Zero Chlorine? Here’s What’s Really Happening

Chlorine keeps vanishing, the water looks fine, and your test kit keeps gaslighting you. We’ve all been there, and the fix isn’t “just add more.” I walk through the real reasons a chlorine pool refuses to hold a residual and the exact order to solve them: restore flow, clear the water, set cyanuric acid in the sweet spot, and only then fine-tune sanitizer. You’ll hear why torn DE grids, clogged cartridges, and starved runtime quietly turn your pool into a chlorine sink — and how a simple filter service can flip the script in 24 hours.

From there, we get into water quality and chlorine demand. Cloudy pools devour sanitizer until you shock, circulate, and clean the filter repeatedly. I unpack the CYA puzzle in plain language: too little and UV burns chlorine off by lunch, too much and chlorine goes sluggish. Using the 7.5 percent rule, we translate cyanuric acid numbers into real free chlorine targets, show when tablets push you past the line, and explain why a partial drain beats chasing a residual that won’t stick. We also spotlight the sneaky players: early-stage algae you can feel but not see, high bather load, phosphates that feed blooms, and nitrates that all but force a refill.

Finally, we cover real-world diagnostics that save time and chemicals. Learn the signs of a leak that keeps diluting your pool, why UV index changes how you plan runtime, and when to test for phosphates or nitrates before you waste another gallon of liquid chlorine. 

• poor filtration and weak circulation causing rapid chlorine loss
• cloudy or green water driving extreme chlorine demand
• shock, continuous run, and filter cleaning restoring clarity
• cyanuric acid sweet spot and 7.5 percent rule
• high UV, low CYA, and fast burnoff
• high CYA making chlorine ineffective and when to dilute
• early-stage algae signs and decisive shocking
• bather load raising daily demand
• phosphates as algae fuel and removal strategy
• nitrates requiring drain and refill
• leaks diluting chemistry and tipping off with shifting levels

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Chapters

• 0:00 Why Chlorine Won’t Hold
• 2:16 Filtration Failures And Quick Fixes
• 4:19 Runtime And Circulation Essentials
• 5:12 Cloudy Water And Chlorine Demand
• 7:52 Shock, Clean, And Restore Clarity
• 8:16 CYA Basics And UV Burnoff
• 9:53 Low vs High CYA And Targets
• 12:28 Hidden Algae And Rapid Drops
• 14:31 Bather Load And Summer Stress
• 16:27 Phosphates, Testing, And Removal
• 17:59 Nitrates And When To Drain
• 19:19 Leaks, Dilution, And Clues
• 20:06 Key Fixes And Resources

