I have been trained in water chemistry for hot tubs and have taken several classes. First, I hace always been taught never to use bleach in a hot tub. Only di-chlor. They are 2 different types of chlorine, different PH and di-chlor hols up the best in hot water. Alot of tub manufactures will void the warranty if they find you are using bleach (or Baqa Spa for that matter!) I have also been taught that if you cant use dichlor since it has to been put in manually on a daily basis, to use the small tri-chlor tablets and not to worry about cyanic acid levels since hot tubs are usually covered and dont need to keep these levels up to protect from the sun. Help me out if you can. I always like to hear and learn more on water chemistry. I try to be as helpful and informative as I can to my customers.
First of all, you should not believe what anybody tells you, including what I tell you, without skepticism or questioning or verification until you feel comfortable you are getting all the facts and know the whole truth to the degree that it can be known. Second, you should review the many posts and threads in this forum, especially in this section on Hot Tub Water Chemistry. You should also take a look at two other forums, The Pool Forum which unfortunately is closed to new registrations and PoolSolutions and TroubleFreePool. Though these are not as focussed on hot tubs, the chemistry of the water is exactly the same. The differences between hot tub and pool water chemistry are related to 1) higher bather load in hot tub (much lower water volume and more sweat from higher temperature), 2) hot tubs are usually covered most of the time and not exposed to sunlight very much, 3) higher aeration due to jets, 4) higher water temperature and 5) hot tubs usually not having plaster/gunite/grout.
I do not work in the pool or spa industry and am just a homeowner with a pool who got frustrated years ago with problems with Trichlor and high CYA and decided to figure out the pool water chemistry (I majored in physics and chemistry in college but I have and continue mostly to work in S/W Engineering and related management). I learned a lot from the aforementioned websites plus my own theoretical investigations and validation against reports on multiple pool and spa forums, peer-reviewed scientific studies (mostly available on the Internet, but also troweling through university library archives), plus my own limited experience. There are quite a few people who know this chemistry and also respond on these forums, such as waterbear who also frequents this forum and knows far more about water testing and practical chemistry than I do. We're all just here to help each other and would welcome industry professionals (more on that later).
Chlorine is Chlorine
The first rule that you should have been taught is that chlorine is chlorine is chlorine, at least when it is in the water. The differences in the various sources of chlorine have absolutely nothing to do with differences of chlorine in the water. The chlorine that is produced in the water after such substances are dissolved is identical regardless of source. The differences between the sources of chlorine are 1) some add chemicals in addition to chlorine, 2) the pH can change differently depending on which source of chlorine is used, 3) some sources dissolve quickly while others dissolve slowly, 4) some are solid pucks or powder/granules while others are liquid or gas.
Now item #1 for the sources of chlorine is VERY important since the "extra" chemicals that come with the chlorine have significant side effects. But before we go there, let's talk first about bleach (or chlorinating liquid which is identical to bleach except for its stronger strength). The primary reason that bleach is not recommended (or voids the warranty) in a hot tub is that using ONLY bleach without any Cyanuric Acid (CYA) in the water (more on that later) would result in over-chlorination that would destroy the hot tub cover much more quickly. The other reason is that it is a liquid that can splash so if you are not careful then you can splash concentrated chemical onto surfaces, but realistically that can happen with any source of chlorine if you aren't careful. You should also add bleach (or chlorinating liquid) slowly over a return flow with the pump running (circulation on, but not jets since you don't want it splashed or aerated) since it is denser than water until diluted/mixed so you don't want it settling in concentrated form.
Differences in Chlorine Sources
It is an over-simplification that Dichlor holds up best when in the water. It is NOT the fact that the source of chlorine is Dichlor, but the fact that Dichlor contains an extra chemical (when dissolved in water), Cyanuric Acid (CYA), that is the reason it holds up best. It makes absolutely no difference whatsoever whether you add Cyanuric Acid directly to the water first (CYA is available in pure form without chorine) and then add bleach vs. adding Dichlor that contains both -- when dissolved in the water they produce IDENTICAL chemicals in the water (ignoring the extra sodium chloride salt in the bleach). If you add bleach with jets on then the chlorine might get aerated more and therefore outgas more than if you added Dichlor (since it does take dilution and mixing of the bleach with the water to get combined with the CYA that is in the water), but adding the bleach either pre-diluted or slowly with circulation and not jets should be similar to adding Dichlor. The pH is different, but I'll talk about that later.
