Pool Chemistry Truths: Chlorine, pH & CYA with Eric Knight

Most pool pros have heard it for years: lower the pH to make chlorine stronger. That’s true in non-stabilized water, but once cyanuric acid enters the picture, the rules change. We sit down with Eric Knight to unpack why the FC-to-CYA ratio, not pH, governs chlorine’s effective strength in outdoor pools—and how that insight can save you time, money, and a lot of acid.

We break down the chemistry in plain language. You’ll hear how chlorine splits into hypochlorous acid and hypochlorite ion, why that balance matters indoors, and how CYA binds most chlorine outdoors to form isocyanurates. With typical CYA levels, the effective kill speed stays nearly the same between pH 7.0 and 8.0, which means chasing an ultra-low pH for “stronger chlorine” is a dead end. Instead, use pH to manage balance on the Langelier Saturation Index, contain its rise with smart alkalinity and calcium hardness, and aim for a CYA level that keeps your free chlorine target achievable.

We also talk real-world strategy: the pitfalls of overstabilization, how high CYA inflates contact times, and why partial drains are sometimes the only fix. To sharpen your program, support chlorine with enzymes to trim oxidant demand, control phosphates to lower growth pressure, and consider secondary oxidation where it fits. The goal is a stable chain: CYA in range, free chlorine matched to that CYA, pH contained for LSI, and demand reduced so sanitizer can do its job.

• FC-to-CYA ratio as the primary driver of chlorine effectiveness in outdoor pools
• Why pH control matters for LSI balance more than sanitization with CYA present
• The equilibrium of HOCl and OCl− in non-stabilized water contrasted with CYA-bound chlorine
• Practical CYA ranges and why levels above 50 ppm complicate free chlorine targets
• Overstabilization risks, longer contact times, and when to drain and dilute
• Using enzymes, phosphate control, and secondary oxidizers to reduce oxidant demand
• Containing pH with LSI strategy instead of forcing low numbers that rebound
• Clear differences between sa

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Chapters

• 2:38 Milestone Season And Guest Intro
• 3:58 PH And Chlorine Basics Without CYA
• 5:54 Adding CYA Changes The Rules
• 9:08 Rethinking PH Control And LSI
• 13:08 Balance Versus Sanitization Explained
• 16:48 Optimizing Chlorine: Ratios, Enzymes, Phosphates
• 20:03 Risks Of High CYA And CDC Guidance
• 23:03 Finding Manageable CYA Levels
• 24:35 Resources, Coaching, And Closing

Transcript

SPEAKER_01: 1:50

Hey, welcome to the Full Black Podcast Show. As I enter my tenth season or nine years of doing this podcast show, at the end of this year, it'll be my tenth, tenth year doing the podcast. I want to thank everyone for sticking with me throughout the years, and the tenth season of the show will allow me to cross the 2 million download mark, which is a true milestone in podcasting. So again, I want to thank you, the loyal listeners of the Pool Guy Podcast Show. In today's episode, I'm going to go back to 2020, where I interviewed Eric Knight, then of Orenda and Hassa. He's now the executive director of Waterscape University. And he's going to talk about the relationship between your planeric acid and the pH in the pool. And it's a really interesting talk that I think is really relevant today. And so let's kick off the 10th season of the Pool Guy Podcast show. Are you a Pool Service pro looking to take your business to the next level? Join the Pool Guy Coaching Program. Get expert advice, business tips, exclusive content, and get direct support from me. I'm a 35-year veteran in the industry. Whether you're starting out or scaling up, I've got the tools to help you succeed. Learn more at swimming poollearning.com. Uh thanks for joining me today. We're going to be talking about kind of a complex subject. It's the relationship between pH, chlorine, and cyaneric acid. And I'll start with the first question here. And if you could just explain briefly the relationship between pH and chlorine's effectiveness.

