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Thread: Maintaining a hot tub the same as a pool?

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    Maintaining a hot tub the same as a pool?

    I'm new to owning a hot tub. I do have a pool and this is my second year using the BBB method. It's been fantastic! Is maintaining the hot tub chemistry the same as a pool? How often should I run the jets? Any feedback would be much appreciated.
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    Re: Maintaining a hot tub the same as a pool?

    Though there are some similarities, there are also some differences. The main difference with a hot tub is that the bather load is much higher due to the small water volume. The other difference is that the hot tub is usually covered so does not get sunlight so with that and the hotter water temperature it usually does not get algae, but bacteria can grow very quickly. So chlorine demand comes from bather waste and means that one needs to add an oxidizer after every soak to oxidize that waste. In addition, in between soaks a disinfectant needs to be added (that part is similar to pools). Note that quantities regularly added to a spa are generally small, in teaspoons and fluid ounces.

    Chlorine
    You can read the sticky on Using Chlorine in a Spa where the basic approach is somewhat similar to using chlorine in a pool. The method describes using Dichlor initially to build up the CYA level to around 30-40 ppm and then to switch to using bleach. One could alternatively add pure CYA. Because the CYA breaks down from chlorine at hot spa temperatures at roughly the rate of 5 ppm per month, one adds more CYA about once a month, usually by using Dichlor for a day or so. One needs to add chlorine every day or two in a spa because at the hotter temperatures the 24-hour chlorine demand is up to 25% FC loss.

    If you have an ozonator, then ozone will react with chlorine using up both so the 24-hour chlorine demand will be 50% of more FC loss so chlorine needs to be added every day in this case. On the plus side, ozone will oxidize some of the bather waste so about half as much chlorine needs to be added after each soak. So an ozonator makes sense if one is using the spa every day or two, especially with higher bather loads (more people or longer soaks), but doesn't make much sense if one only uses the spa on weekends or infrequently.

    For the chlorine method when using bleach, one needs to have the TA be much lower, usually around 50 ppm, and must use 50 ppm Borates for additional pH buffering. This is particularly true if one uses spa jets since they generate a lot of aeration.

    Just as with pools, one can get a saltwater chlorine generator such as the ControlOMatic TechniChlor though the salt level of 2000 ppm is lower than in pools that usually have 3000 ppm. The generator is useful for maintaining a background dose of chlorine, but one usually still adds bleach after their soak to handle the bather waste and of course one must start off with some CYA in the water (either from pure CYA or by using Dichlor initially).

    Bromine
    An alternative to using chlorine is to use bromine because there are slow-dissolving bromine tabs available that can maintain a background dose of disinfectant in the spa. See Using Bromine in a Spa. One usually also adds sodium bromide to create a bromide bank so that adding an oxidizer (such as chlorine) after each soak converts bromide to bromine. An ozonator will also oxidize bromide to bromine so may be enough for a background dose without needing bromine tabs, but ozone also oxidizes bromine to bromate so one needs to replenish the bromide bank over time (and unfortunately it is not easy to know the bromide level). Because bromine is not as strong an oxidizer as chlorine, one usually needs to shock with chlorine on occasion if one isn't using chlorine as the regular oxidizer (especially if an ozonator is not being used).

    Non-halogen
    For those that want a non-halogen system, Nature2 with its silver ions is used with non-chlorine shock (MPS) for disinfection and is approved by the EPA. This system only works at hot spa temperatures (i.e. it is not EPA-approved for pools) and because MPS does not oxidize everything that chlorine does, one usually needs to shock with chlorine on occasion (especially if an ozonator is not being used). There is also another EPA-approved non-halogen system that uses Baquacil/biguanide/PHMB, but as with pools we don't recommend it because of its high cost and side effects though in a spa the issues with white water mold are easier to deal with since one can more easily change the water.

    Common Attributes
    Regardless of which system is used, the rough rule-of-thumb for how much oxidizer is needed after each soak (with no ozonator) is that every person-hour of soaking in a hot (104F) tub requires roughly 3-1/2 teaspoons of Dichlor, 3-1/2 fluid ounces of 8.25% bleach, or 7 teaspoons of non-chlorine shock (43% MPS). With an ozonator, usually half or less of these amounts are needed. One also needs to add a disinfectant regularly in between soaks. The real rule is to add whatever is needed so that one starts out with 1-2 ppm FC (2-4 ppm total bromine) for their next soak and that one never gets to zero disinfectant in between soaks.

    The Dichlor-then-bleach method will usually have the water last at least twice as long compared to the Dichlor-only method because the CYA won't build up. So in that sense, it's similar to the situation in pools that use Trichlor and have problems vs. using TFP methods that don't. The difference is that the problems in pools is usually algae while in spas it's that the water gets dull/cloudy. Also, with spas, there is a much greater risk of getting an infection from uncontrolled bacteria growth since they grow more quickly in spas and spas are more likely to get to zero disinfectant since the usage rate is higher and most people don't add enough disinfectant or add it frequently enough. The rough Water Replacement Interval (WRI) when using the Dichlor-then-bleach method is the following:

    WRI (days) = (2/9) x (Spa Size in Gallons) / (# of person-hours per day)
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    Mod Squad pooldv's Avatar
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    Re: Maintaining a hot tub the same as a pool?

    It is similar. I've been using the How do I use Chlorine in my Spa (or pool)? for about 18 months and it great!
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    Re: Maintaining a hot tub the same as a pool?

    Thanks for the links. That really helps. It's a little overwhelming. I'm sure Ill figure it out. I did the pool!!
    18K gal, IG Gunite, Sand Filter, TF-100 test kit

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