Transcript

SPEAKER_00: 1:03

And welcome to the Pool Guy Podcast Show. I recently did a podcast on why a salt water pool would have zero chlorine or is not holding chlorine. So I thought I would cover why a regular chlorine pool, quote unquote, we're using trichlor tablets, liquid chlorine or cal hypo, or dichlor, would not have or would not have the ability to hold chlorine maybe day to day or week to week. 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. Since there are so many factors that will play into this equation of why the pool's not holding chlorine, I'll start with some basic ones that you can eliminate and this may solve the problem. One of the most basic is that you have poor filtration going on in the pool. And this is something that you don't think about too much. But if you're out in the field and you've ever experienced this where you had a D filter that had torn grids and D was leaking leaking back into the pool, you may notice that the water quality starts to deteriorate rapidly. And also the chlorine level may zero out pretty rapidly in a pool. In this case, where the filtration system is not working. Maybe you have a cartridge filter where it's a single cartridge filter and you haven't changed it for two years, or the customer hasn't changed it for two years, there's very little water flow. And you'll notice that the chlorine isn't holding in that pool as well. The moment you fix the D grids, replacing all eight grids in some cases, and recharging them with fresh DE, you'll notice immediately, almost immediately, that the chlorine level holds in that pool tremendously better than when the filter wasn't working. When you replace that single cartridge filter with a brand new cartridge, you're gonna notice immediately that the chlorine level is holding in the pool. So the filtration of the pool is one of the primary factors. If you have poor filtration, you're not going to have the chlorine being distributed or circulated or being effective in the pool, which means other things in the pool are going to, of course, destroy the chlorine level, and that's why the filtration is really important. And then, second to that, in conjunction with the filtration, is the pool's runtime. If you have a brand new filter with brand new DE in the grids and you run the pool one hour a day, I can almost guarantee you that that pool is not going to have a consistent nor significant chlorine level. If you run that pool, of course, eight to ten or twelve hours a day, or the VS pump, if you're running it 18 hours a day, you're going to have a better chance of having a good chlorine level in that pool day in and day out, week in and week out, than if you're running that pool 2 hours a day or three hours a day. So circulation and filtration are sometimes the primary reason that the chlorine level is zeroing out or not holding properly day to day. Another big one is the water quality. If the water quality is impacted in any way, it's going to affect the chlorine level in the pool as well. What I mean by impacted is if you look at the pool and you can't see the third step because the water is so cloudy that you can't see past the third step, and you certainly can't see the pool bottom because the water is cloudy. There's a lot of stuff in that water that's going to destroy the chlorine level in that pool. And so, besides filtration and circulation, the overall initial water quality of the pool is a big factor in the chlorine holding in the pool. I'm sure you've done this before if you've done green pool cleanups, you've had a green pool, let's just say it's a 15,000-gallon green pool, and it's at a level where there's like mosquito larvae and tadpoles forming in the pool, and you shock that pool, you put in let's just say 20 gallons of liquid chlorine and 10 bags of cal hypo, and you bring the chlorine level up to I don't know, 200 parts per million, something like crazy like that. Clean the filter, run the pool for 24 hours, you get back there the next day and check the chlorine level, and you're like at 5 parts per million, or you're at 2 parts per million, or you're at zero, and you put all that chlorine in there. But of course, the pool should look a lot better, it should be like a milky blue, where you can probably see the top step. All the green algae has been killed. There's a lot of dead algae dust on the bottom. All the mosquito larvae and tadpoles have miraculously disappeared, they don't exist anymore, they've been, you know, kind of consumed and evaporated by the chlorine or fall into the bottom of the pool, and you have no chlorine in the pool, and you put a ton of chlorine in there, but there was so much stuff in the water that it quote unquote used up all the chlorine during that process. And so if you haven't experienced that, you probably will when you do green pool cleanups, is that you put in so much chlorine that you think you're gonna go back there the next day and it'll be like at 20 parts per million when it's actually at zero, and that's what I mean by initial water quality. If the water is cloudy, that means that there are things in the water, combined chlorine is one of them that makes the chlorine ineffective, and it's going to cause the chlorine to zero out in a lot of cases. So just be aware that along with filtration, circulation, water quality is also a big factor in how long the chlorine lasts in the pool. The more cloudy the pool is, the more murky it is, the more green it is, or yellow it is with algae, the less likely the chlorine level will be consistent and be able to be sustainable in that pool. So, of course, you can shock the pool to a shock level of 20 to 30 parts a million, clean the filter, run the pool, get it back to blue again, and then the chlorine should hold pretty well in that pool, given the fact that the water quality has improved dramatically with the process of shocking it, filtering it, cleaning all the gunk out of the bottom of the pool, and getting it back to a pristine, clear blue pool. Another factor that kind of goes with the water quality is how much cyaneric acid is in that particular body of