CYA in salt vs non-salt pools

[BowserB]

I see recommendations of 30-50 ppm CYA for regular chlorine pools and 70-80 for salt chlorine pools. Given that CYA "protects" the chlorine from UV and at the same time reduces the effectiveness of chlorine, why the different levels for salt vs non salt pools? I've searched the CDC website, and the only info I can find says (for sanitation, as algae is not addressed at CDC) that chlorine ppm should be doubled if cyanuric acid is in the water. Note CDC does not mention CYA levels, just its presence. Incidentally, CDC says 1 ppm chlorine takes 10.6 days to kill cryptosporidium. Again, no info is given on higher chlorine concentrations.

So original question, why is CYA recommended at much higher concentrations in a salt-chlorine pool? I'm considering a salt chlorine generator, so this is of particular interest to me.

 

[mknauss]

Pool Water Chemistry

This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...

www.troublefreepool.com

The above is from the Deep End. So reader beware.

In a nutshell, a SWCG adds chlorine in small amounts over a long period. You will see that the FC/CYA Levels uses a lower % of CYA for FC when you have a SWCG. That is explained in the thread linked above. Because you use a lower FC versus the CYA level, a higher CYA level keeps the FC protected from the UV. The SWCG cannot 'keep up' during high UV periods. The higher CYA also allows your SWCG to not have to 'work as hard' to maintain the FC level in the pool during high UV periods.

 

[JoyfulNoise]

Be careful with industry and government regulations as they often leave out pertinent information in their analysis. While the inclination to "trust the experts" is understandable, in the case of CYA and FC, the government and industry gets the science all wrong. The key to understanding TFP methodology is that the science and chemistry of CYA and chlorine teaches us that, as long as the ratio of FC to CYA is held constant, it doesn't really matter what the absolute values of either component is (within reason). TFP teaches that manually chlorinated pools typically need an FC/CYA ratio of about 7.5% while SWG pools tend to need a ratio closer to 5%. In the case of an SWG pools, experience has shown that raising the CYA levels tends to work more effectively for all the reasons that Marty has alluded to above. So, as long as you maintain a 5% FC/CYA ratio, your pool will be sanitary and your SWG will work more efficiently at higher CYA. SWG's have a finite lifetime (anther from 6000-8000 hours of operational life) and so anything an owner can do to reduce output % and run time means that the SWG will last longer. Higher CYA levels allow one to run an SWG at lower output.

 

[BowserB]

Pool Water Chemistry

This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...

www.troublefreepool.com

The above is from the Deep End. So reader beware.

In a nutshell, a SWCG adds chlorine in small amounts over a long period. You will see that the FC/CYA Levels uses a lower % of CYA for FC when you have a SWCG. That is explained in the thread linked above. Because you use a lower FC versus the CYA level, a higher CYA level keeps the FC protected from the UV. The SWCG cannot 'keep up' during high UV periods. The higher CYA also allows your SWCG to not have to 'work as hard' to maintain the FC level in the pool during high UV periods.

Thanks, Marty and Matt. If read the chem geek's charts correctly, in order to get the same level of disinfection of 3ppm chlorine and CYA of 30, you would need nearly 9ppm chlorine at CYA of 70. Or for a less extreme difference, disinfection of chlorine at 3ppm at CYA 50 is comparable to 5ppm at CYA 70. So in the end, more CYA makes the chlorine last longer, but that longer-lasting chlorine is less effective. That of course, is old news and should be equally valid for a pool using liquid chlorine and one using a SWCG. If the SWCG can't "keep up" with chlorine demands with high UV, then why wouldn't you just use a higher output SWCG, rather than reduce the effectiveness of the chlorine with high cyanuric acid?

Pardon me for questioning conventional wisdom, but chem geek's posts seem to justify my questions. Going to a SWCG and dumping 700 pounds of salt into my pool seems like an expensive-to-reverse decision that I'd rather get right the first time. I'm going to research more, but it seems a little like one should maybe get a SWCG with double the capacity of the pool. Thanks again for the replies and link to chem geek's "Deep End" analysis.

 

[Msch99]

They do recommend double here. 10k pool get SWG rated 20k. We have swg and won’t go back. Once things are dialed in there isn’t much to do except test. Family says they like the feel better, but that is subjective.
Most say the cost averages out about the same. Higher up front for salt and SWG. Less daily maintenance though.

