How To Break Chlorine Lock

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Chlorine lock isn't the end of the world but it is a common problem for swimming pool owners. Chlorine lock means that the chlorine in the pool is useless which means the water isn't being sanitized. It can also indicate the presence of chloramines which give off a chlorine smell. Below are a few ways to deal with this issue.

Step by Step

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Step 1

How does chlorine lock occur? Chlorine lock can occur when there is too much cyanuric acid (also referred to as conditioner or stabilizer) in the water. This occurs when too much stabilizer is added to the water or when the swimming pool isn't being partially drained and refilled periodically. Chlorine lock can also occur if the pH is unbalanced. The quickest way to determine if a chlorine lock is present is to perform a test for total chlorine and free chlorine. Total chlorine is a measure of all the chlorine in the water and free chlorine is the chlorine that actually sanitizes the water. The two results should equal each other under normal conditions. if they don't you likely have chlorine lock. For example a free chlorine reading of 3 and a total chlorine reading of 3 is fine. If you have a free chlorine reading of 3 and a total chlorine reading of 7, there is a problem.

Step 2

There have been many discussions questioning the existence of chlorine lock and how to resolve it. While there are many ways to resolve a chlorine lock, I'll only be covering a few. The first method is to partially drain and refill the pool. Using this method will take some time depending on how badly the chlorine is locked. There is no math equation for this method. You simply need to partially drain, refill, retest and repeat (if necessary).


Step 3

Another way to break chlorine lock is to shock the pool with a non-chlorine shock. Non-chlorine shock will oxidize the chlorine in the swimming pool. To determine the amount of non-chlorine shock you will need, you will need to do some math. You will need to subtract the free chlorine from the total chlorine and multiply that by the number of gallons in the pool divided by 10,000 then multiply by 2. To make it easy it looks like this: (TC-FC) x (#of gallons/10,000) x 2 = amount of non-chlorine shock. If we use a total chlorine reading of 5 ppm and a free chlorine of 3 ppm on a 15,000 gallon pool, the equation would look like this: (5-3) x (15,000/10,000) x 2 = x. Therefore: 2 x 1.5 x 2 = 6. I would need to add 6 lbs. of a non-chlorine shock to the pool.

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Step 4

Yet another way to resolve a chlorine lock is to reach what is called breakpoint chlorination. Breakpoint chlorination is the point where the chemical bond that ties nitrogen, chlorine and ammonia together by using large amounts of chlorine. This will cause the chlorine residual to drop rapidly. Reaching a true chlorine breakpoint is vital to breaking a chlorine lock. If breakpoint is not reached using this method, the problem will only get worse. To accomplish a breakpoint, 7.6 free chlorine molecules are used to break apart an individual combined chlorine molecule. The amount of product you will need will vary depending on the type of shock you are using. Aside from knowing your total chlorine, free chlorine, number of gallons in the pool, and the weight of a gallon of water (8.34 lbs.), you will also need to know the type of pool shock you are using. Before attempting this method, the pH needs to be between 7.2-7.4. This equation will be a two part process. The first step will be determining the ppm of chlorine needed to reach breakpoint: (TC-FC) x 10. Using the same amount above, the equation would look like this: (5-3) x 10 = x ppm or 2 x 10 = 20 ppm. I will need 20 ppm of chlorine to reach breakpoint in this 15,000 gallon pool. The next step is to determine how much product I will need. I'm using calcium hypochlorite in this example and need 2 oz. per 10,000 for a 1 ppm increase. I'll need to multiply 2 oz. x (gallons in pool/10,000) x (ppm needed/ppm dosage) which simply put is: 2 oz. x (15,000/10,000) x (20 ppm/1 ppm) or 2 oz x 1.5 x 20= 60 oz. To convert ounces to pounds, I divide by 16: 60 oz./16= 3.75 lbs. So to reach breakpoint in my 15,000 gallon pool I will need 3.75 lbs. of calcium hypochlorite. 

