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Chlorine Lock in a Pool: What Is It And How Do You Break It?

Are you continuously adding chlorine to your pool, but nothing seems to happen?

The bad news is, you may have what’s traditionally referred to as “chlorine lock”, a chemistry issue that seemingly renders your chlorine useless even though it shows up on your test results.

The good news is, this article will explain exactly what chlorine lock is, what causes it, how it differs from chlorine demand, and how to break chlorine lock and chlorine demand in your pool.

What is Chlorine Lock?

Chlorine lock is a somewhat misleading term for when chlorine no longer appears to be doing its job despite seeing a suitable free chlorine level.

In reality, this is usually due to your cyanuric acid level.

Cyanuric acid is a chlorine stabilizer; it regulates chlorine by combining with and temporarily “locking up” some of your chlorine to protect it from sunlight and prevent it from getting used up too quickly.

This process emulates a slow release of sanitizer, but too much cyanuric acid in your water can cripple your chlorine. This is called “over-stabilized chlorine” and it can effectively leave your water without enough “active” sanitizer.

Essentially, chlorine lock doesn’t mean you have a problem with your chlorine, but more so a problem with high cyanuric acid.

Important: If your test results show your cyanuric acid level isn’t actually too high, you’re probably experiencing a separate issue called chlorine demand. You can jump to the relevant section by clicking here.

Why is Chlorine Lock Bad News?

When you add chlorine to your water, two things begin to happen:

  1. The cyanuric acid in your pool immediately binds to as much chlorine as possible, creating a reserve.
  2. The remaining chlorine is free to get to work attacking and neutralizing contaminants in your water.

Under normal conditions, the amount of remaining chlorine (not tied up by cyanuric acid) is more than enough to keep your water properly sanitized and free of algae formations.

This is when cyanuric acid works in your favor, as it allows just enough chlorine to get the job down without over-expending.

When your cyanuric acid level is too high, however, there’s not enough active chlorine left over to keep your water clean and healthy, which often leads to aggressive algae blooms that are seemingly “immune” to chlorine.

What Causes Chlorine Lock? (High Cyanuric Acid)

The ideal cyanuric acid level for pools is 30 to 50 parts per million (ppm).

Unfortunately, there are a number of ways you could be adding cyanuric acid to your pool without even realizing it.

For example, if you’re using chlorine tablets, they’re likely going to be trichlor tablets that contain cyanuric acid.

A single trichlor tablet will increase your cyanuric acid level by 3-4 ppm, per 10,000 gallons of water. Multiply this over dozens of tablets and this factor alone could take you well over the recommended limit.

And if you regularly shock your pool (which you should be), you might be using a dichlor shock which also contains cyanuric acid.

Some pool owners also add pure cyanuric acid with the rightful intention of stabilizing their chlorine, completely unaware of all the other ways this acid is being introduced to their pool water.

Finally, cyanuric acid doesn’t break down in water, so your stabilizer level will continue to increase as you add more of it to your water, be it intentional or not.

How Do You Break Chlorine Lock? (Lower Your Cyanuric Acid)

The only way to fix chlorine lock, which is really just high cyanuric acid, is to reduce your cyanuric acid level.

While there are products on the market that claim to lower cyanuric acid levels, the results are unpredictable and expensive, with many pool owners claiming they made no significant difference.

The only practical way to lower cyanuric acid in your pool is to partially or fully drain your water, then fill it back up with fresh water.

After that, you need to prevent chlorine lock from returning by keeping tabs on your stabilizer level, including all potential sources of cyanuric acid such as chlorine tablets and trichlor pool shock.

What If It’s Not Caused by Cyanuric Acid?

Maybe your cyanuric acid level isn’t high enough to warrant a chlorine lock, or maybe you don’t use a chlorine stabilizer in your pool at all.

In that case, if you’re still having issues with chlorine effectiveness in your pool, you’re almost certainly experiencing a high chlorine demand issue rather than a chlorine lock issue.

Chlorine demand is sometimes used interchangeably with chlorine lock, and while the symptoms may seem similar on the surface, the chemistry behind the two conditions is actually very different.

Chlorine Lock vs Chlorine Demand (And How to Identify Them)

When you have a chlorine lock issue, the chlorine is still technically in your water; it’s just in a very weakened state.

In fact, chlorine that is “locked up” by cyanuric acid still shows up as free chlorine in a pool water test.

Chlorine demand has nothing to do with cyanuric acid.

When you have high chlorine demand, the chlorine is used up almost immediately as it enters the water. This is due to an extremely high level of contaminants, the most common being green algae or yellow algae.

Since used chlorine will not show up as free chlorine on a pool test, you can use this key piece of information to identify between chlorine lock and chlorine demand during testing.

To summarize:

  • If your test results show free chlorine, you likely have too much cyanuric acid in the water (chlorine lock).
  • If your test results show no free chlorine, you likely have too many contaminants in the water (chlorine demand).

How to Fix Chlorine Demand

Fortunately, fixing chlorine demand is easier because you don’t need to drain your pool to adjust the cyanuric acid level.

Instead, you need to shock your water to kill all the contaminants gobbling up your daily chlorine consumption, which just means adding a high amount of chlorine (10x the usual level) to reach breakpoint chlorination.

Breakpoint chlorination will rid your pool of all contamination and effectively “reset your water”, allowing you to escape chlorine demand and maintain a stable chlorine level.

Throw Away the Key

If your chlorine is staying in your water but doesn’t appear to be doing anything, chlorine lock (as in high cyanuric acid) is the likely cause.

All you need to do is lower your cyanuric acid level by draining and refilling your pool until your cyanuric acid level falls back into the recommended range, then add your chlorine as before.

If the chlorine won’t stay in your water, however, you’re likely dealing with a chlorine demand problem instead, often pointing to serious contamination such as algae blooms.

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