I understand not wanting to blindly follow other folks recommendations, but I will say TFP has worked for me and tons of other success stories in the forum. So I follow theirs without questioning much. A friend purchased same pool as us, fights it every year. I wouldn’t swim in it even when it is “good.”

 

[AusPhil]

If the SWCG can't "keep up" with chlorine demands with high UV, then why wouldn't you just use a higher output SWCG, rather than reduce the effectiveness of the chlorine with high cyanuric acid?

Ultimately there is more to it but yes it is my view that the biggest possible SWCG is a good thing. The TFP ranges specify a higher FC range for non SWCG pools and this allows for the bigger swings between dosing periods without dropping under the minimums. What interests me is that time of dosing is not taken into account with respect to CYA levels .... certainly this would fall into advanced use and hence outside the 80/20 rule of keeping it simple I suppose

Pardon me for questioning conventional wisdom, but chem geek's posts seem to justify my questions. Going to a SWCG and dumping 700 pounds of salt into my pool seems like an expensive-to-reverse decision that I'd rather get right the first time. I'm going to research more, but it seems a little like one should maybe get a SWCG with double the capacity of the pool. Thanks again for the replies and link to chem geek's "Deep End" analysis.

Personally i run a RJ45 cell on a 7k gallon AGP ... way over the 2x rule and more like 6x ... but it runs 1/4 of the day at usually 50% though with the cover on that reduces to 40%or30% ..... Go large and enjoy the SWCG, i would never go back to manual dosing.

 

[wireform]

One point left out is the SWCG is a "maintain" method meaning you have to have all your numbers where they belong and also use liquid chlorine to bring it to the FC level. At that point the SWCG kicks in and only needs adjustments for the non usual pool event like pool parties with heavier bather loads. The larger the SWCG the faster it maintains a given FC so 2x or 3x pool capacity will not over tax the cell giving it longevity. Pump run time only needs to be on long enough to achieve the FC level. The rest of the pump time is only op preference and isn't needed for the FC level.

 

[C.Johnson]

Some numbers from the field:

I have several salt pools that I service weekly, and can tell a huuuge difference in ease of maintaining adequate FC levels between 30-50ppm and 70-80ppm CYA. Our IC40s need to be run close to 80-90% with the lower CYA range, and even that isn't adequate, with occasional algae outbreaks. Bumping the CYA up to 80ppm eliminates that problem and we need to bump the run-time down dramatically to keep from having high FC levels.

 

[goz63]

BrowserB I am really glad you asked that question because as a new pool owner that just switched to a SWG didn’t make sense. C. Johnson that experience helps. A follow on question is the pool company (I know I don’t trust them either) that put in my SWG said to run at 40-50 to prevent chlorine lock from the CYA. Is that a thing? By The Way the manual for my SWG says CYA of 80 preferred for whatever that’s worth.

 

[C.Johnson]

Just to clarify, by "running it at 40-50" you are talking about ppm CYA and not the active % on the generator correct?

If so, their advice is good for non-salt pools, but the recommended level is higher for the reasons stated above. I'm not 100% on the exact mechanism as to why it is so much harder to maintain FC levels with the lower range; I suspect something to do with the cell generating hypochlorous acid only and the rate of UV degradation with the smaller amounts added at any time, but perhaps that is a discussion for the Deep End section of the forum.

Suffice to say, Normal CYA range in salt pool = hard.
Switching to recommended levels for salt pool = life became easy

 

[JamesW]

Higher CYA might make the water more opaque to UV light.

However, that hypothesis still needs to be tested.

Higher CYA helps reduce cell scaling because it's a non-carbonate form of acidity, which contributes to the total acidity.

 

[BowserB]

Goz63, I'd be curious to know how your CYA of 40-50ppm is working out for your FC levels. I see that your Hayward T-Cell9 is a 25,000 gallon capacity unit running in your 10,000 gallon pool. Of course it's winter, and UV is reduced compared to summer, but I see you're at about the same latitude as we are here in the Houston area. Is CYA at 50 and your 25k capacity cell able to stay ahead of the sun? Or have you followed the manual instead of the PB? At least your guy put in a salt system. Mine practically shouted that with salt he couldn't guarantee that anything would last a year!

