What is a Chlorine Lock and How do you Break It

Written By Timothy Te

05th November 2020

Have you ever walked by a pool and you get a whiff of a strong chlorine or chemical smell and thought to yourself “Oh, they put in too much chlorine in the pool”? Well, this is actually quite the opposite. A strong chlorine smell is often a sign of having a lot of Chloramines in your pool. These are chemical compounds that are the result of the combination of organic material (sweat, pee, etc) and chlorine disinfectants.

Why is this relevant? Well, the presence of a lot of chloramines says a lot about the sanitising power of the chlorine in your pool. Chloramine build up means that there is either not enough chlorine in your pool to destroy them or your chlorine isn’t working as it should be.

What does this have to do with chlorine lock?

A chlorine lock happens when your free chlorine reading is wildly divergent from your total chlorine level. Free chlorine is the amount of chlorine that is actually “working” to sanitise your pool while total chlorine is the total amount of chlorine in your pool. They should be roughly the same or very close in order to get the proper sanitising and when the total chlorine is much higher than the free chlorine.

Warning: A lot of people will mistakenly think that the chlorine smell is too much chlorine and will even stop treating their pool, this is a huge no-no.

What exactly is a chlorine lock?

In layman’s terms, a chlorine lock is when your chlorine is rendered useless and no matter how much chlorine you add, it won’t work and free chlorine levels won’t go up during testing. One of the main causes for this is having too much cyanuric acid or stabilizer in your pool. While cyanuric acid protects your chlorine from UV rays, too much of it will render your chlorine ineffective and useless.

How to deal with a chlorine lock

There are a few ways to deal with a chlorine lock and we’re going to look over some of the best ways to deal with it short of changing out your pool water.

Partially Draining your pool

If your chlorine lock is caused by too high levels of cyanuric acid, the only way to deal with it is to partially drain your pool. Cyanuric acid doesn’t dissipate so draining it out is the only cost-effective way to do it. We recommend using a submersible pump or a venturi pump to do this to protect your pool pump from burning out or running dry once the water level has dropped below your skimmer levels.

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Take out about ¼ of your pool water and then refill your pool water with fresh water. After this, allow the water to circulate for at least 8 hours before checking your chemical levels again. The cyanuric acid levels should have dropped and your chlorine levels should have dropped significantly as well. Try adding chlorine and check to see if your free chlorine levels are going up as normal.

If this doesn’t work, try it one more time and that should do the trick. This is the simplest way to deal with a chlorine lock, but is very time consuming and consumes a lot of pool chemicals as you’re throwing out a large portion of your pool chemicals along with your water as you do this.

Update: Using a Cyanuric Acid Reducer

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This is a fairly recent addition to Mr Pool Man's online pool shop. This enzyme compound allows you to lower your cyanuric acid levels without going through the hassle of partially draining your pool and rebalancing your pool chemicals all over again! learn more through out product spotlight on the Bio-Active Cyanuric Acid remover here.

Achieving Breakpoint or break even Chlorination

The other way to break a chlorine lock is to achieve what is called breakpoint or break even chlorination. Simply put, you’re going to be adding enough chlorine to oxidize everything in the pool that’s causing the lock. This is a pretty complex process which involves some accurate computation which must be done correctly simply because if you’re not reaching breakpoint chlorination, you’re just going to be making the problem worse.

For a more details and a step-to-step guide on how to achieve breakpoint chlorination to break your chlorine lock, see our guide here on breakpoint chlorination.

P.S. If the graph above gave you a headache, don't worry, you're not alone! We've simplified the computations involved in achieving breakpoint chlorination in our guide so be sure to check it out!

Preventing Chlorine Locks

The best way to prevent a chlorine lock is to constantly monitor your chlorine and cyanuric acid levels. Make sure that your cyanuric acid levels do not overshoot the recommended levels and only use unstabilised chlorine when your cyanuric acid levels are close to the maximum recommended levels. Also make sure to keep your chlorine levels on the high side of the recommended levels. Now if you don’t have a lot of time to keep on monitoring your chlorine levels, you may want to consider getting a salt water chlorinator like the Water TechniX Atomic Salt Water chlorinator.

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Having a salt water chlorinator takes the hassle and effort of monitoring your chlorine levels. Simply plug it in and it will continuously chlorinate your water, avoiding the build-up of chloramines in your pool and preventing a chlorine lock. This also reduces your need to constantly monitor your cyanuric acid levels as the chlorinator is constantly adding chlorine to your pool, making chlorine dissipation a side note.


The next time you detect a strong chlorine smell from your pool, test your water to see if your free chlorine levels are close to your combined chlorine levels. If caught early, proper addition of unstabilised chlorine or shocking your pool can head off a chlorine lock before it happens. Or again, if you want a more hands-off approach to preventing a chlorine lock, consider getting a salt water chlorinator.

Worried about things you’ve heard about salt water chlorinators? You shouldn’t be! Check out our demystifying salt water pools blog post to see that many of the myths people say about salt water pools are just that, myths.

Do you have any questions about this topic or the featured products? No worries, we're here to help! Drop us a question down below and we'll get back to you ASAP.

Happy swimming :)

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Written By

Timothy Te

Pool Guru at Mr Pool Man

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