Bob Lowry on Dialing in Free Chlorine the Right Way

Your pool can look fine on paper and still be losing the chlorine battle. We dig into the frustrating “problem pool” scenario where you’re pouring in gallons of chlorine and the free chlorine (FC) reading barely budges, then break down what’s really happening when chlorine gets consumed faster than you can add it. With Bob Larry’s help, we walk through how to think like a detective: look at the environment, the deck, landscaping, runoff, bather load, and even the odd stuff people don’t mention like rinsing tools or chemicals in the pool. 

From there, we get practical about chlorine demand, including the idea of negative chlorine demand where your first big doses are basically paying off a hidden deficit. We talk through a proven cleanup strategy: raise FC to a strong level and keep it there for 24 to 48 hours so the pool actually finishes oxidizing contaminants and knocking out algae, instead of slipping back into the same cycle. We also touch on an underrated cost saver for commercial pools and busy backyards: getting swimmers to rinse off first can dramatically reduce what chlorine has to burn through. 

Finally, we tackle the classic complaint: “My chlorine is 3 to 5 ppm but algae keeps coming back.” The missing link is cyanuric acid (CYA) and how it changes chlorine effectiveness. We explain the 7.5% of CYA rule for setting a reliable free chlorine target that prevents algae, plus why consistency between service visits matters so much in warm, sunny conditions. If this helped, subscribe, share the episode with a pool owner or tech, and leave a review so more people can stop wasting chlorine and start keeping water clear.

We troubleshoot the pool that burns through chlorine and still reads near zero, using a detective mindset to track down hidden sources of chlorine demand. We also explain why “normal” free chlorine targets can still allow algae, then land on a practical CYA-based chlorine rule that keeps pools clear.  
• identifying real-world chlorine killers like yard debris, runoff, deck wash water, and heavy bather load  
• spotting sneaky contamination from cleaners, solvents, and tool rinsing  
• understanding early algae growth, biofilm, and why algae is mature when you finally see it  
• recognizing negative chlorine demand when big doses barely move the test result  
• holding a high chlorine level for 24 to 48 hours to fully oxidize organics and kill algae  
• reducing chlorine usage by having swimmers rinse off before getting in  
• using the 7.5% of CYA free chlorine target to prevent algae  
• keeping free chlorine consistent between service days to avoid fast blooms  
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Transcript

SPEAKER_00 0:14

Hey, welcome to the Best of the Pool Guy Podcast Show. In this episode, Bob Larry is gonna go over some mistakes you're gonna make with your free chlorine level. He'll talk about the 7.5% ratio cyaneric acid to free chlorine. 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. Let's let's talk about some problem pools here. You know, I get this all the time from I don't know the the I don't see the pools myself, but people will email me or or text and say they have this pool, they've they're just putting in tons

Welcome And Chlorine Mistakes

SPEAKER_00 1:03

of chlorine, you know, two gallons a day, three gallons a day. Maybe it's not that large of a pool, 10,000 gallons or 8,000 gallons, and it's just not holding chlorine. They've checked, you know, the nitrates. There's no nitrates reading on the test strip, which is probably the only way to test in most cases, right? And so what would you say? I mean, they say the filters have been cleaned, but the pool is just not turning around and it's not holding chlorine, and it's just becoming a problem pool. What would be the main issue they should look at when they get a pool like that?

SPEAKER_01 1:36

Well, you know, I

Problem Pools That Will Not Hold FC

SPEAKER_01 1:37

think a pool that's using a lot of chlorine, you you need to to be a bit of a detective, if you will. A lot of times it can be the homeowner's fault. And it's something that either the homeowner's doing or one of the workmen around the house is doing, that they are cleaning something in the pool, they're blowing when they mow the grass, the grass clippings are getting into the pool, when the guy next door is fertilizing, you know, something is dropping into the pool. They can also have a pool that's surrounded by vegetation or landscaping, and that stuff is dropping into the pool, and everything that comes into the pool is organic and it needs to be, it needs to