SPEAKER_02: 3:26

Sure. So when chlorine goes into water, no matter what type of chlorine you use, trichlor, liquid chlorine, calhypo, uh, what happens is you get this dissociation reaction where hydrogen either binds to it or leaves it. And what happens is you have two different types of chlorine. You've got the strong killing form of chlorine called hypochlorous acid. And then you've got the weaker, like a hundred or more times slower and weaker form called the hypochlorite ion. And these two are always functioning in an equilibrium, either 50-50, 60-40, 70-30 at all times. And the pH controls that ratio. So the lower your pH, the higher the percentage of the strong killing form of chlorine. And it's the opposite, the higher the pH, the lower that percentage. So the higher your pH, the weaker your chlorine. But all of that only applies to a non-stabilized pool. And this is the trap because so many people in the pool business have always been taught that the pH must be lower to have stronger chlorine. And that's true if there's no stabilizer. But David, how many outdoor pools do you know of that have zero stabilizer in them?

SPEAKER_01: 4:48

Yeah, none.

SPEAKER_02: 4:49

Pretty much none. I mean, in the state of New York, the commercial ones don't, but let's be realistic, this kind of goes out the window because the strength of chlorine is now commanded by the free chlorine to cyaneric acid ratio, which I think we're probably about to discuss.

SPEAKER_01: 5:06

Yeah. So this the next question is um let's add the variable of cyaneric acid into the equation now. Um how does that change the relationship between pH and chlorine's effectiveness?

SPEAKER_02: 5:17

So uh I have a chart here, if I can share my screen uh so that your viewers can see it. So let me share my screen.

SPEAKER_01: 5:25

And for those of you who are listening to the YouTube version of this, you're gonna see the chart appear in the video here. And if you're listening to the podcast, you can find this chart very easily. I have it in a Google Drive file. You can get to the link for the image while you're listening to the podcast by in the Google search box. You just have to type in rb.gy forward slash G Z O R R E. Again, that's rb.gy forward slash G Z O R R E. And that should take you to the Google Drive file of the image that um Eric is talking about right now in the podcast.

SPEAKER_02: 6:11

And let me know if you can see this chart.

SPEAKER_01: 6:14

Yeah, I can see it.

SPEAKER_02: 6:15

Okay, so the left side, as I was just explaining, the red line uh is hypochlorous acid, HOCL. The lower the pH, the higher the percentage, stronger chlorine. So at 7.2, it's uh about 65 to 66 percent, it looks like, of strong chlorine. The rest, a much lower percentage of weak chlorine. But up here at like 7.8 or 7.9, it's a much higher percentage of the weak chlorine, you know, over a hundred times slower and weaker, this hypochlorite ion, which is this green line OCL negative. This is what we're all used to. At around 7.5, it's 5050. But that's with no stabilizer. So, to your question, with stabilizer, let's come over to the right side. Now find the red line. David, do you see it?

SPEAKER_01: 7:08

Yeah, at the bottom.

SPEAKER_02: 7:09

At the bottom. And at 7.0 pH, it's about 3%, as opposed to here, where it's 75% without stabilizer. Now it's about 3% with 30 parts per million CYA. And most outdoor pools have more than 30 parts per million CYA anyway. So can you really tell the difference between 70 pH and 80 pH on this chart with the red line?

SPEAKER_01: 7:36

No, it's pretty equal.

SPEAKER_02: 7:38

It's pretty equal. I mean, it goes down a little bit, but it's less than 1% difference. So it's it's pretty much negligible. And that is what we determine as the strength. I'm using air quotes, the strength of chlorine is actually remarkably the same between 7.0 and 8.0 pH, and even up to 8.5, 8.6, if you look at that chart. And that's because the vast majority of the chlorine has been bound up by cyanuric acid, which is this blue line at the very top. So now these are, I think they're called isocyanurates. This is basically chlorine attached to cyanuric acid. This is the vast majority of what is in your pool. It's not all bad though, because I know it looks like your chlorine, oh my gosh, it's so weak. You know, it's it's only 3%. That's not necessarily true. What only matters here is the difference between the different pHs. The pH no longer controls the swing, is what I'm trying to get at. Because you do have plenty of chlorine, it is slowed down because it has to release itself from cyanuric acid to go oxidize or sanitize. So that's what I'm trying to get at is stabilizer stabilizer takes over, whereas pH no longer controls the strength or speed of your chlorine, stabilizer does.

