CYA Levels in the average pool

[Agentbolt]

The previous owner of the pool that came with my house would be, in my mind, the "average" pool owner. He went to the pool store about once a month and bought about 60 bucks worth of supplies, including an OTO test kit, some cal-hypo shock, and some Trichlor tablets. I'd say he's a common pool owner, I think a lot of normal folks don't bother with the algaecide unless they run into issues, with how expense the stuff is. And the pool looked great when I got it, very sparkly clean and pleasant to swim in.

Even if you'll grant that the average pool owner uses weekly algaecide to prevent the inevitable CYA-induced algae bloom (which I rather doubt, I'll bet most people wait until the algae bloom actually HAPPENS, but I don't have a lot of anecdotal evidence either way) I don't understand why most pools....work, for lack of a better word. Here's the parts that confuse me:

A) I've never seen anyone mention any kind of upper CYA limit where maybe it stops accumulating. The pool I inherited had a CYA level of somewhere between 150 and 200. I'm not great at reading the test, and the dilution method probably messes up the results somewhat, but still, this guy had been using 3-4 trichlor pucks at a time for YEARS. And this is in AZ, the pool never got drained for a closing. The guy would just try to make sure he was maxing out the OTO meter and be on his merry way, the entire year. It's a backwashed sand filter, so I'm sure that helped some, but we're talking a 1-2% drain MAYBE, done once a month or so.

So if the average CYA puck introduces about 10 ppm chlorine and maybe 6ppm CYA, why wasn't the CYA like 500 or something? The chlorine was getting burned off by the sun, but the CYA kind of maxed out apparently and just kind of found a place it liked despite the continual addition of more CYA, as far as I can see.

When I was googling "High CYA" after I inherited the pool, I saw plenty of people complaining of levels of about 100-150 or so, but no one really mentioned getting much higher than that. Even just in 1 pool season, if people go through maybe 40 Trichlor pucks (I have no idea what the average person's consumption is, but just one puck eroding at a time wouldn't be putting in enough chlorine to keep the FC level correct, so I'd assume most people use at least 2, but I could be wrong of course), just that, right there, should be giving people a CYA level of about well above 200. Even people who close their pools with a partial drain don't completely empty the pool, so the cycle doesn't start at 0 next season.

B) How is it that nobody really knows about the CYA/chlorine relationship? This is a niche site, like I said, I consider the average pool owner about as savvy as the average computer user. They don't know or care what the difference between SATA II and SATA III is, they just listen to the Best Buy guy suggest a computer and they buy it. (maybe I'm being massively dismissive of how educated most pool owners are) Everyone says the pool store folks don't know about (or choose not to mention) the CYA stuff, so if just dumping in pounds of stabilized chlorine over the months is an inevitable problem waiting to happen, why are most pools basically okay (as in, not a green mess)? Are most people's pools just massively overstablized, and they keep the algae problems at bay with Polyquat 60 forever? I can't even FIND algaecide at some places when I'm shopping, but even the local bodegas have trichlor pucks. If algaecide is basically just as important as chlorine because of everyone having too much CYA, you would think it'd be a more prevalent item. Basically with how much it can mess stuff up, I'd just figure more people would understand what CYA is and what it does.

Sorry, this was a long and rambling post, but I think you get the overall idea. I don't think my pool looks or feels any BETTER than it did when the prior owner had it. Granted, I spend a lot less than he did, but if everything I've learned about CYA and pool store chemicals is right, the pool should've turned int a disaster at least once a year or so when he was in charge of it, and it didn't.

 

[257WbyMag]

As long as you are using stabilized chlorine, CYA never stops accumulating. Most people don't mention CYA levels above 100 typically because you really can't reliably measure it above 100.

Most people don't know about the chlorine CYA relationship because most people don't expect that they need to get too involved with the chemistry side of things. The average pool owner relies on the pool store to troubleshoot for them when they have a problem. A sort of "tell me what product I need to buy and put in the pool for XYZ problem?" Pool stores don't really have a reason to get people to learn about their pool water management because when someone does eventually learn it, it costs the pool store money (in lack of sales).

I'm not sure that I would say that most pools are okay. Unless one knows what is happening in their water by testing accurately, then there is no way to know whether the water is ok or not. I'll tell you what...after coming to TFP, I can safely say that the majority of pools that I have seen at hotels and public places are seriously lacking in proper management. I have seen more dirty pools than clean.

