemming from my hot tub. The hot tub is constantly covered when not in use and I'm wondering if I should be able to just let it sit without having to add chlorine if it is not used (even for weeks at a time). It seems logical, but is not the case - so I'm wondering if the logical conclusion is that I do have organics in the water -- but it would seem they would all be killed off after days of adding chlorine and no use of the tub and I'd eventually get to equilibrium.

    BTW, I understand that 'sunlight' really means UV light - important to note since many pools and hot tubs are now using UV Sanitizers as the latest way to keeping your water clean. My hot tub does have a UV Sanitizer but I've disabled it.

    Actually, that's another interesting question: Does anyone think that UV sanitizers kill different bacteria or kill it more efficiently? In other words, are they worth having in a pool or not? I did read on this forum that having one with a SWG makes sense since the SWG can replace the zapped chlorine - but does that just mean the SWG makes running one less painful or that having one really does help water purity?

    Thanks for any insights.
    19,000 gal fiberglass
    Pentair pump/controller (SunPlay) w/ Intellichem & Intellichlor SWG
    Advantage cleaning system
    Geothermal heat

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    Re: Overnight chlorine test: should I lose chlorine w/ clean water and no sun?

    I can't answer your hot tub questions because I've never had one that is covered. I can tell you what I know about UV sterilizers. They are not a very effective means of sanitation, relative to oxidizers such as ozone and chlorine. The concept of UV sterilization makes sense, but often the units used are way undersized and more often than that, the flow is usually too fast to provide adequate contact time. I use UV sterilization in my line of work every day, and it's very tricky to size a unit appropriately, even on a small loop, such as a hot tub. They are more of a bacteria "preventer" if none are already present, not so much a bacteria killer. They will kill all bacteria with adequate contact time, but that rarely happens. I wouldn't waste the money.
    Curtis- 20k gallon IG gunnite/plaster with hot tub/waterfall and "grotto"
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    Mod Squad JVTrain's Avatar
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    Re: Overnight chlorine test: should I lose chlorine w/ clean water and no sun?

    You will lose some chlorine even in a covered hot tub. Despite being covered, bacteria and other spores, various other organics are always being introduced and the temperatures tend to be elevated near 100 F. So there will always be some consumption of chlorine as well as some break down of the chlorine itself regardless of exposure to anything. There is a half-life rate of decay on hypochlorite just the same in your spa as there is in the jug and it's increased by the higher temps. 2 ppm loss per day is fairly normal in a covered spa even when not in use. You will have to add chlorine at least every other day or so even if it's not used and covered.
    Joel - TFP Moderator - Minnesota - **Become a TFP Supporter!** Helpful Links: ABCs of Pool Water Chemistry - SLAM Procedure - Chlorine/CYA Chart
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    Re: Overnight chlorine test: should I lose chlorine w/ clean water and no sun?

    Thanks for the responses. I did disabled the UV on the hot tub because it did not seem to do anything for my water and since it kills the chlorine, I think it's actually a nuisance. I also have a large UV sanitizer on my salt water pool. Hmmm... interesting idea to nuke that as well but it's not a nuisance since I have a SWG. It is one more thing to winterize here in WI though... speaking of which...
    19,000 gal fiberglass
    Pentair pump/controller (SunPlay) w/ Intellichem & Intellichlor SWG
    Advantage cleaning system
    Geothermal heat

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