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: 06/13/2017 23:51 PM  Inyo Product Specialist

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: 06/13/2017 9:43 AM 

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  Posted: 06/02/2017 11:00 AM  Inyo Product Specialist

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

Anonymous  Posted: 05/31/2017 17:24 PM 

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: 05/22/2017 16:35 PM 

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: 08/15/2016 18:02 PM 

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: 08/13/2016 18:27 PM 

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  Posted: 06/18/2016 8:57 AM  Inyo Product Specialist

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: 06/16/2016 18:54 PM 

What if cyanuric acid is low?

Inyopools  Posted: 06/14/2016 11:46 AM  Inyo Product Specialist

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: 06/12/2016 18:33 PM 

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 10:40 AM 

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  Posted: 06/18/2014 11:26 AM  Inyo Product Specialist

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: 06/18/2014 9:15 AM 

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: 05/30/2014 22:36 PM 

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!!