SPEAKER_01: 9:03

Got it. So all the guys that are trying to keep their pH at 7.2 to make the chlorine much more effective or faster at killing um the viruses, germs, bacteria in the water with the cyaneric acid in there, there's no real reason to keep it down at 7.2 because at 8.0 with 30 parts per million according to this chart, the chlorine's going to have the same killing speed no matter what the pH is. Is that correct?

SPEAKER_02: 9:28

Or very close to it. I mean, there are it does slightly go down, but it's negligible difference. So you look here. I mean, here's 7.2. Can you even see a difference between 7.2 and 8.0 in terms of the strength of chlorine?

SPEAKER_01: 9:40

Very very small percentage. So the guys that are just thinking, like uh the old school thinking like myself, that the pH is a big factor in the chlorine's killing ability, um, that becomes kind of that kind of throws it out the window when you look at this chart with 30 parts per million of cyaneric acid in it. So we're actually probably wasting money, time, and effort trying to keep the pH low all the time in the pool. Sure.

SPEAKER_02: 10:07

Uh that's certainly the case. And uh that chart, by the way, is all available on our blog, which is blog.orendatech.com. There's a search bar, just type in CYA and hit enter, and you will see uh we probably have 10 articles about it. That chart's in a lot of those articles. I think there's kind of two sides to what you just said. When people are trying to correct pH, don't be correcting pH because of the strength of chlorine. There are reasons to adjust pH, but I think the mistake that we see most often is people try to control pH. It doesn't want to be controlled. pH is a reactionary chemistry, it's going to bounce away from you like a rabbit. So instead, what we want to do is we want to teach people to try to contain pH. Use things like the LSI to your advantage and understand that physics is going to pull the pH up to 8.2 anyway. So if you know where the ceiling is and you can set the floor with something like calcium hardness, you can contain the pH. So my message to your viewers is don't necessarily lower your pH to 7.4 below just for the sake of thinking it's for chlorine. It's not anymore. Maybe go to 7.6 and have more of an LSI-based strategy because the pH dramatically impacts the LSI, which is the saturation index. And one thing I'd like to tag onto that is there are really two sides to water chemistry. There's sanitization, which is keeping the pool clean, clear, algae-free, chlorination, safety, all that stuff. And then there's balance. And people conflate the terms. They think, oh, my chemistry is balanced because I have a lot of chlorine in there. They're largely unrelated. Balance is the saturation of calcium carbonate as measured on the LSI, or if you choose, you could use like a Hamilton index or Reisner. That's balance. And pH is really, really, really important for balance. It is not important for uh sanitization in a cyanuric acid pool. But say an indoor commercial pool that has no stabilizer in it, yes, pH controls the chlorine strength directly. So it's it's still very important there.

SPEAKER_01: 12:27

Yeah, so I'll just we'll touch on that one more time. When you say balance, so the LSI will indicate if the pool is corrosive or if the pool is scale forming, and pH is one of the factors in that. I don't want to I don't want to gloss over that. So people know that you mentioned that just by balancing the water, people think since they have sanitizer in the pool, that the water is safe and balanced, but it's not really balanced in the way of the LSI, where it could be a corrosive water versus a scale-forming water.

SPEAKER_02: 12:56

Yes, you you could have you could have a safe, clean pool that's horribly out of balance, and you could have a green, disgusting, algae-filled pool that's perfectly balanced. I mean, it's not likely, but I mean it's possible. So it we have to look at those things as two different things. So for your homeowners who are listening, or the pool service guys, if you have aggressive water on the LSI, this is the water that destroys surfaces by etching, it destroys heat exchangers, salt cells, uh, it destroys vinyl liners. They start fading and cracking. Same with fiberglass pools. Aggressive water is hungry for one thing and one thing only. Do you know what that one thing is? Calcium's and it will stop at nothing to find it. So that's balanced, right? So we don't want to confuse the two. And so, yeah, I don't think going down to 7.4 is necessary because the pH is trying to get up to 8.2. That's a huge swing on the LSI. You're gonna be a lot better off going down only 7.6 or even 7.7 and letting that pH rise a lot slower because it's closer to its end goal.