Related question. Does anyone here have sacrificial anodes installed as part of your SWCG? I know that technology is legit. The chemical plant I worked for had them for their underground pipes in the tank farm. It's the chemistry behind "galvanized" in garbage cans and other steel products. In a swimming pool SWCG, it seems like the salt cell unit itself might be a good place to include the anode, or possibly at the skimmers. Any experience here? We've visited and stayed on Galveston Island enough to have seen what even "salt air" does to steel and other metals. I know salt concentration in a pool is only 1/10 that of the ocean, but we also have chlorine, another corrosive element, at work destroying algae and bacteria plus pretty much anything else that it touches. I want to do this SWCG project right the first time and don't want to be dependent on just the installer's expertise and knowledge.

Thanks again, guys, for taking the time to share your knowledge and experience.

 

[Jimrahbe]

B,

I have three saltwater pools here in the DFW area. One has been a saltwater pool for about 10 years, one of 7 years and the one at my house about 5 years.. They all work great, nothing has failed, or shown any signs of damage or deterioration due to the saltwater.

I have seen a few posts from the members that have Anodes, but they have all been in the North East of the US. I have never heard of anyone in Texas having one.

If I owned another 100 pools, they would all be saltwater pools.. Or they would be filled with dirt..

Thanks,

Jim R.

 

[wireform]

Goz63, I'd be curious to know how your CYA of 40-50ppm is working out for your FC levels. I see that your Hayward T-Cell9 is a 25,000 gallon capacity unit running in your 10,000 gallon pool. Of course it's winter, and UV is reduced compared to summer, but I see you're at about the same latitude as we are here in the Houston area. Is CYA at 50 and your 25k capacity cell able to stay ahead of the sun? Or have you followed the manual instead of the PB? At least your guy put in a salt system. Mine practically shouted that with salt he couldn't guarantee that anything would last a year!

Related question. Does anyone here have sacrificial anodes installed as part of your SWCG? I know that technology is legit. The chemical plant I worked for had them for their underground pipes in the tank farm. It's the chemistry behind "galvanized" in garbage cans and other steel products. In a swimming pool SWCG, it seems like the salt cell unit itself might be a good place to include the anode, or possibly at the skimmers. Any experience here? We've visited and stayed on Galveston Island enough to have seen what even "salt air" does to steel and other metals. I know salt concentration in a pool is only 1/10 that of the ocean, but we also have chlorine, another corrosive element, at work destroying algae and bacteria plus pretty much anything else that it touches. I want to do this SWCG project right the first time and don't want to be dependent on just the installer's expertise and knowledge.

Thanks again, guys, for taking the time to share your knowledge and experience.

check this thread for similar content

 

[JoyfulNoise]

So in the end, more CYA makes the chlorine last longer, but that longer-lasting chlorine is less effective.

This statement is exactly wrong and why you are confused. This is the standard interpretation by the industry and by people that do the epidemiological studies of CYA and chlorine.

The idea that chlorine is less effective at higher CYA is predicated on the complete misunderstanding of the equilibrium chemistry of chlorine in water. When you put chlorine in water with CYA present there are three (3) "free" chlorine compounds present - chlorine bound to CYA (chlorinated cyanurate), hypochlorite anion (OCl-) and hypochlorous acid (HOCl). At a pH of 7.5 the ratio of HOCl to OCl- is approximately 1:1 . When cyanurate is present at any concentration above ~ 20ppm or so, nearly 95% of the chlorine is bound to the cyanurate ion as a chlorinated cyanurate, however, the equilibrium ratio of HOCl:OCl- remains the same.

Chlorinated Cyanurates on their own are weak disinfectants.

Hypochlorite anion (OCl-) is a mild disinfectant that has a fairly long CT kill profiles and is very sensitive to UV photolysis.

Hypochlorous acid (HOCl) is a very powerful disinfectant and oxidizer with short CT kill profiles.

Since water with 2.5ppm FC and 30ppm CYA has almost exactly the same amount of HOCl (~ 35ppb) in it as water with 5.5ppm FC and 70ppm CYA, there is no "less effective" chlorine. The pathogens in that water would experience the exact same CT kill profiles as HOCl is what is responsible for the disinfection. As the HOCl gets used up, chlorine is released from the cyanurate to compensate and those release rates are very fast. So the only thing CYA does is act as reservoir for chlorine and those bound states are much less affected by UV radiation.

