Chlorine Lock in Your Pool and How to Fix It

Michael Keenan


Pool Water Chemistry

I’m going to explain everything you need to know about chlorine lock and chlorine demand in your pool. There are different interpretations of this issue but the problems described are the same – as are the ways to fix these problems. Have a good read …

When you have a swimming pool, you already know how important it is to keep it loaded with the right chemicals and the right combinations of those chemicals. Otherwise, it can be uncomfortable at best to swim in the pool, and downright dangerous at the worst.

Besides chlorine, one of the most important chemicals to put in your pool water is the cyanuric acid, or CYA, which essentially causes your chlorine to work much better, in part by protecting its molecules from being destroyed by the UV rays of the sun.

chlorine lock and demand

With both of these chemicals, you have to make sure they are in the correct amounts. If you put too little in the pool, the chemicals won’t have a chance to do their jobs. If you put too much, on the other hand, it can cause burning or stinging to swimmers’ eyes and skin area.

Including both chlorine and CYA in the proper amounts is important, and one of the things it can lead to if you don’t do this is chlorine lock. Find out how to lower the CYA level in your pool in this tutorial.

What Is Chlorine Lock?

First, if your chemicals are out of sync in any way, chemical compounds called chloramines can form in your pool’s water. In many cases, this buildup causes chlorine lock, which happens when your total chlorine and your free chlorine aren’t in equal amounts the way they should be.

To make it worse, chlorine lock often results in a bleach-like smell, which can make many pool-owners think they don’t have enough chlorine in the water, or that the chlorine isn’t active.

Furthermore, since the chlorine level can read “low” when you test it, this can cause a lot of pool-owners to think they need to add more chlorine. When the level keeps reading like it’s low over and over again even with the addition of more chlorine, they can wonder what they’re doing wrong.

So, there are two problems at this point. The first is that your chlorine level reads low no matter how much chlorine you add, and the second is that in this circumstance, it isn’t more chlorine you need because the actual problem is that your CYA has the chlorine locked up so it can’t do its job.

This is mostly because there is too much chlorine in your pool, instead of not enough chlorine.

Chlorine Demand?

What some people call chlorine demand is basically the same problem as chlorine lock. It’s as it sounds – your pool is demanding more available chlorine to keep the pool healthy.

Things like increased use of the pool, heavy rain or increased organic matter in your pool can increase the demand for active chlorine.

How to Fix Chlorine Lock or Demand?

Just what do you have to do to fix the problem of chlorine lock? Below is a step-by-step list of instructions to follow to get rid of the problem. Don’t get too caught up in the lock or demand issue – the cure is the same process!

Drain part of the water out of the pool.

There is no hard-and-fast rule regarding how much water to drain out of the pool, but many people start by draining about one-fourth of the water out, then refill the pool with fresh water. See my article on how to drain an above ground pool for some details on how to do this.

Keep doing this and testing the water until the total chlorine and free chlorine are in equal parts. It may take several times, but eventually it will work and these two numbers will be in sync.

Add non chlorine shock to the pool.

If you add non-chlorine shock to the water in your swimming pool, the water will be oxidized, which results in both cleaner water and water that restores the balance of total chlorine and free chlorine.

You simply add this product to your pool, then wait a few hours and test the water again to make sure the free chlorine and total chlorine are the same. You may have to repeat the process several times until the numbers come out right.

Regarding the formula to use when adding the non-chlorine shock, here is a good “recipe” to follow:

(Total chlorine – Free chlorine) x (# of gallons/10,000) x 2 = amount of shock to add).

Instead of non-chlorine shock, add chlorine shock.

You can also add chlorine shock to break chlorine lock. In this case, you need to achieve breakpoint chlorination. In other words, large amounts of chlorine need to be added so that the chemical bond of the chloramines is broken up.

I have a detailed guide covering how to shock your pool – quickly and easily!

HTH Super Shock Treatment Swimming Pool Chlorine Cleaner
BENEFITS: Increases the chlorine level in swimming pools to kill bacteria and algae to keep your pool water clear and ready for fun all season long.

To do this, the first thing you have to do is check the pH balance in your pool and make sure it is between 7.2 and 7.6. This means the pH level in your swimming pool is just right.

You can use several different formulas to determine how much chlorine you need to add so that breakpoint chlorination occurs, but if you just follow the directions on the product like you would if you were normally shocking your pool, this should work just fine.

Once again, you’ll have to wait several hours, then test the chlorine levels to make sure they are in the right proportions after this is done.

If this doesn’t work, you have one of two choices: continue adding the pool shock until the number comes out correct, or simply use another method to eliminate the problem.


Chlorine lock may be time consuming to break, but just like keeping the chemical levels in your water at certain numbers, this is an extremely important task. After all, you want more than just clean and comfortable water in your swimming pool.

You also want water that is safe for all swimmers, and for this to happen, all of your numbers (pH level, free versus total chlorine, and so on) have to be exactly right.

Chlorine lock causes the chemicals in your pool to work incorrectly, and one of the results is that none of the chemicals you’ve added to your swimming pool will have the ability to kill off any of the bacteria or algae that are in the water.

To learn when you have chlorine lock, you have to pay attention to the signs, and once you discover what’s wrong, you have to remedy the situation immediately.

In addition, if this problem occurs repeatedly and you start to get frustrated, you might want to consider lowering the amount of CYA you use the next time you chlorinate.

If your swimming pool has a strong bleach smell, that’s your first sign that something is wrong, but don’t assume that you don’t have enough chlorine in the water because the problem could be something else entirely.

Last update on 2024-06-17 / Affiliate links / Images from Amazon Product Advertising API

michael keenan author

Author - Michael Keenan

I'm Michael Keenan the owner and creator of the Outdoor Care Guide. I'm a trained horticulturist with over 30 years of experience in pool care, plant care, and landscape care! Seemed like a good idea to share - I think I can make your life easier and save you some time and money!