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


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.

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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Inyopools  Posted: 5/17/2018 

Hello Blinded - The method of adding more chlorine to break chlorine lock sounds weird but it does work. There is no easy method but draining and refilling is the route that I usually recommend.

Blinded  Posted: 5/15/2018 

I am just jammed up on this Chlorine issue. I have a 62,500 gallon pool. ph is 7.5 free Chlorine is .9 and total is 9.8 Hardness is 342 Alk is 241 They told me to put 68 pounds of Calcium Hydrochlorite in. I am just baffled. They keep saying the pool will break. So why put more chlorine in. I double shocked it. Going to hope something changes. I need devine help. Blinded

InyoPools Product Specialist Dennis R.  Posted: 8/29/2017 

Jimmymag - It sounds like you are at the point where you have to replace 18" to 24" of your water. If your nitrates are high that could be causing what appears to be chlorine lock. See "Anonymous" posted on 5/31 for a similar problem and solution.

Jimmymag  Posted: 8/27/2017 

I have a problem similar to the one that Ray posted on 8/23/17. I have 26,500 gal. plaster lined pool. I have been using liquid sodium hypochlorite (10%) which I had to add a gallon every couple of days to keep the level above 2ppm. The ph gradually increased to 7.8. I added some muriatic acid to bring the ph down a little. After doing this, a problem developed with the chlorine. The FC level dropped to zero. To shock, I added 2 gallons and checked the FC after a couple of hours and it was about a three so, I added 2 more gallons late in the day Over night the FC reading was zero. I went to pool supply to have water tested. They said the the cyuratic acid level had dropped to zero. They recommend using 5 lb. of di-chlor to superchlorinate. I did this and the previously clear water turned cloudy like Ray’s did. I ran the pump for about 16 hours but the water was the same. I added some old clarifier that I had and that cleared the water. I superchlorinated with the liquid and again the FC dropped to zero in 12 hours (overnight) . Pool supply recommend something they call “Purge” . Added that and ran pump for 24 hrs and thoroughly cleaned filter. I had water tested and ph still 7.4 and cy acid and all other tested good. I superchlorinated with liquid and again overnight the FC was zero and the water is sparkling clear with no signs of algae. Pool supply retested and all readings were good except that phosphates were 300. They had no answer to the chlorine problem. I shocked again with the same result. Back to pool supply and they ran an additional test for nitrates. Nitrates were high and they said there is no chemical treatment for nitrates and that replacing some of the water was the only way to lower nitrates. There is no chlorine odor My question for you is… could the nitrates be causing the chlorine problem? Thanks, Jim

InyoPools Product Specialist Dennis R.  Posted: 8/25/2017 

Ray - What is your stabilizer level. If it's 0-20, you will have to add more to protect the chlorine from being burned away by the sun. If it's at the 100 level, you may have too much stabilizer in the pool which locks up the chlorine and gives you a 0 reading. The only way to reduce stabilizer is by draining a foot or so of the pool's water.

Ray  Posted: 8/23/2017 

I have a 33,000 gallon in ground plaster pool. I actually have a 2 part question. My first question is regarding chlorine. For some reason my pool zeros out on chlorine weekly. I am not sure why. All my test readings always come back fine except the chlorine. Any knowledge on why that is? My 2nd question is regarding cloudy water. My pool was fine a week ago and then out of nowhere my pool looks like a milky white. I can see the bottom in the shallow end of the pool, but cannot see the bottom in the deep end. I tried non-chlorine shock, water clarifier and ran the filter for 24 hours during this process. Nothing changed. Any other ideas would be great. Thank you very much.

InyoPools Product Specialist Dennis R.  Posted: 8/15/2017 

jjcresci - I'm not sure I have enough information to know what your problem is. If you have drained a significant amount of your pool water, like 1/3, you shouldn't have chlorine lock - if you even did. Your chemical numbers like ok except for your FC. How long have you had no chlorine in your pool? If it's more than a couple of days, your pool should have become cloudy by now. Did your chemical readings come from a local pool store? If not, I would have them check your FC. The function of non-chlorine shock is to oxidize chlorine. If you have no FC, I'd question its use. If your pool is starting to cloud over, I'd add a couple of bags of regular shock to see if that brings your FC up and clears your pool.

jjcresci  Posted: 8/13/2017 

my pool was fine until my timer went.had to be off 4-5 days .I replaced timer. My pool was cloudy. So i got it running within 48 hrs it was clear. now pool has 0 fc.
about 10,000 gallons
ph 7.6
Alk 130
stabilizer 80 always been 80
calcium 200
this is a vinyl liner pool that is crystal clear ,but 0 fc will not hold FC and no alge at all
any ideas i did drain water and i'm thinking of useing non chlorine shock.

InyoPools Product Specialist Dennis R.  Posted: 7/25/2017 

Jennhurley59 - Not sure what is going on here. "Chlorine lock" means that the chlorine in your pool is ineffective - cannot eliminate algae. Even if you were able to break the chlorine lock without adding chlorine, I don't know how you avoided growing algae without adding chlorine for two months. If you are now seeing algae growth, I would shock the pool and begin adding regular levels of chlorine plus add whatever chemicals you need to balance the pool.

Jennhurley59  Posted: 7/24/2017 

My pool had "chlorine lock", cya levels through the roof we drained and refilled for weeks. Along with adding no chemicals, no algae grew for over 2 months. Now pool is finally turning green, is this a good thing?