Environmental Sources That Burn Chlorine

SPEAKER_01 2:24

be oxidized. Pools that they look great, you know, people want a pool to look sometimes like a jungle and stuff, you know, and and while that looks cool, everything that's on that plant, as soon as it rains or gets some water on it, it drips into the pool. You know, and the wind blows and blows all the spores and everything else off of that, and it gets in the pool. You need to look at the environment and perhaps even what's going on. You know, we had a guy at a commercial pool, we found out that the guy that was washing the deck at night turned around, just took the hose and washed the deck, but he hosed it off so the the rinse water from the deck ran into the pool. You know, so everything that was on the deck hit the pool and the and it would use up all of the chlorine. You know, they'd start out in the morning with zero chlorine. I think, first of all, you need to be a detective. And and even bather loads, you know, you may find out that that all of a sudden, you know, this pool has become the neighborhood spot for kids, you know, and on Saturday afternoon or five o'clock, you know, two or three days a week, they get 10 or 20 kids in the pool. You know, so I think you need to be aware of all of those kinds of uses first. And you can start to then look at what's using up the chlorine, but and it could be just something that somebody either put in the pool or or thought they were doing good or something, and dump something into the pool. You know, we had a guy that was that was using the pool to clean his tools, and he would clean his tools with with some kind of a cleaner and then go dip them in the pool to rinse them off, you know, and you get whatever solvent he was using into the pool. And solvent uses up chlorine like crazy, you know, and it doesn't go very far away. So there's things like that. There's also the fact that you could have a biofilm that's growing someplace, you could have an algae bloom. You know, by the time you see algae, it's really colonized and already mature. Because many times algae gets started, you can't even see it. You know, it only gets you only get to see it when it builds on top of itself and gets dense enough that you know it's no longer transparent. It's pretty mature by the time you see it. So you could have algae in the pool, you could have a high cyanuric acid level, and in which case you've got so much cyanuric acid in the pool the chlorine's not able to do everything. So there is this thing they call chlorine lock,

Hidden Growth And High CYA Clues

SPEAKER_01 5:16

which chlorine lock kind of means that it locks up the chlorine so it doesn't work, but what I'm saying is it locks up a lot of the chlorine, so not very much of it is working. So there is that. You can have a high cyanic acid level. And also, once something starts to grow, it does have then a chlorine demand. And so if you have an algae, if you have algae in the pool, you not only need algae to get chlorine to get rid of the algae, but you do need chlorine to get back to a good chlorine reading. It is possible, as you probably have noticed if you've been servicing pools, you probably noticed that sometimes you add a lot of chlorine and only get a little reading. And the reason is that you can actually have a negative chlorine demand. What that means is you could add an amount of chlorine just to get back to zero. And that happens a lot, you know, where you've got something growing in the pool that used up all the chlorine, and then there's a negative demand, and you may need 10 parts per million of chlorine just to get back to zero. You're gonna need 10 to get it, to get it, you know, to get another reading. So, you know, it could take a lot of chlorine to do that.

SPEAKER_00 6:38

Yeah, it makes a lot of sense because right now I'm doing some painting around the house, and I noticed that when I'm painting over a darker color, you can still kind of see it. So you're gonna need more than one coat to cover it, and probably the same concept with the pool that has a negative chlorine demand, you can need more of it to cover it, and then finally you'll get it to where all the dark color of the paint is gone. And that's something that you don't think about often, but I have seen pools where you pour in a ton of chlorine and then you read do the reading and you're only getting like one or two parts per million, when it should be 20. Um, so that does happen.

SPEAKER_01 7:09

If if they suspect that, one of the one of the remedies is to get a very high chlorine level, but keep it that way for 24 to 48 hours, and then let the chlorine level go back down. So you run the chlorine level up to maybe 25 parts per million or 40% of CYA, whichever is a higher number, and put that in the pool. But after you put in the 25 ppm, two hours later, check it and make sure you still have 25 ppm. And if you don't, add more chlorine to get back up to 25 and keep doing that until you get a chlorine level that doesn't drop. And that usually takes 24 to 48 hours, and when you do that, then you've

Holding Shock Level For 48 Hours

SPEAKER_01 7:58

killed everything in the pool or oxidized it. So that is something that you may want to consider. Also, one of the really big things that they did a study on in England was that if they got bathers to just rinse off, not take a shower with soap and water, but just rinse off before they get in the pool, they actually saved almost 60% of the chlorine bill by getting the bather just to rinse off. Because you've got all that sunblock and sweat and all that stuff on your body, and if you rinse that off before you get in, then chlorine doesn't have to work on it.

SPEAKER_00 8:40

That'd be a good invention for commercial pools, having like a shower right by the step area that everyone's gonna be.