SPEAKER_01: 14:09

Okay, and so you know, the biggest takeaway, I think, from this for the listeners is that you should look at pH more as a balancing tool for the LSI versus if this is speaking of a pool with cyaneric acid in it, of course, uh, versus one as an indoor pool without it. So if the pool has cyaneric acid in it as a stabilizer, you're looking more for pH to not control the effectiveness of chlorine, because now the chart showed us that with the cyaneric acid level at 30 parts a million, there's not a lot of difference in the kill speed at 7.0 and 8.0. So what you should be doing as far as adding acid to lower the pH down is for the LSI balancing and not necessarily to increase the effectiveness or the kill rate of the chlorine.

SPEAKER_02: 14:52

Correct. The the ways that you optimize the killing effectiveness of chlorine is to manage the free chlorine to cyanuric acid ratio. And and that is what really matters in terms of sanitization. And then outside of that, you can remove the oxidant demand with enzymes or a secondary oxidizer like ozone, and you can remove nutrients like phosphates that slow down the growth and reproductive rate of contaminants. But really, when it comes to sanitizer, you what really controls chlorine effectiveness is cyanuric acid.

SPEAKER_01: 15:28

And you know, I'm a big fan of chlorine enhancers nowadays. I I've I really jumped in a bandwagon. I was an old school pool guy that I didn't really pay much attention to it or or give it much credence. But I think by enhancing the effectiveness of free free chlorine, especially with the in lieu of the chart you showed about the cyaneric acid level interfering with the kill rate of the chlorine, I think by enhancing that free chlorine by using enzymes and phosphates is a key factor in maintaining a good balanced pool, balanced in the and in the way of sanitizer being effective. Um and I think a lot of pool professionals are starting to realize that that the guys that were ignoring the chlorine enhancers are starting to see the the need for it. And the chart really illustrates the fact that pH can no longer be um the key element in making chlorine effective. It's just the free chlorine is what is what we should be focusing on.

SPEAKER_02: 16:23

Well, you're you're right. Well, the free chlorine to cyanuric acid ratio. And I just want to say that we didn't make any of this up. Um Renda certainly didn't. We didn't even do any of the research directly. Uh I just read from people who have done the science, uh, people like Richard Falk and Dr. Alan Meyer and the council ad hoc committee for the Model Aquatic Health Code, all these little acronyms of these agencies that are doing this research for the industry. All I'm trying to do is be the messenger and distill this information. I didn't make up the chart, but when I ask the people who did make the chart what it means, is it's a paradigm shift, guys. We have to think a little bit differently here. Cyaneric acid has a lot of benefits to it, but too much cyaneric acid doesn't give you any more sunlight protection, but it's all the detriments that go with overstabilization. So that's what we want to avoid. We want to avoid too much CYA. We don't want to have none.

SPEAKER_01: 17:26

And the Pool Guy Podcast Show is teamed up with UPA to bring you affordable, reliable liability insurance starting at just$64 a month. Get up to$2 million in coverage, and the members of my coaching group get an even lower rate. Protect your business and your peace of mind. Sign up with UPA today by clicking the link in the podcast description. Yeah, so look the person that has a pool with a cyaneric acid of 100-150 parts per million, they're gonna have a harder time, especially with that chart. It was only at 30 parts per million on that chart. So I can imagine at 150 parts per million, how slow is that chlorine's effectiveness?

SPEAKER_02: 18:05

Well, it that's a great question. I don't know the answer. I'm sure there's parts that can do it, and you can extrapolate. If you look at the data um and there's a lot of data available, there's charts that show the kill time, or the it's called CT, contact time. And it goes up a lot. I'm not sure if it's exponentially, but for instance, Cryptosporidium is a very chlorine-resistant bug that commercial pools really have to worry about in case there's like a diarrheal accident with a child. That problem can shut down an entire swimming pool and they have to be forced to drain. So the CDC gets involved and they look at the cyanuric acid, and they came up with a uh, I don't know if it's a regulation, but basically a limit, a practical limit that if you have more than 15 parts per million, 15, 1.5 parts per million CYA, you have to drain your pool down to below 15 parts before you can effectively sanitize it because you couldn't even measure the contact time required. So it goes, it goes through the roof the higher your CYA. And it's not linear, it's it's it's kind of parabolic. It goes up really fast the higher and higher you go. So if you have a hundred see 150 CYA or above, there's really only one thing Arenda would recommend you do, and that's drain a portion of your pool, if not all of it, to get that that number down because it's it's got total control over your pool at that point.