So why is it important for SWGs?

As mentioned, SWGs add chlorine to the water at very slow rates. By increasing the CYA level, you make the water more opaque to UV transmission and thus reduce the photolysis of hypochlorite. You have a bigger reservoir of chlorine that has much lower UV loss and therefore is more stable. When you lower the CYA level, chlorine gets used up more rapidly and the SWG has to be run at a higher output duty cycle and for longer time. Because SWG's have finite operational lives (they typically only last 6000-8000 hours), you want to use them in a way that runs them as little as possible. By doubling the size of the SWG and using higher CYA levels, you get the same effective sanitation with less loss of chlorine and thus your cell lasts for more seasons.

 

[goz63]

Joyful, dang that’s a lot of chemistry but very helpful and makes a lot of sense. Thank you for the detailed explanation.

 

[goz63]

Goz63, I'd be curious to know how your CYA of 40-50ppm is working out for your FC levels. I see that your Hayward T-Cell9 is a 25,000 gallon capacity unit running in your 10,000 gallon pool. Of course it's winter, and UV is reduced compared to summer, but I see you're at about the same latitude as we are here in the Houston area. Is CYA at 50 and your 25k capacity cell able to stay ahead of the sun? Or have you followed the manual instead of the PB? At least your guy put in a salt system. Mine practically shouted that with salt he couldn't guarantee that anything would last a year!

Related question. Does anyone here have sacrificial anodes installed as part of your SWCG? I know that technology is legit. The chemical plant I worked for had them for their underground pipes in the tank farm. It's the chemistry behind "galvanized" in garbage cans and other steel products. In a swimming pool SWCG, it seems like the salt cell unit itself might be a good place to include the anode, or possibly at the skimmers. Any experience here? We've visited and stayed on Galveston Island enough to have seen what even "salt air" does to steel and other metals. I know salt concentration in a pool is only 1/10 that of the ocean, but we also have chlorine, another corrosive element, at work destroying algae and bacteria plus pretty much anything else that it touches. I want to do this SWCG project right the first time and don't want to be dependent on just the installer's expertise and knowledge.

Thanks again, guys, for taking the time to share your knowledge and experience.

Bowser,
My cell was running at 50% for 4 hours a day. My pool is partly shaded by my house this time of year, pool on north side if house. My FC was 4.5 and rise to 6. I have dialed down the SWG to 40%. Starting to use a pool blanket so that will help. New to all this so just reporting my findings. By The Way the pool guy did not put salt in. I did. Took 160lbs. SWG says 3000ppm. Ordering a salt reagent kit to supplement my TF100 kit. Speed stir arrives tomorrow.

 

[sean.a.hyde]

This statement is exactly wrong and why you are confused. This is the standard interpretation by the industry and by people that do the epidemiological studies of CYA and chlorine.

The idea that chlorine is less effective at higher CYA is predicated on the complete misunderstanding of the equilibrium chemistry of chlorine in water. When you put chlorine in water with CYA present there are three (3) "free" chlorine compounds present - chlorine bound to CYA (chlorinated cyanurate), hypochlorite anion (OCl-) and hypochlorous acid (HOCl). At a pH of 7.5 the ratio of HOCl to OCl- is approximately 1:1 . When cyanurate is present at any concentration above ~ 20ppm or so, nearly 95% of the chlorine is bound to the cyanurate ion as a chlorinated cyanurate, however, the equilibrium ratio of HOCl:OCl- remains the same.

Chlorinated Cyanurates on their own are weak disinfectants.

Hypochlorite anion (OCl-) is a mild disinfectant that has a fairly long CT kill profiles and is very sensitive to UV photolysis.

Hypochlorous acid (HOCl) is a very powerful disinfectant and oxidizer with short CT kill profiles.

Since water with 2.5ppm FC and 30ppm CYA has almost exactly the same amount of HOCl (~ 35ppb) in it as water with 5.5ppm FC and 70ppm CYA, there is no "less effective" chlorine. The pathogens in that water would experience the exact same CT kill profiles as HOCl is what is responsible for the disinfection. As the HOCl gets used up, chlorine is released from the cyanurate to compensate and those release rates are very fast. So the only thing CYA does is act as reservoir for chlorine and those bound states are much less affected by UV radiation.