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(1 to 40 of 102)

 Posted: 8/1/2021  Latest

FC = 12 ppm (too high) CC = 1.9 ppm TC = 13.9 ppm pH = 7.7 Cyanuric Acid = 68 ppm I need to bring free chlorine to 2-4ppm range, turned my salt cell all the way down, but free chlorine staying stubbornly high
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InyoPools Product Specialist  Posted: 8/3/2021  Latest

The two routes I would use are draining a portion of your pool, refilling with fresh water, or using a chlorine neutralizer to wrangle in the chlorine level.
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 Posted: 8/3/2021  Latest

Thank you
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InyoPools Product Specialist  Posted: 8/4/2021  Latest

You're welcome! Glad to help.
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 Posted: 7/25/2021 

Cloudy water and can't raise free chlorine level. Have followed advice of local pool store but problem persists. Current test strip measurements are Total Hardness 250-300, Total Chlorine 3-5, Free Chlorine 0.5, pH 7.0, Total Alkalinity 100, Cyanuric Acid 0. What would you do if this were your pool? Thanks for your suggestions.
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InyoPools Product Specialist  Posted: 7/27/2021 

No wonder there isn't any chlorine in your pool. You have cloudy water, low pH, and no stabilizer. You HAVE to balance those other categories before chlorine can efficiently clean your water. How to Balance Your Pool WaterYour local pool company should have told you that throwing chlorine into a pool with those numbers is useless.
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 Posted: 7/17/2021 

just triple shocked our pool (cal hypo) last night hoping to raise free. total is 7 (from 3) free is 2 (from 1). so looks like chlorine lock. water is crystal clear, perfect. no signs of bacteria etc. run filter 12 hours a day. shocked it at dusk. est method to get thigs ack to normal? should i consider having water tested at local pool place? using the Remington copper solar shock device, would that e playing any role? pool near alot of trees\pants\dirt yet root cleaned 1-3 times a day, o visale dirt etc. thank you for your time.
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InyoPools Product Specialist  Posted: 8/4/2021  Latest

What are the results of your latest water chemistry tests? In order to know what to fix, we have to know what is broken.
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 Posted: 7/8/2021 

My pool looks like milky water. Have had water tested several times and adjusted chemicals accordingly. Could it be chlorine lock? Last pool test showed normal everything. Most frustrating pool season to date!!!
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InyoPools Product Specialist  Posted: 7/14/2021 

If you show normal free chlorine levels then you don't have chlorine lock. You may want to check you aren't mixing two or more chemicals that are causing the cloudiness. See this guide - Bad Pool Chemical Combinations
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 Posted: 7/2/2021 

Our pool is crystal clear but we can't get the chlorine levels up after shocking or running the chlorinator in the max setting. I do see small spots of green algae but not significant. We have a DE filter and the pool is 22 year old, 30,000 gallon, gunnite pool. I've just cleaned the filter and re-shocked the pool with 4 bags of super-shock (chlorine) and still no luck. Our water test results are: Free Chlorine=.84 ppm, Total Chlorine=.84 ppm, pH=7.3, Hardness=306ppm, Alkalinity=80 ppm, Cyanuric Acid=92 ppm, Phosphate=1127 ppb. We are treating for Phosphates, which were at 1385 ppb.
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InyoPools Product Specialist  Posted: 8/3/2021  Latest

Your cyanuric acid is way too high. If you have a traditional chlorine pool, it should be 30-60 ppm. CYA can hinder chlorine if it is out of balance. You need to drain a portion of your water to get it in to check. Shocking the pool will only go so far if the other water chemistry is out of whack.
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 Posted: 6/16/2021 

Pool was cloudy had water rested ,chlorine was high so I haven't put any chemicals in for a week let filter running steady and chlorine is still high but water is crystal clear
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InyoPools Product Specialist  Posted: 6/17/2021 

If the water is clear and the chlorine is high, I would continue to wait for it to drop. Chlorine gets used up, usually rather quickly this time of year. What are your chlorine levels (free and combined chlorine)?
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 Posted: 5/6/2021 

Hello, I think I have chlorine lock as well. My total chlorine is 3 ppm and my free chlorine is reading 0 ppm. The CYA is 100ppm. The pool was in the exact same condition last fall when we closed it. We drained half of the water and the pool was full when we opened it this spring. We back washed and drained a third of the water. We then refilled it. Shocked it. We find ourselves in the exact same position. The water is cloudy, But it has only been a few days since we uncovered it. Any advice? It is a small fiberglass pool holding 8000 gallons.
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InyoPools Product Specialist  Posted: 5/13/2021 