water, that pool account, surface account, how much cyaneric acid is in that pool, because it actually can go both directions. Now there's a sweet spot, and my sweet spot for most pools is 50 to 100 parts per million of cyaneric acid. That's kind of this the area where the chlorine is going to be most effective. You know, after 100 parts per million, things start to go a little sideways for you, especially if you're using Bob Lowry's 7.5% formula, which I'll touch on in a second. And you can also have too little cyaneric acid. Anything below 20 parts per million, or even 10 parts per million, or even zero cyaneric acid, will cause problems with the chlorine holding in the pool. The reason why cyaneric acid was invented was that back in the day you would have to actually go to the pool every day to put chlorine in there because there was no way to protect it in the summer from the sun's UV rays burning off the chlorine in the pool. The cyaneric acid is kind of like sunscreen. It will block the it will slow down the UV rays that will destroy the chlorine in the water to a point where the chlorine is going to stay in there that normally would only stay in there for three hours with no cyaneric acid at all, where you can have it actually stay in the pool for 20 hours. And so that's why the cyaneric acid was created, and that's to extend the life of the chlorine in the pool. Before that, again, you would just have to add chlorine every day to the pool, and now you can not need to do that because the cyaneric acid will actually stretch out the chlorine. Now, if you're in an area like Texas where the UV index is at the highest point in the summer, you have to factor that in as well because even with cyanaric acid in the pool water, with the UV index at the maximum, it's going to burn off the chlorine even faster than if it was at a minimum UV with the cyanaric acid level to say 80 parts per million. So the higher the UV level, the more the chlorine will even burn off, even if it's being protected by the cyaneric acid, because it's not a end-all fail-safe, and there's going to be some burnoff depending on how high that UV index is, regardless of the cyaneric acid level in the pool. Of course, if you have a cyanic acid level of 20 parts per million in the pool, it's going to be pretty much useless because it's too low to protect chlorine over a certain part per million. The old school formula is that at 30 parts per million, it'll protect chlorine to three parts per million. At two at 20 parts per million, it'll protect chlorine to two parts a million. And at 50 parts per million, it'll protect chlorine up to five parts per million. I think that's a fair formula, even though it's not totally scientific. Chlorine's going to have a normal, of course, degeneration in the pool water. And Bob Lowry says that normally you'll lose one part per million per day. So if you're at the beginning of the week and you're at eight parts per million, by the time you get to that same pool a week later, you're going to be at one part per million, unless you're adding chlorine to that pool to maintain that level at eight parts per million daily. Now let me address too much cyaneric acid in the pool water and the effect that has on the chlorine. So there's a formula that Bob Larry came up with, and that's your free chlorine should be 7.5% of your cyaneric acid level in the pool. So if your cyaneric acid level is at 100 parts per million, you're going to need to maintain a free chlorine level, a minimum free chlorine level of 7.5 parts per million for that chlorine to be effective in the pool. If your cyaneric acid level is 200 parts per million in that pool, you're going to need to maintain 14 parts per million of free chlorine in that pool for the chlorine to be effective. And that's why in a lot of cases, your pool will zero out in chlorine. You may get to that pool and you may be at 5 parts per million of chlorine, and then you add some chlorine to bring it up a little bit, and you get back to the next week, you're at one part or you're at zero. But that pool has 200 parts per million of cyaneric acid in it. At five parts per million, you're way below the 14 parts per million necessary to maintain that pool and make the chlorine effective. And so you can kind of see how it's a head scratcher sometimes when you're at a pool and the chlorine looks fine, you're at maybe eight parts per million, and you think that you're fine, so you don't add any chlorine to the pool, you get back to the next week and you're at zero because that pool actually has a cyanaric acid level of 200 parts per million, and you need it 14 parts per million for that chlorine to be effective, if that makes sense. Regardless, if you follow Bob Lowry's formula, just know that the higher the cyaneric acid level in the pool, the higher the free chlorine level needs to be in the pool for the chlorine to be effective to prevent algae growing in that pool. And as you know from being in the industry, algae is a huge consumer of the chlorine in the pool. There's actually a potential for algae being in the pool that you can't see immediately. And it's one of those things where you can maybe feel it if you put your hand in there and rub it on the surface, and you can feel a little bit slimy, but you can't actually see the yellow algae forming or green algae forming because it's in the early stages of the developing. But even in the early stages, it's consuming a huge amount of chlorine and it's causing the chlorine level to drop rapidly in the pool. In fact, if you have ever done this, and I've been guilty of this, you get to a pool, there's maybe some yellow algae on the steps and deep end, and you just simply brush it off, and maybe you'll bring the chlorine, you just put one or two tablets in the floater because the chlorine level is already at three parts a million, you're not really thinking any big deal. It's a little bit of algae, you'll just brush it off. But have you ever gotten back to a pool like that the next week? And the entire deep end is covered in algae, all the walls are covered, and the chlorine, of course, is at