When you add an amount of Dichlor that produces 10 ppm Free Chlorine (FC) in water, it also adds 9 ppm Cyanuric Acid (CYA).
When you add an amount of Trichlor that produces 10 ppm FC in water, it also adds 6 ppm CYA.
When you add an amount of Cal-Hypo that produces 10 ppm FC in water, it also adds 7 ppm Calcium Hardness (CH).
When you add unscented bleach or chlorinating liquid (both are sodium hypochlorite and differ only in strength) or Lithium Hypochlorite or chlorine gas to water, they do not add any CYA nor do they increase CH. The bleach, chlorinating liquid and lithium hypochlorite add 8 ppm sodium chloride (salt) for every 10 ppm FC and this is above and beyond the 8 ppm salt for every 10 ppm FC that is produced from ANY source of chlorine since the usage of chlorine produces chloride. Cal-Hypo adds somewhat less salt. Trichlor and Dichlor add no "extra" salt beyond that produced from the usage of chlorine.
The thing to keep in mind is that while chlorine gets used up and gets converted to chloride so more needs to be added, the CYA does not break down and just continues to build up as you add more products that contain CYA (e.g. Dichlor, Trichlor).
You can see this link for a cost comparison of chlorine sources.
Chlorine and Cyanuric Acid (CYA)
Though it is true that Cyanuric Acid helps protect chlorine from being broken down by the UV rays of sunlight, it is not true that this is its only side effect. The CYA protects chlorine from sunlight via two effects. One is a shielding effect since CYA absorbs UV so that lower depths of water are not exposed to as much UV when CYA is present. The other factor is that CYA combines with chlorine to form a series of compounds called chlorinated cyanurates, but are conceptually chlorine attached to CYA. These compounds are more resistant to breakdown from the UV rays of sunlight so this factor protects chlorine even at shallow depths. However, the chlorinated cyanurates are not effective sanitizers nor algaecides -- they are much, much more inert compared to chlorine that is unbound to CYA (specifically, compared to hypochorous acid). You can learn more about the chemistry of CYA and chlorine in this thread where you will notice that the equilibrium constants for the chemistry were known back in 1973 and published in 1974. Some manufacturers and others in the pool and spa industry deny that this chemistry has an effect in "real pools" and they refer to a study to back up their claims which I talk about in this thread.
You have no doubt heard that stabilized chlorine, that is chlorine in the water with CYA present, is less effective than chlorine without CYA and that high levels of CYA make chlorine even less effective. What you probably haven't been told is by how much. It isn't just a factor of two or so difference. It's orders of magnitude -- factors of 10 or more. This has not only been shown in numerous bacteria, algae, protozoan cyst studies, but also shows up in oxidation-reduction potential (ORP) which of course relates to oxidation and not disinfection. Though there is some variation in the studies in terms of higher CYA levels and disinfection rates, there is absolute consistency in higher CYA levels having lower disinfection and the difference between having CYA and not having CYA is huge. The disinfecting form of chlorine that is the most powerful is hypochlorous acid. If you look at the link I gave above on the chemistry of CYA and chlorine, you no doubt recognize the traditional industry graph of hypochlorous acid and hypochlorite ion at various pH, but this graph is very deceitful because it does not take into account what happens in the presence of CYA. At a pH of 7.5 with 30 ppm CYA in the water and a Free Chlorine (FC) level of 3.5 ppm, over 97% of the chlorine is attached to CYA while about 1.5% is hypochlorous acid and 1.5% is hypochlorite ion. Only the hypochlorous acid is the highly effective sanitizer and oxidizer and that is only 1.5% of what is measured as FC!