InyoPools Product Specialist Dennis R.  Posted: 7/13/2017 

Lifetimelinda - I'd check your CYA reading again. Adding more CYA to your pool if it has a 100+ reading is going the wrong way. Too much CYA can lead to chemical lock. I'd lean more towards draining 20% or your pool (generally 12' to 18") then adding non-chlorine shock (step 2). See "Anonymous" comment 4 comments down.

Lifetimelinda  Posted: 7/11/2017 

The pool store is saying I have a chemical lock. My readings were chlorine 3, ph below 7, phosphate 1000 and cynaric acid 100++. They suggested I drain 3/4 of my pool and start over. Went for second opinion and they said I had no stabilizer,start with getting it up, then work on alkalinity and then ph. We are pool newbies. So confused so starting with the stabilizer. We have small pool, just 12000 gal. Any thoughts.

InyoPools Product Specialist Dennis R.  Posted: 6/13/2017 

hamnut - It sounds like you have dumped a lot of chlorine in the pool already without much success. I would try draining a foot to 18" of your pool water, then adding shock like "Anonymous" did as stated in the third comment down.

hamnut  Posted: 6/13/2017 

I was told I have chlorine lock over the weekend by my pool store. here are the numbers from their testing:
PH: 7.8
Alkalinity: 19
Free Chlorine: 0
Total Chlorine: 6.6
Hardness: 281
CYA: 150

They told me to add 18 pounds of shock. I added 20#s Sunday night and left the pump running all night. I checked the pool water last night again the Free Chlorine is 0 and the Total Chlorine is is about 7 (I use test strips but I do have a reagent test kit.)
PH looked about the same as did the Alkalinity. CYA was still above 100.

I have read about "Chlorine Lock" for the 2 hours and there is a wide variety of opinions. So I am about to add liquid Chlorine. I can get it at the pool store for about $5/gal and get Clorox Bleach at the local food store for about the same price. I am going to try 6 ga. at first. I think I want to stay away from the shock due to my hardness being high.

Does this sound like a good plan?

InyoPools Product Specialist Dennis R.  Posted: 6/2/2017 

Anonymous (chlorine block) - Thank you for your feedback! It's always good to get alternative approaches.

Anonymous  Posted: 5/31/2017 

Thank the Lord, we now have FC! After putting nearly 25 pounds of shock into our pool to no avail, Leslie's pool employee was the answer to our prayer. I told her that our pool was eating chlorine and she said we needed to check for nitrates. The water was positive. She told us to drain the pool about 18 inches to 2 ft. and fill it back up. We did and followed with a super shock. It held and was still 10 the next day!

Rod n Aurora  Posted: 5/22/2017 

I am draining my pool as I read these comments. Spent a lot of money on chemicals including chlorine free shock and was never able to get ph correct. Finally supply company guy checked my water and gave me the good news...chlorine block. Hope this effort solves my issues now need to determine proper amount of stabilizer when refill is complete. Thanks to all who have gone down this road and provided valued input on finding a solution. Have a great summer.

Anonymous  Posted: 8/15/2016 

ive never ever heard of stabilizer stopping chlorine from sanitizing, and i work at a pool store where our everyday job is to fix these types of problems, testing the water and adjusting the chemical balance and whatnot. do you end up accumulating a larger ppm of chlorine over time because of it? yes. The only thing weve ever found that really stops chlorine from sanitizing is phosphates and nitrates. before you try any of these wacky equations, get a test specifically for phosphates and nitrates. if its either of these, depending on the level, get a phosphate remover and it should take care of the problem. however, there is nothing on the market thats proven to remove nitrates. but we have experimented and found that a strong, commercial grade phosphate remover does help with the nitrates

MarkyMark  Posted: 8/13/2016 

Chlorine lock due to high stabilizer is a heated topic in the pool maintenance industry. I personally believe it is largely a myth and/or overstated. There have been real world studies done on residential pools measuring the amount of stabilizer vs the amount of bacteria and algae present. The studies show that stabilizer levels all the way up to 400 ppm had no impact on chlorine's ability to fight bacteria and algae.

InyoPools Product Specialist Dennis R.  Posted: 6/18/2016 

low CYA - High cyanuric acid level is one of the contributing causes of chlorine lock. It is also caused by high levels of contaminates that build up over time when water in the pool isn't changed out.

Anonymous  Posted: 6/16/2016 

What if cyanuric acid is low?

InyoPools Product Specialist Dennis R.  Posted: 6/14/2016 

Catdog - You have too much "stabilizer" not "sanitizer". Stabilizer, or cyanuric acid, is added to help reduce the loss of chlorine due to direct sunlight.

Catdog  Posted: 6/12/2016 

I don't understand? If I have chlorine lock, which I do think that is what it is, and it's because too much sanitizer, then why is my sanitizer reading low?

DinAustin  Posted: 11/12/2015 

I have found that draining the pool and starting fresh is the easiest and most cost efficient way to take care of the problem. After spending a few hundred dollars on chemicals I did eventually have to drain the pool. I live in Cedar Park Texas and water rates are extremely high. The cost of refilling my pool was so much cheaper.

InyoPools Product Specialist Dennis R.  Posted: 6/18/2014 

Lmk - You have too much "stabilizer" not "sanitizer". Stabilizer or cyanuric acid, is added to help reduce the loss of chlorine due to direct sunlight.

Lmk  Posted: 6/18/2014 

I don't understand? If I have chlorine lock, which I do think that is what it is, and it's because too much sanitizer, then why is my sanitizer reading low?

Anonymous  Posted: 5/30/2014 

Thanks, Last year had same problem. I super chlorinated last year, this time I will try the chlorine free method. Draining is tghe last option. I have a huge pool, over 50 k gallons, neighbors will complain.
Thanks for the choices!!