So why is it important for SWGs?

As mentioned, SWGs add chlorine to the water at very slow rates. By increasing the CYA level, you make the water more opaque to UV transmission and thus reduce the photolysis of hypochlorite. You have a bigger reservoir of chlorine that has much lower UV loss and therefore is more stable. When you lower the CYA level, chlorine gets used up more rapidly and the SWG has to be run at a higher output duty cycle and for longer time. Because SWG's have finite operational lives (they typically only last 6000-8000 hours), you want to use them in a way that runs them as little as possible. By doubling the size of the SWG and using higher CYA levels, you get the same effective sanitation with less loss of chlorine and thus your cell lasts for more seasons.

This is an amazing explanation, thank you.
Based on this, how do you set the upper bound for CYA? Increasing it gives you a bigger chlorine capacitor (clearly I'm an EE, not a ChemE).
Is the upper bound based on FC being so high it starts to mess up tests like pH? Is the upper bound based on needing a massively high FC for things like SLAM?

 

[BowserB]

Matt, you are the man! Your posts here plus the one Wireform refererenced on anodes, could be the answers to any SWCG question. Interestingly enough, your pool size and equipment are very similar to my own, except you have a retired suction side Kreepy Krauly, and I have a retired pressure side Kreepy Krauly (plus its associated NOISY booster pump.) I also contacted Brian Boothe at Pentair to find out if either of their SWCGs were compatible with the IntelliConnect installed by my pool builder. It turns out both of their product lines are compatible, and his guys at the office recommended the IC40 for my pool. Same as yours. I won't pretend to fully understand the details you have offered--it's been 57 years since my high school chemistry class--but you do make sense and your conclusions are consistent with others I've read. So thanks. As to anodes, your post on that is helpful, too. I'm pretty old, so there may be a good chance that I'll be dead before corrosion is an issue, although I'm still going to use your info and inquire of my installer to see what he knows. It won't be the original pool builder, though, since he is a nonbeliever (who didn't even know how to set up the IntelliConnect box and finally suggested I buy a separate remote for the pump!) There is an authorized Pentair dealer and pool builder here in Katy, located not a mile from my house. He doesn't build pools in Katy--says the permits and inspections are a problem, so his company builds pools in River Oaks and Memorial mostly (even if you don't live in Houston, you can probably guess what that means.) He did take care of a Pentair warranty issue for me, though. I'll give him a call next week.

Thanks again to all who replied. At least I'm going into this project with some knowledge, and as "Auntie Mame" proclaimed, Knowledge is Power!

 

[JoyfulNoise]

This is an amazing explanation, thank you.
Based on this, how do you set the upper bound for CYA? Increasing it gives you a bigger chlorine capacitor (clearly I'm an EE, not a ChemE).
Is the upper bound based on FC being so high it starts to mess up tests like pH? Is the upper bound based on needing a massively high FC for things like SLAM?

The upper bound is really set based on practicality. If you maintained the 7.5% FC/CYA ratio, you could, in theory, run the CYA as high as you want. But there is diminishing returns in terms of UV protection once you get above 120ppm and there is a huge downside - if you get algae, a SLAM would become very costly. Shock levels promoted by TFP are essentially a 40% FC/CYA ratio. So if your CYA is much above 90ppm, a SLAM would require HUGE amounts of liquid chlorine. This would make remediating a green pool very costly. For winterized pools, CYA can present an unwanted challenge - there are bacteria that can get into a winterized pool and use the CYA as an energy source. In the process of consuming the CYA, the bacteria will convert it to ammonia. Ammonia requires very large quantities of chlorine to neutralize it (roughly a factor of 10X so a 9ppm ammonia load will require nearly 90ppm FC to neutralize it). So, if a winterized pool had large quantities of CYA in it, that could be a ticking time bomb in terms of ammonia generation.

All the recommended levels posted on TFP come from well over a decade of practical user experience (TFP has been online since 2007 and the old Pool Forum was around ten years before that using a very similar philosophy) in tens of thousands of pool. The data here is very trustworthy...