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

Why Does Chlorine Lock Happen? The most common cause of chlorine lock is an overabundance of cyanuric acid (CYA). Cyanuric acid is commonly referred to as a "stabilizer" and is vital to maintaining healthy chlorine levels in your pool. Stabilizer is found in liquid, granular, and tablet forms of chlorine; and acts as a "chlorine sunblock," preventing burn-off by the sun. 

The recommended range for your pool's CYA level is 30-50 ppm for traditional chlorine pools and 60-80 ppm for saltwater pools. When your CYA creeps above this range, that's when it goes from an aid to an inhibitor.

An imbalance of pH levels can also cause chlorine lock.

Click Here to View Pool Shock

Step 2

How to Test for Chlorine Lock - Determining the severity of your chlorine lock can be done by calculating the Combined Chlorine of your water.  Requires DPD Test Kit

Combined Chlorine = Total Chlorine (TC) - Free Chlorine (FC)
  • Good Result: 4 TC - 4 FC = 0 CC
  • Bad Result: 7 TC - 3 FC = 4 CC
The good result means our water’s chlorine level is healthy; the bad result means there is more “dead” chlorine than active, which causes Chlorine Lock.

Click Here to View the Taylor Water DPD Test Kit

Step 3

Chlorine Lock Prevention Method 1 - Solution by Dilution - the most effective way of breaking chlorine lock is diluting the concentration of CYA in your pool. You can lower CYA levels by draining a percentage of your pool then refilling it with fresh water. This method is the only guaranteed way of lowering pool water’s CYA significantly in one fell swoop.

By draining the pool, you have more control over the amount of CYA more control over how CYA is purged.

Step 4

Chlorine Block Prevention Method 2 - CYA Reducer - CYA reducing chemical agents are a relatively new product that removes the need of draining and refilling your pool. A CYA reducer may be your best choice in preventing or treating chlorine lock if you live in a water-restricted area due to drought or general climate.

A CYA reducer is usually made of the chemical compound Aluminum Sulfate, which acts as a coagulating agent called a flocculant. The flocculant is essentially dirt particle flypaper; everything sticks to it until it becomes a larger mass. Eventually, that coagulated material sinks to the bottom can vacuum to waste.

For in-depth instructions on using Aluminum Sulfate, you can find them here: Lowering Your CYA Without Draining.

Click Here to View a CYA Removal Kit

Step 5

Chlorine Lock Treatment Method 3 - Non-Chlor Shock (Oxidization) - Non-Chlorine Shock, also called Non-Chlor, uses oxidation to break down contaminant compounds in your pool water. Oxidation is different from sanitation, whereas oxidation breaks down chemical compound sanitization to kill bacteria.

You will need to subtract the Free Chlorine (FC) from the Total Chlorine (TC)  and multiply that by the number of gallons in the Pool divided by 10,000, then multiply by 2. 

How to calculate the amount of Non-Chlor Shock Needed:

(TC - FC) ✖ (Pool Size in Gallons ➗10,000 gal.) ✖ 2

The equation for our Pool:

(7 - 3) ✖ (20,000 gal. ➗10,000 gal.) ✖ 2


4 ✖ 2 ✖ 2 =16 pounds of Non-Chlor Shock

Click Here to View our Non-Chlorine Shock 

Step 6

Chlorine Lock Treatment Method 4 - Breakpoint Equation - Breakpoint chlorination is the chemical bond that ties nitrogen, chlorine, and ammonia together by using large amounts of chlorine, causing the chlorine residual to drop rapidly. Reaching an actual chlorine breakpoint is vital to breaking a chlorine lock. If a breakpoint is not reached using this method, the problem will only worsen.

Step 7

Chlorine Breakpoint Equation (Cont.) - It takes 7.6 free chlorine molecules to break apart an individual combined chlorine molecule to accomplish a breakpoint. 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


(121 to 132 of 132)

InyoPools Product Specialist  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  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.

 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?

 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  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.

 Posted: 1/25/2019 

I love how you try to make them believe that it’s most likely going to be that their stabilizer levels are too high. Pardon me ladies and gentlemen, but I’m a true pool professional with over 19 years of experience in this business, as I am the assistant manager of a small family pool supply store. Now, in 19 years of dealing with anywhere from 10-50 customers per year with this exact problem, and in those 19 years, only twice has it had anything to do with Cyanaric Acid. Nitrate and Nitrite level is 99.98% of the time your problem. So how about we quit using algebra to fix our pools and do everyone here a favor, take it from a man that is a third generation swimming pool supply store manager and future owner. Here’s a table for you: Pool size. Chlor needed 0-10,000 gal. 4-gal 10k-20k gal. 8-gal 20k-30k gal. 12-gal 30k-40k gal. 16-gal 40k-50k gal. 20-gal Ok, I think that you can see how that works, but I must also advise you to make sure you aren’t buying liquid chlorine from box stores or large chain stores (family leisure) their chlorine is usually old liquid chlorine that is labeled as 12.5% liquid chlorine is actually bottled at, between 18-20%. Yes that does mean that buying your liquid chlorine at Walmart is basically like buying water that had chlorine in it last year. So visit a reputable local swimming pool supply shop in your area and buy the necessary amount of liquid chlorine into the pool with the pump running while the sun sets, then check your total and free chlorine levels the next morning. They will both be sky high and no one should enter the pool until those levels get below 5 ppm

 Posted: 7/29/2020 

After 3-4 days of 0 reading for chlorine I was convinced we had the block. I didnt not want to drain the pool until after I saw if your method would work. My pool is 14000 gals so I used 8 gals of liquid chlorine from Lowes. I put them in at night and woke up this morning to a chlorine reading of 5 and clear water! We are getting in the pool today!!

InyoPools Product Specialist  Posted: 9/3/2020 

Heck yeah!

InyoPools Product Specialist  Posted: 1/25/2019 

Hello APP - We appreciate the feedback. However, a high cyanuric level (CYA) will definitely impact chlorine. Nitrates and Nitrites have nothing to do with "chlorine lock". If your chlorine is low, either from not adding enough or a high CYA level, algae can bloom and feed on nitrates and nitrites. However, the root cause is not nitrites and nitrates. If you don't believe us, check out an article titled "New Thinking: Chlorine/Cyanuric Acid in Balance". It's written by a chemical specialist, Robert Lowry, and backed up by a theoretical chemist. Our friends at would love to have this conversation with you. They firmly believe in the issues with high CYA. You may find a thread titled "High Nitrates" very interesting.

 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!!