I would shock it a time or two more and also use a heavy-duty flocculant to clear up that cloudiness. You may also want to try a fun little product like Pool First Aid which is a good catch-all product for these situations.
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Anonymous  Posted: 8/25/2020 

I have a 4,000 gal vinyl pool. Something weird is going on. Cya numbers dropped rapidly over the past two week. Had been up around 100 and they dropped to around 60. Brought the stabilizer back to around eighty and all the other numbers are fine but the pool week not seen to hold chlorine. I literally put in over a gallon of chlorine in to this little pool yesterday and I'm still getting a reading of zero or 0.5 on chlorine. Checked the numbers at the store too in case my test strips were bad, but they're not. I'm stumped. Any ideas? I'm wondering if chlorine lock would tend to give a reading of no chlorine or if you know of something that would drop cya numbers rapidly. Thanks!
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InyoPools Product Specialist  Posted: 8/27/2020 

What are your Free Chlorine and Combined Chlorine numbers? Is your water clean, hazy, or milky looking?
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 Posted: 8/12/2020 

I recently had an issue with green water, so I decided to shock the pool with liquid chlorine as per instructions for 19k gallon pool. The free & total chlorine came up to 8 over night & within 24 hours the pool was crystal clear. 3 weeks later the pool is still clear, but the free chlorine is 3 & total still 8 & I have added no additional chlorine since the shock. I live in Bali, Indonesia, sun shines every day & the pool gets the sun for at least 8 hours a day. What is my problem
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InyoPools Product Specialist  Posted: 9/1/2020 

I suggest using some non-chlor shock to bring down your chloramines. Another option is a liquid shock. But your combined chlorine is so high because all that chlorine you added is getting used up.
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 Posted: 8/6/2020 

Free Chlorine, 0.07, Total Chlorine, 0.46, ph 7.1, Cyanuric Acid 141, Phosphates 3152 The local pool place recommended draining about two feet of water out the pool (one foot at a time, refill and recheck). I have yellow algae due to low chlorine but I'm in chlorine lock looks like.
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InyoPools Product Specialist  Posted: 9/3/2020 

Your CYA levels are way too high. The CYA level should be 30-60 for a non-salt pool, and 60-80 for salt pools. Levels that high inhibit chlorine from sanitizing your water. The only way to get that down is to drain a portion of your water and refill with fresh.First, get the CYA down, then add shock, and maintain a healthy chlorine level.
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 Posted: 8/6/2020 

we are struggling with CYA levels and chlorine lock. we have a 15 x 48 intex pool about 4000 gallons. Water was clear up until a week ago after some heavy rains. We use strips regularly to test levels and its been fine up until now. Pool store test said CYA was high. to partially drain, refill, partially drain, refill and add some product they sold us to help. Have done the partial drain and refill a few times now. We were told to use stabilizer when we set the pool up so my husband was adding it weekly, not realizing that the chlorine tablets already had stabilizer in it. So we probably built up a high level. We've had heavy rain the past day with the latest tropical storm, and will have more rain today and tomorrow. Test strip shows free chlorine is almost none. Do we now add stabilizer free chlorine? More shock?
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InyoPools Product Specialist  Posted: 9/3/2020 

To be able to tell you the course of action, I would need to know the results of the water tests. When it comes to water chemistry, the test results do most of the work for you. Once you know what is low and high, and by how much then you can begin the fun stuff.
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 Posted: 8/3/2020 

I have an algae (think green) issue i can't get under control. Last test results were 5.97 Free Chlorine and 5.97 total chlorine (just shocked with cal-hypo), 7.7 PH, CYA 97, calcium hardness 165. I quit using 3" chlorine tabs a few weeks to try and get CYA to start coming down thinking that was issue. But after each shock the algae comes back, even next day sometimes. Feel like I have chlorine lock. Any advice?
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 Posted: 2/17/2021 

First importantly? Make sure you use a stiff brush and completely brush the sides and bottom of the pool thoroughly to actually kill the algae. It's a common problem people have with Alge issues. The number one cause is not brushing the pool algae off and then super chlorinate or add algeacide chemicals. It comes back. Also test for phosphates. Phosphates are the food for Alge. It's a bit costly and runs about 70 bucks us dollars for a gallon of phosphate remover. Keeping phosphates down and the cya levels in check will stop the lock and alge growth.
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InyoPools Product Specialist  Posted: 9/3/2020 