zero because the algae, that little bit that you saw and brushed off, was not really all the algae that was in the pool. All the algae was actually forming and you couldn't see it. And by you not shocking the pool, caused the algae to bloom to a point where everything was consumed, all the chlorine was consumed, and the allergy just took off. So that does happen, and that is a reality out there. So maintaining the proper cyaneric acid level is key. Too low, you're gonna have the chlorine burning off too fast. Too high, you're gonna have the chlorine being ineffective in the pool. And I know it's kind of one of those things where the cyaneric acid is a great tool and it's a great asset to the industry. But anything use in excess, if you eat cake every day for a year and you have pancakes for breakfast and you have waffles with chicken for lunch, I can almost guarantee that you're gonna have some pretty bad side effects from your eating habits. And everything in moderation is key. How you eat and how much cyaneric acid you utilize in the pool are all factors in the chlorine sustaining itself and being viable and being somewhat sustainable in the pool. So the higher the cyaneric acid level again, the more chlorine you're gonna have to have in the pool for it to be effective. Now I didn't mention high bather load, and that's kind of an obvious use of chlorine. If you do commercial pools, you know that you're fighting a losing battle sometimes trying to maintain that chlorine level in the summertime. Same thing in a backyard pool with heavy use, you may be fighting a losing battle in that case. But just know that in reality, the higher the bather load, the more chlorine is going to be used up in the pool, and that's just something that's a given in the industry. Something that I think I should address here is phosphates in the pool water. This is something that particularly has been a problem, growing problem over the years, because a lot of city municipalities use orthophosphate to keep their pipes from rusting. A lot of household products has phosphates in it. People go swimming and it goes off into the pool. Some of it may even just blow into the pool. So phosphates are a source of food for the algae and something that causes the chlorine to be used up rapidly because the algae is feeding on the phosphates and growing in the pool. And something you can do to, of course, eliminate the phosphates in the pool is to put a phosphate remover in the pool, maybe use a weekly dose of it. You can test for phosphates, and if it's anything over 300 parts per billion, you have to treat it and take care of it before it gets worse and becomes a greater problem in the pool. You can also use a pull-rex in there, and with a pull rex, it doesn't really matter what the phosphate level is because it's a mineral technology. It'll actually prevent the chlorine from having any problems at the phosphate level. But if you really wanted to cure that pool of the phosphates, you definitely would want to use a phosphate remover to remove the phosphates in the water, and thereby removing a food source for the algae and remove one of the reasons why chlorine doesn't hold in the pool, and that's because of the higher phosphate level in the pool itself. And lastly, I'll touch on nitrates. Nitrates are kind of like the kiss of death for the pool water. If you have a pool that has nitrates in it, and again, this can come from sources that blow into the pool. Typically, it happens in more agricultural areas where you have a problem with nitrates. But if you do have a pool where you're putting in a ton of chlorine, and I've had this happen to me, the pool was perfectly fine, never had a problem with it, and then just one week it just started to have problems holding chlorine. And so it's one of those things where I was putting in, you know, four gallons of liquid chlorine, and I was putting in extra tablets to kind of hold the chlorine. It wasn't helping because the cyanaric acid level was going up at that point, and I was just putting a ton of chlorine and I couldn't figure out why the pool kept zeroing out. Filter was clean, everything was fine, and then I did a nitrate test, and I of course it came back that it had nitrates in it, and basically the nitrates were destroying the chlorine level in the pool, and there's really no real cure for it except draining the pool and refilling it to get rid of the nitrates in the water. Not super common, but if you have a pool where you're trying everything you can to keep it and it's still dropping, nitrates may be the cause of that. So get some nitrate test strips and test your pool for that, if that is what you're facing. And one thing that probably could be obvious, and it could not be obvious, is if the pool has a water leak, that would cause the chlorine, of course, to be diluted with the leak. And typically, if a pool is leaking water, you're going to see the calcium hardness levels as well as the cyaneric acid levels. If it's a salt pool, you're going to see the salinity level dropping in the pool. You're going to see the pH rising rapidly in that pool as fresh water is being added all the time, especially if it has an autofill. So a leak is going to have some definite signs besides not holding chlorine. You're going to notice that the other numbers are also changing, as well as the chlorine being zeroed out every week with the water loss in the pool. So be aware of the water loss, it's a big factor as well. But I think I covered most of the factors here of why when you test the pool water, you're going to have zero chlorine. And there are a few more that I didn't cover here, but most of these are going to be the cases you're going to run into. And once you solve these problems, their chlorine level should hold pretty well in the pools that you're servicing. If you're looking for other podcasts, you can find those on my website, singlepoollearning.com. On the banner, click on the podcast icon. There'll be a drop-down menu of over 1800 podcasts if you're there. 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 the rest of your week and God bless.