SPEAKER_01: 19:37

Yeah, it's interesting. I really think that the CYA, um some people call it a debate in the industry, but there's really not much debate if you look at the data. The higher the CYA, the more problems you're gonna have with everything. And not only that, but you have to have a higher free chlorine level and with the higher cyaneric acid level. So um all those are huge factors in this. Um so someone listening to this podcast, where can they go to get more information about uh the chart that you showed, um, all this all the educational uh material that Arenda makes?

SPEAKER_02: 20:09

Certainly. Uh you can access all of it through the Arenda app, which is a free app. It's available in the Play Store and uh the Apple Store. Um in the menu you can see articles. You can search C Y A and get to it. That taps into our blog on our website. If you prefer on a desktop, you can just go to our website orendatech.com, O-R-E-N-D-A-T-E-C-H dot com. Tap on the blog, all the information is there. Type C Y A in the search bar, you'll see all the cyaneric acid uh articles about it. We've got 10 or more, I think. We have a lot of videos about CYA as well where we explain this. And again, this data does not come from us, so we hyperlink out to our sources so you can see it for yourself. Um but you know, the opinions that I've shared today have been vetted because I've spoken to the people who wrote the charts, and I've I I understand what they're trying to say in the charts because they can be kind of intimidating to look at without any context. So, what I explained to you today is accurate to the best of our knowledge. And if anything like that changes, I'll be sure to update our blog or anything else. Because you know, sometimes when there's something new like this, you gotta take the time to slow down and make sure that it is accurate and everyone's sure about it. This is pretty settled, it seems like.

SPEAKER_01: 21:32

Yeah, I think so. I I think you know the adjusted aquinity thing has been something. That it's pretty much settled now, too. And that was pretty new for a while there, too. And I think cyaneric acid is something that as an industry, um, it's like a double-edged store. We do need it because we can't be at the account every day to add chlorine to the pool, like back in the 50s or 60s. I don't know when pools were started, run out of time. Um, but you also you have to realize that this is something that will bind up the chlorine in the pool, and there's got to be some kind of effect with that. It's not something that you can ignore as a factor.

SPEAKER_02: 22:09

Yeah, and again, you it's stabilizer itself is not the problem. Over stabilization is the problem. It's too much, right? Because if you have a good ratio, you get sunlight protection, chlorine is still fast enough to do its job, you know, it's all good. And that this is neg this is debatable. You could make an argument either way. Arenda's opinion, the optimal level is around 30 parts per million, 15 to 30. Uh, you can go as high as you want. After 50, it gets really hard. After above 50 parts per million CYA, it's very hard to maintain enough free chlorine to keep that ratio optimal. But I mean, you can you can do it. If you want 100 parts, okay, but you got to maintain 7.5 parts per million free chlorine all week. Good luck. That's that's a really high number of chlorine. But back to what I was saying earlier, to kind of pull this together, there's two sides to water chemistry, right? There's sanitization, keeping it clean, healthy, all that stuff, chlorination, and then there's balance. Cyanuric acid dramatically impacts both. There's only a few other factors that do. For instance, pH impacts both, but with cyanuric acid, pH is kind of almost negligible on the sanitization thing. Uh without CYA, of course, it's very important. But what does calcium hardness have to do with sanitization?

unknown: 23:34

Right?

SPEAKER_02: 23:35

What does TDS have to do with sanitization? Not that much. But cyanuric acid is profound on both sides of water chemistry. So we have to maintain levels that can be managed.

SPEAKER_01: 23:47

Yeah. Well, I appreciate that, Eric. And I thank you for your time to discuss this and to really unpack it in the understandable way for the listeners. And if you're looking for other episodes, of course, you can go to my website, swinging for learning.com, and you're gonna see about eighteen hundred episodes available. Just click on the podcast icon and the banner. And if you're interested in the coaching program that I offer, you can learn more at PoolGuyCoaching.com. Thanks for listening to this podcast. Have your rest of your week and God bless.