How to Break Chlorine Lock

Sometimes you walk by your swimming pool and get a strong whiff of chlorine or chemical smell. It makes you think there’s too much chlorine in the pool. The opposite is actually true. Strong chlorine smells emanating from your swimming pool are a result of chloramines taking over. In simple terms, chloramines are the chemical compounds that result from organic contaminants and chlorine being mixed together.

Why does this matter? If you have too many chloramines, it implies that the chlorine levels in your pool are too low to destroy them or that your chlorine isn’t working the way it should. Don’t mistake the smell of chlorine to mean you should stop adding sanitizer to your pool. This will cause problems.

How to Break Chlorine Lock

What is a Chlorine Lock?

Chlorine locks happen when your total chlorine level doesn’t match your free chlorine levels. Free chlorine is the volume of “working” chlorine that is actually sanitizing your pool, while total chlorine is the total amount of chlorine present in your pool. Therefore, your total chlorine and free chlorine levels should be close to the same amount.

A chlorine lock means that your chlorine isn’t working. If your pool is in a “lock” situation, you can add as much chlorine as you want to the water without increasing your free chlorine levels. This is said to be caused by having too much cyanuric acid or stabilizer in your water.

Cyanuric acid is designed to prevent chlorine breakdown from UV rays, but too much of it renders your chlorine ineffective at sanitizing.

Chlorine lock is another term for chlorine demand. Unfortunately, the term chlorine lock can be misleading, and it is theorized that the problem is actually that your demand is too high.

Can you have too much stabilizer in your pool? Absolutely. You can have too much of any chemical in your pool. But if you follow the manufacturer’s instructions for adding a stabilizer and test your water regularly, this shouldn’t be an issue.

What is Chlorine Demand?

The effectiveness of chlorine is finite. High bather loads, extra debris, or increased organic contaminants in your pool water will require more chlorine to sanitize the water. Therefore, the more you use your pool, the more chlorine your water will require. This is chlorine demand.

What’s the Cause of Chlorine Demand?

Chlorine demand (or lock, if you prefer to call it that) often occurs in the spring when you first open your pool for the season. This is because your pool water sits stagnant and unused over the winter and accumulates significant amounts of organic contaminants.

These contaminants take a lot of chlorine to disappear. This situation can also occur if you haven’t been regularly testing and maintaining your pool water.

Heavy rainfall in an uncovered pool changes the water composition and dilutes the chemical volume. If you have recently had a rainstorm and find that you’re having difficulty maintaining chlorine levels, you have a chlorine demand situation.

Runoff is a common culprit as well. Fertilizers and pesticides from your yard can rinse into your pool and throw off the chemical balance.

Diagnosing Chlorine Demand

If you have added chlorine to your pool or completed a chlorine shock, and your chlorine levels don’t rise upon testing, you have chlorine demand.

Curing your Chlorine Demand

Super chlorinating your pool water is the best solution for curing chlorine demand. Isn’t that what you do when you shock your pool? Yes, but in this case, you’re going to use calcium hypochlorite and triple chlorinate it.

To do this, you will need to add three pounds of shock for every 10,000 gallons of water.

Before you start, ensure your stabilizer level is sitting between 30 ppm and 50 ppm. Shocking at night will also increase the effectiveness of super chlorination and prevent UV rays from breaking down all your hard work.

What if Your Cyanuric Acid Levels are too High?

If your stabilizer levels truly are too high, there are a couple of ways to deal with it.

  • Partially drain your pool.
  • If you have a chlorine lock situation with too much stabilizer, the only way to truly fix it is to partially drain the pool. Unfortunately, stabilizer doesn’t dissipate well, so you should use a submersible pump to take out some of your water volumes. You will want to remove about ¼ of your pool’s volume. Then refill it with fresh water.
  • Let your pool water circulate for about 8 hours before adding any additional chemicals. Your stabilizer levels should drop off and your chlorine levels should be lower as well. Now you can add more chlorine and watch for your free chlorine level to increase. If you don’t see success, try running through the partial drain process again. This is the easiest way to deal with chlorine lock, but it is time-consuming and labor-intensive.
  • Try a cyanuric acid reducer
  • This is a fairly recent addition to the world of pool chemicals. It’s available online and is an enzymatic compound that helps lower your stabilizer levels without draining and refilling your pool.

Achieving break-even chlorination or the breakpoint of your lock

Your goal in breaking chlorine lock is to achieve a breakpoint or break-even chlorination. You want to add enough chlorine to oxidize absolutely everything in your pool that might be causing your chlorine lock or demand. This is a very time-consuming process and requires a lot of chemical volumes. So, while you might want to fix the problem this way, it’s usually easier and more cost-effective to try partial draining or a reducer.

Are Chlorine Lock and Chlorine Demand the same Issue?

No, they’re not the same problem, but they have the same symptoms.

Chlorine demand is solved by adding chlorine to your pool until it registers on your test strip. It’s the result of your pool using more sanitizer to clean the water and the pool simply needing a higher volume of chlorine.

Chlorine lock can also be cured by adding more chlorine to your pool until it registers on your test strip. Hmmm, it sounds exactly the same, doesn’t it?

It doesn’t really matter if your chlorine levels are low because your other chemical levels are too high or if your chlorine is low because of increased demand. The answer is that you need more chlorine in your pool. Period.

Prevention of Chlorine Lock and Demand

Preventing chlorine lock requires regular water testing and monitoring of free chlorine levels. You also want to monitor your water pH and stabilizer levels to keep them even.

Alterations in your water chemistry change the way chlorine works, so everything needs to be maintained in balance. Try to keep your chlorine level on the high side of recommended levels to account for periods when demand might be higher.

The other way to avoid having to constantly adjust chlorine levels is to install a saltwater chlorinator.

Saltwater chlorinators are designed to take the hassle out of monitoring chlorine levels. They continuously chlorinate your pool water and avoid chloramine buildup. By using salt, you don’t have to constantly check the chlorine levels in your pool.


If you start to smell chlorine coming from your pool, check your chlorine levels. If you catch the imbalance early on, it’s easy enough to add free chlorine or shock your pool water to stop a chlorine lock before it happens. If you want a more maintenance-free solution to preventing chlorine lock, a saltwater chlorinator can help.

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