SPEAKER_01 8:47

You could even have what they call a sun shower there, where you could have perhaps a a reservoir or a bag of water that the the sun warmed up so that you're not taking a cold shower, you're kind of getting a maybe a lukewarm shower before you get in.

SPEAKER_00 9:09

Another question I get often is I've got a pool, I test the chlorine, it's showing three, four, five parts a million, but I'm still getting algae on the steps, the deep end. I can't get rid of the algae in the pool. What can I do to get rid of that? It's just it seems like a permanent fixture in the pool.

SPEAKER_01 9:25

Well, and this is true. What happens is we get so much cyanuric acid in the pool that that we don't have enough chlorine to prevent algae. Well, I actually have formulas and stuff to show how much chlorine you need in your pool. But and the math is a little hard to do without without seeing it, but you know that that 3% of the chlorine that's in the pool is the part that's free to kill anything.

Algae Despite Good Test Results

SPEAKER_01 9:57

And you might remember that at a pH of 7.5 that only 50% of your chlorine was in the killing form of chlorine called HOCL, right? HOCL, hypochlorous acid. And so that means that only 1.5% of your chlorine is in the killing form. And that means that if you had two parts per million of chlorine in the pool times 1.5% is only 0.03 parts per million of HOCL. And what we found, and I've researched this till I'm tired of researching it, is you need about 0.05 parts per million of HOCL to kill algae. If you're only keeping two parts per million of chlorine in your pool, you haven't got enough chlorine to kill the algae. The industry standard, and Leslie's and everybody else, says that you need two to four parts per million of chlorine in your pool. And what I say is that doesn't always work. And the reason it doesn't always work is because of that math I just gave you. And so one of the things that I tell people is if you want to know how much chlorine you need in your pool, the answer is you need 7.5% of your cyanuric acid level. And so whatever your cyanuric acid level is, you need 7.5% of it to prevent algae. If you've got 50 parts per million of cyanuric acid, you're gonna need almost four parts per million of chlorine in your pool, not two. So two won't do it, but four will. So you need to be aware of that. But once you get algae, you can use an algae and brush and you can buy all these kinds of algae side and stuff like that. But a minute ago

The 7.5 Percent CYA Rule

SPEAKER_01 12:06

I mentioned to you about running the chlorine level up to either 25 parts per million or 40% of cyanuric acid. And what I'm saying is that works great for killing algae. And so if you get the level up, you have to maintain that level. So you can't just run it up and then a day later, you know, go back to using the pool. You have to check it and make sure that whenever the chlorine level drops down by more than one or two ppm, that you bring it back up so that you maintain 25 parts per million of chlorine for at least 24 or maybe 48 hours if the chlorine if the algae had been there for a long time. But once you kill the algae and it's gone, now you've got a pool that's completely disinfected with no algae in it. And if you keep 7.5% of CYA as the free chlorine level in your pool, you'll never have algae growing in it again.

SPEAKER_00 13:08

And I think one of the points of that, and I yeah, I think we've talked about that many a times, and I I would like to nail that into people's heads that this is really crucial in pool maintenance. If you do pool service, and let's say you're there on a Wednesday, and someone has a party on a Saturday or Sunday, and then the pool zeros out, or maybe it's at one point per million on Monday and Tuesday, those two days are enough with the coin level below the level necessary for algae to grow in there for the water to turn pretty quickly. And so it's a consistent level every day, every time you're testing the pool, right?

SPEAKER_01 13:43

Yes, that's right. It especially in the summer, if you get a little wind blowing and stuff like that, you're gonna get algae spores in the pool. And they pop open and start growing because there's plenty of sunlight, there's

Keeping Chlorine Consistent And Closing

SPEAKER_01 13:57

plenty of food in the pool for them, and there's there's no chlorine. So you can get chlorine, you can get algae growing in a day or two. And that's even one of the reasons with a startup, you know, on day three, we want to start getting some chlorine in there.

SPEAKER_00 14:13

If you're looking for other podcasts, just go to my website, swimming for learning.com. On the banner's a podcast icon. There'll be a drop down menu with over 1900 podcasts for you there to listen to at your leisure. If you're interested in the coaching program, you can learn more at pullguycoaching.com. Thanks for listening to this podcast. Have a rest of your week. God bless.