So what does this all mean? It means that if you use Dichlor every day in a spa adding, say, 4 ppm FC per day, then after 3 months you will have added 4*0.9*30*3 = 324 ppm CYA -- about 100 ppm CYA per month. This means that the chlorine in the spa becomes less effective over time. Though the biggest drop in chlorine effectiveness occurs with relatively small amounts of CYA (so basically after the first few days of Dichlor addition), after that the amount of disinfecting chlorine is roughly proportional to the ratio of FC to CYA. Roughly speaking, if you double the amount of CYA, you halve the amount of disinfecting chlorine (assuming constant FC). Another thing you were probably not told in your training is that the Free Chlorine (FC) test does not measure only the disinfecting chlorine (hypochlorous acid) nor the hypochlorous acid plus hypochlorite ion, but in fact it also measures the chlorine attached to CYA. This is because chlorine releases from CYA fairly quickly (the hydrolysis half-life of one chlorinated cyanurate species is 0.25 seconds while for another it's 4 seconds).
You might say, well since it gets released in the FC test, it must be effective for killing pathogens and preventing algae. That would be incorrect. The chlorine attached to CYA is like being held in reserve -- CYA essentially acts like a hypochlorous acid buffer. However, this reserve is not "active" and does not directly participate in the reactions that oxidize or disinfect (at least not to any great extent). The rate of chemical reactions is based on the INSTANTANEOUS concentration of the chemicals in the reaction, not on the amount in "reserve" that can be released more slowly by conversion of related compounds. The rate of killing pathogens, inhibiting algae and oxidizing organics and ammonia depends on the hypochlorous concentration. All the chlorine bound to CYA does is act as a reserve replenishing the hypochlorous acid as it gets used up -- so you don't run out of chlorine as quickly, but it does NOT speed up the chemical reaction. It's like having a large army reserve without rifles and having a much smaller number of active soldiers with rifles. The rate of killing is dependent on the number of soldiers with rifles -- the reserve just means that when such active soldiers get killed that new ones can take their place from the reserve (that is, the rate of killing tends to remain constant instead of slowing down if active soldiers get killed, but the overall rate of killing is far, far less than if all soldiers were "active" instead of mostly being in "reserve").
If you don't have any CYA in the water at all, then adding even small amounts of chlorine produces a LOT of disinfecting chlorine (hypochlorous acid) and essentially you over-chlorinate the water. This is, in effect, what happens in most indoor swimming pools since CYA is not used in that water and it is annoying as hell since my wife's swimsuits degrade (elasticity deteriorates; they are fade resistant so only fade a little) over just one winter season of use in an indoor pool while in our outdoor pool that has CYA her swimsuits only show very slight signs of wear after 4 summers of use. Her skin and hair are also much more affected by the chlorine in the no-CYA environment. In fact, I believe that many of the problems with asthma and respiratory problems with indoor pools are not just due to poor air circulation and lack of sunlight, but also due to not using CYA (or using too much CYA which has different problems) since higher chlorine levels produce more disinfection by-products -- a fact well-known in the water treatment industry. I haven't proven this with real-world data and only have breakpoint chlorination models that indicate this, but there is a study in Europe that I've told to look at CYA levels as a factor and hopefully they will do that. It is the industry mantra that "CYA only protects chlorine from sunlight so isn't needed in indoor pools" that has led to these practices. At the other extreme, the industry mantra that "CYA doesn't matter; only FC matters" has many outdoor pools develop algae when Trichlor is used over time without additional algaecide or increase in FC level to match the rising CYA level.