 

[linen]

Are you sure the previous owner didn't do partial drains then refill? It seems sometimes that is the "dirty little secret" that often you don't hear a puck chucking pool owner admit too or not even realize/admit they are doing it occasionally. It is not uncommon for pool stores to recommend partial drain and refill hidden amongst their other advice/ products that get pushed.

 

[JasonLion]

There is all kinds of useful information about swimming pools, like the chlorine/CYA relationship, that does not get taught anywhere. With most of the pool chemical companies making money selling trichlor, isn't any incentive for them to undermine their primary product, nor does it benefit any of the other parts of the pool industry to do this kind of research and education. This kind of information does eventually get out there, but it takes many decades before it has any real impact. Measured in decades, the correct understanding of the chlorine/CYA relationship is quite new, with most people in the pool industry still pushing the old rules of thumb from before CYA true behavior was properly understood.

 

[woodyp]

I'm still waiting for the "Miracle Puck" to hit the market. Wouldn't something that has our good old chlorine combined with 1/3 parts of CYA, salt, and calcium or something inert to replace the usual dosage of CYA be nice?

 

[chem geek]

CYA can be lowered or not rise as quickly when using Trichlor from water dilution, but that not only comes from intentional partial drain/refill, but also from backwashing sand filters and from rain overflow which in some areas can happen in the summer as well as winter. There is a slow oxidation of CYA by chlorine as well. There can also be bacterial conversion of CYA into ammonia and/or nitrogen gas when chlorine levels are zero but that's not a very controllable process.

High CYA levels without proportionately higher FC levels do not guarantee algae growth. Algae also need nutrients to grow and some pools are naturally deficient in phosphates and/or nitrates (less likely with the latter if the pool is regularly used). Some Trichlor pucks include copper that can inhibit algae growth. Many people shock their pools weekly and that can partially make up for too low an FC/CYA ratio during the week.

At very high CYA levels with moderate FC levels, the chlorine bound to CYA may have some small algae inhibiting effect since chlorine bound to CYA oxidizes some chemicals about 100 times more slowly than hypochlorous acid (HOCl) alone (see this paper). So 5 ppm FC with 300 ppm CYA at pH 7.5 may only have 0.0064 ppm HOCl, but the 5 ppm FC that is mostly bound to CYA has an oxidation rate for monochlorodimedone equivalent to around 0.05 ppm HOCl so dominates. I don't think that this factor of 100 works for algae inhibition (partly because the main chemical is negatively charged so like hypochlorite ion it is repelled by cell surfaces which are negative), but there is still probably some similar effect for algae but with a higher factor (perhaps 300?). The net effect of this is that at very high CYA levels the FC alone appears to determine algae inhibition but it still takes a substantial FC, probably at least 8 ppm under conditions with plenty of nutrients.

Finally, though some pools don't develop algae in spite of the higher CYA levels, there are many, many pools that do. In my own pool years ago when I used Trichlor pucks, the first year was fine, but around the middle of the second year I had unusual chlorine demand and the water started to turn dull/cloudy in the beginnings of an algae bloom. This happened when the CYA level hit around 150 ppm with an FC of 3 ppm. My pool has a mostly opaque cover so the chlorine usage was low and I have a cartridge filter so had no backwashing nor rain overflow (the pool cover pump put water into the drain, not in the pool). So the precise level when algae grows varies by pool because specific conditions vary by pool. The recommendations on this forum handle nearly all pools with a failure rate of far less than 1 in 1000 pools (probably closer to 1 in 5000 or better).

I'm still waiting for the "Miracle Puck" to hit the market. Wouldn't something that has our good old chlorine combined with 1/3 parts of CYA, salt, and calcium or something inert to replace the usual dosage of CYA be nice?

That would be nice, but unfortunately the chemistry does not allow for that. Lithium hypochlorite is a solid that won't add CH nor CYA, but it dissolves too quickly (though I haven't seen any data of it being compressed into pucks) and it is very expensive. Cal-Hypo increases CH and pucks tend to fall apart if additional binder chemicals aren't used. Trichlor and Dichlor both increase CYA. Sodium hypochlorite needs to be in water in diluted form as the solid (sodium hypochlorite pentahydrate) is very reactive so is not used commercially.