What was the chlorine reading before you shocked the pool? Taking a chlorine reading right after adding shock can eschew numbers to misleading conclusions. Also, that CYA level is still too high.
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 Posted: 8/1/2020 

free chlorine .78 total chlorine 1.18 combined 0.4 PH 7.4 phophates 693 can't get rid of the algae? Any advise?
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InyoPools Product Specialist  Posted: 8/21/2020 

Your pool's cyanuric acid (CYA) levels are not listed. Proper CYA levels are essential to the sanitizing efficiency of the chlorine in your pool.What are your CYA levels?
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 Posted: 7/29/2020 

I have been fighting a chlorine block for 2 weeks this has cost me to have cloudy water. I've been to the pool store twice. First time told me to add 16oz of algaecide then one gallon chlorine weight 12 hours add another gallon. The water is still cloudy. Went back the second time told me to add pound of pH up and the water is still cloudy. how do I get my water back to crystal clear so I can see the bottom of my pool?
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InyoPools Product Specialist  Posted: 9/9/2020 

What are the results of your latest water chemistry test? That info is what we need to determine what steps are needed to correct your issue.
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 Posted: 7/20/2020 

i have a 3000 gallon above ground pool with 14 on chlorine and i do not know what to do....to keep chlorine low
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InyoPools Product Specialist  Posted: 9/9/2020 

When you say the chlorine is 14, is that the Free Chlorine, Combined Chlorine, or the Total Chlorine?First, let’s make sure you understand the differences between free chlorine, combined chlorine, and total chlorine and how you measure each.Free chlorine (FC) shows the level of disinfecting chlorine available to sanitize your pool. FC is the part of the chlorinated water that hasn’t interacted with contaminants, yet.Once chlorine interacts with compounds such as urine, sweat, bacteria, etc they form chloramines. Chloramines are what we refer to as combined chlorine (CC). The CC levels are chlorine that no longer has disinfecting power. Essentially, it is useless.Total chlorine (TC) is simply your FC plus your CC. Your TC should not be less than the FC level.
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 Posted: 5/30/2020 

I would like to use the chlorine break point method, but I have no free chlorine or available chlorine. If I test it right after I add chlorine, there is a little, but if I test 30 minutes later, there is none at all.
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InyoPools Product Specialist  Posted: 10/23/2020 

What are the results of your most recent water chemistry test?
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 Posted: 4/7/2020 

CYA is at 187 with free chlorine at 0.69 and total chlorine at 1. PH at 7, calcium hardness at 469 and phosphates at 343. Told to drain and refill to bring CYA down but I am also seeing yellow stains on walls and black spotting on floors. Will draining help? Or should I be doing smth else? Thanks
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InyoPools Product Specialist  Posted: 4/8/2020 

The staining is likely black and mustard algae due to no chlorine in the water. You know the CYA is too high and the guide covers why that is bad. Lower the CYA to a reasonable level, then add the chlorine so you can begin the cleaning process. Unfortunately, there really isn't a workaround to this issue; balance your water chemistry, add chlorine, and brush your walls to get rid of the stain and algae.
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 Posted: 3/25/2020 

28K GAL POOL. Free Chlorine .43 PPM, Total Chlorine 1.33 PPM, Combined Chlorine .9 PPM. PH, Hardness, Alkalinity, Cyanuric Acid, Cooper Iron, PHosphate are all good. My pool stays cloudy. How to fix?
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InyoPools Product Specialist  Posted: 3/31/2020 

First, it doesn't seem like you have very much chlorine in the pool. Have you been treating your pool for an algae bloom? If so, have you added algaecide that contains copper? If so, that could be a cause of the milky water.Second, are you just now opening the pool and have to work to get it clear, or has the pool been blue and only recently turned cloudy? Have you added any clarifiers, flocculants enzyme cleaners to work on clearing the cloudiness?
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 Posted: 3/20/2020 

So, my free chlorine is 0.83 and my total chlorine is 6.35. Stabilizer is at 36 so it’s not high. How do I fix this?
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