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Thread: Chlorine tablet run pool - lowest CYA that will work?

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    Aquatica's Avatar
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    Chlorine tablet run pool - lowest CYA that will work?

    still got a few traditional pools running on tablets.

    whats the lowest CYA I can run these pools on. Looking for a good starting point. I feel 30-50 is too high when using tablets. would a lower CYA than 30 work better with tablets?

    Thanks.

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    mas985's Avatar
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    Re: Chlorine tablet run pool - lowest CYA that will work?

    With tablets, I wouldn't add any CYA and just let it come up on it's own which it will do very quickly. Early on the chlorine might get used up faster but it gives you more time later in the season when CYA gets fairly high.
    Mark
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    Re: Chlorine tablet run pool - lowest CYA that will work?

    Quote Originally Posted by mas985
    With tablets, I wouldn't add any CYA and just let it come up on it's own which it will do very quickly. Early on the chlorine might get used up faster but it gives you more time later in the season when CYA gets fairly high.
    Thanks. I see makes a lot of sense. but in a newly filled pool might be good to add a little liquid chlorine to bring FC up and then let tablets take over. or am I just wasting liq chlorine?

    could use a floater instead of adding to skimmer..will make tablets last longer especially during start.

    Given the FC/CYA ratio tablets should be fine without any CYA in the water. once FC 1-3ppm should be enough for sanitization?

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    Re: Chlorine tablet run pool - lowest CYA that will work?

    (I posted this after Mark. It's a different perspective, mostly because I'm concerned that initially with no CYA you may get to near-zero chlorine levels and allow algae to grow. This might be overly conservative since the chlorine level will build up overnight and even the small amount of CYA in the first few days will be better than nothing at all.)

    You can go as low as you want, but the lower the CYA level the faster the chlorine usage from breakdown by the UV in sunlight. So for uncovered pools exposed to lots of sunlight, I wouldn't go any lower than 20 ppm and even then you'll likely use 60% or more of your FC in one day. With 30 ppm, you may lose more like 50% in a day. The tablets only dissolve at a fixed rate, though that is somewhat adjustable via the openings in the floating feeder or by the number of tablets you use. So this means that you need to have the openings and number of tablets higher when the CYA level is lower in order to keep up. As the CYA level rises over time, you should be able to reduce the opening size or number of tablets in the feeder.

    If there is any way to get water dilution, be it from rain overflow, backwashing, or intentional dilution, then that's the best way to keep CYA in check in which case higher CYA levels can be used. Nevertheless, the amount of dilution needed is still pretty substantial so I can understand why you want to take the approach of starting out with low CYA.

    I would not use Trichlor tablets in the skimmer unless they are designed to not dissolve with no water flow (most are not) as they are too acidic.

    The problem with no CYA in the water isn't the FC/CYA ratio, but rather the FC getting used up faster than the Trichlor tabs can keep up, but perhaps with enough in a floater at full throttle, maybe it will be OK.
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    Re: Chlorine tablet run pool - lowest CYA that will work?

    Quote Originally Posted by chem geek
    (I posted this after Mark. It's a different perspective, mostly because I'm concerned that initially with no CYA you may get to near-zero chlorine levels and allow algae to grow. This might be overly conservative since the chlorine level will build up overnight and even the small amount of CYA in the first few days will be better than nothing at all.)

    You can go as low as you want, but the lower the CYA level the faster the chlorine usage from breakdown by the UV in sunlight. So for uncovered pools exposed to lots of sunlight, I wouldn't go any lower than 20 ppm and even then you'll likely use 60% or more of your FC in one day. With 30 ppm, you may lose more like 50% in a day. The tablets only dissolve at a fixed rate, though that is somewhat adjustable via the openings in the floating feeder or by the number of tablets you use. So this means that you need to have the openings and number of tablets higher when the CYA level is lower in order to keep up. As the CYA level rises over time, you should be able to reduce the opening size or number of tablets in the feeder.

    If there is any way to get water dilution, be it from rain overflow, backwashing, or intentional dilution, then that's the best way to keep CYA in check in which case higher CYA levels can be used. Nevertheless, the amount of dilution needed is still pretty substantial so I can understand why you want to take the approach of starting out with low CYA.