So no CYA is bad because the disinfecting chlorine concentration is too high and too much CYA is bad because the disinfecting chlorine concentration is too low, so how much is really needed? We don't really know for sure, but from the data from thousands of multiple pool and spa forum users plus disinfection (CT) rate data it appears that very low disinfecting chlorine amounts are needed to kill most pathogens, but higher amounts may be needed to prevent hot tub itch and certainly higher amounts are needed to prevent algae growth (mostly in pools since spas generally don't get enough sunlight for algae to be a problem). The rough rule of thumb is that, without an algaecide (or phosphate remover), you need a minimum FC level of 7.5% of the CYA level to prevent algae growth, at least up to phosphate levels of 3000 ppb or so (above that, it might make more sense to use a phosphate remover unless higher FC levels are maintained). Pools that have extra systems such as saltwater chlorine generators (SWG) can usually get away with an FC level of 4.5% of the CYA level to prevent algae (due to superchlorination in the SWG cell that helps kill free-floating algae). To prevent hot tub itch in spas, I've been recommending 4 ppm FC with 20 ppm CYA so a 20% ratio, but that may be too conservative and perhaps 50 ppm CYA might be OK, but we don't have enough data yet to know. Certainly, 100+ ppm CYA is going to not be sanitary enough even with the more reasonable (lower) CT values of some pathogens (though some others are very easy to kill even at 100+ ppm CYA). And besides, it makes absolutely no logical sense whatsoever to vary the disinfecting chlorine level over the 3 months one uses a hot tub. Finally, high levels of CYA and therefore low disinfecting chlorine levels also slow down the breakpoint reaction of chlorine with ammonia so one finds more Combined Chlorine (CC) if you don't keep the CYA level in check.
To get to 20 ppm CYA in a hot tub, you would add Dichlor at a rate of around 4 ppm FC (2 teaspoons in 350 gallons) per day for about a week and then switch to unscented bleach (3 fluid ounces of 6% bleach in 350 gallons) after that, assuming that chlorine usage is around 4 ppm FC per day on average.
Chlorine and pH
Now let's turn to pH. It is generally recommended NOT to use Trichlor in hot tubs because Trichlor is highly acidic. It's bad enough in pools, but in spas the acidity can be more harmful, mostly because the water volume is much lower so you can make a mistake and get the pH too low much more easily. One of the myths in the industry, however, is that Dichlor is pH neutral (or nearly so) and that bleach or chlorinating liquid are high in pH. Though this is technically true, it is deceitful and misleading because it does not account for what happens when the chlorine then gets used up. I talk about this in the chemistry link I gave earlier, but essentially the breakdown of chlorine (by sunlight, disinfection or oxidation) is an acidic process that almost exactly compensates for the initial rise in pH from bleach or chlorinating liquid addition. When taking this into account, Dichlor is actually acidic and Trichlor is very acidic while the hypochlorite sources of chlorine (bleach, chlorinating liquid, Cal-Hypo, Lithium Hypochlorite) are close to pH neutral.
So you might ask, why does the pH rise if I use bleach? The reason is that the pH rises is that pools and spas are intentionally over-carbonated and this excess of carbonates (which is most of what is measured in Total Alkalinity, TA) outgasses as carbon dioxide. When this happens, the pH rises (with no change in TA for technical reasons I won't get into here). The relative rate of outgassing of carbon dioxide is a function of pH and TA as shown in this table where you can see that the rate is higher at lower pH and higher TA. The rate is also higher when there is more aeration, so when running the jets. The procedure to lower the TA level therefore takes advantage of these facts by adding acid in conjunction with aeration all done at low pH as described in this post. If you use a lower TA below 80 (but above 60 in spas and 50 in pools) and target 7.7 as a goal pH, you can significantly reduce the rate of pH rise. Or you can just not aerate and use a cover more frequently, but for spas aeration (via jets) is part of the experience.
[EDIT] Also, there is an initial rise from bleach addition and until the chlorine gets consumed (FC drops) the pH will be high so one should start at a pH of 7.5 before doing bleach addition or one should add acid and then bleach to more closely simulate what occurs with Dichlor (minus the added CYA). [END-EDIT]
What You Can Do
Thank you so much for asking questions and wanting to be helpful to your customers. I'd like to educate the resellers and retailers since it seems to me that the manufacturers are not doing so, but when I've approached these groups (both manufacturers and retail chains), including trade groups like the APSP, I generally get rebuffed or ignored (if not at first, then after some correspondence). I don't mind if they want to present their opinions along with the data I present, but I'd at least like to make sure you have all the information you need to help your customers. So to the extent that you can go back to your training instructors and industry professionals and start questioning them about this stuff, then maybe there will be some positive movement towards more thorough education.