    I would not use Trichlor tablets in the skimmer unless they are designed to not dissolve with no water flow (most are not) as they are too acidic.

    The problem with no CYA in the water isn't the FC/CYA ratio, but rather the FC getting used up faster than the Trichlor tabs can keep up, but perhaps with enough in a floater at full throttle, maybe it will be OK.

    Thanks Chem. great reply and info. Yes I see. Just took over a pool with very low CYA. Lower than 30. I'm guessing 10ppm. would say 0 but there is a very slight clouding in the test tube. The pool had one tab in the skimmer and an FC of 3ppm. which is not bad. also CC was 0.

    This got me thinking about my other tab run pools. summer is coming up and I need to lower CYA. right now UV index here is low but come summer UV is very high and FC will burn off fast.

    We have some Chinese tablets. they seem to last longer than reg and are cheaper. plus they wrap each one in plastic.

    First thing I do now is test for CYA and CH. If level is high water needs replacement otherwise I refuse to take a pool on. Once you have a pool and it has high CYA and or CH some pool owners refuse to replace water. its like pulling teeth!

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    Re: Chlorine tablet run pool - lowest CYA that will work?

    I am just going by my own experience when I started my pool. I couldn't run the SWG for a month so I just threw in a floater without adding any CYA and there was no issue with algae but I can't speak for every part of the county. Also, when I refill, I don't turn on the SWG right away so I go without chlorine for a day or two without chlorine and still get no algae. This winter the water temp has been below 50 for several weeks now and since the SWG has been off, I haven't added any chlorine and still no algae but the water is much colder. I find that with warmer water, it takes a couple of days without chlorine for the algae to start to show up.
    Mark
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    Re: Chlorine tablet run pool - lowest CYA that will work?

    How long and what times of day the pump is running makes a big difference when CYA is really low. If the pump is running 100% of the time, trichlor works fairly well starting from zero CYA. Chlorine is added constantly to replenish what is lost. At lower CYA levels you lose chlorine quickly, but need less to be effective. Then at night you get a kind of mini-shock effect. As long as chlorine is constantly being replaced it tends to work out. On the other hand if the pump only runs at night, half the run time and none when you really need it during the day, there are likely to be problems.
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    Re: Chlorine tablet run pool - lowest CYA that will work?

    Quote Originally Posted by JasonLion
    How long and what times of day the pump is running makes a big difference when CYA is really low. If the pump is running 100% of the time, trichlor works fairly well starting from zero CYA. Chlorine is added constantly to replenish what is lost. At lower CYA levels you lose chlorine quickly, but need less to be effective. Then at night you get a kind of mini-shock effect. As long as chlorine is constantly being replaced it tends to work out. On the other hand if the pump only runs at night, half the run time and none when you really need it during the day, there are likely to be problems.

    makes sense. I normally run pump from 10am to 6pm. or longer but during the hottest time of the day.

    on this new pool fill all levels balanced perfectly...pH: 7.6, TA: 70 and CH:330

    decided to shock new water clear with a couple gallons of 12% liq chlor and added 2 tabs to skimmer. might add some more liq chlor if need be. levels balanced out nicely and getting ready to add salt and CYA for when new SWG is installed.

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    Re: Chlorine tablet run pool - lowest CYA that will work?

    If you are going to use tabs raise the TA to 120 because your PH is gonna tank with the TA that low.

    I used tablets last summer and started with a CYA of about 20. It never got above 50, but we had alot of rain/dilution/splashout issues. TA was 100 at the start and PH kept dropping so monitor both if you go that route.
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    Re: Chlorine tablet run pool - lowest CYA that will work?

    Thanks for the replies. Much appreciated.

    I think what I will do is start with cya of 10 and monitor FC but may need to increase cya to 20ppm. also use floater wide open, 1 tablet and see what happens. got a 8,000 gal pool so if I can make 1 tab in a floater work weekly that would be great. also taking over a 15,000 gal pool with spa attached. going again to start with 10 cya and 2 floaters..one in the spa and one in the pool. Think since its summer now I will run pump 12 hrs from 7am - 7am..the hot part of the day.

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