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How to Cure Chlorine Lock and Chlorine Demand

By Matt Giovanisci | Updated: June 15, 2021

Lock and Demand. They’re actually a couple of problems you might run into with your pool water. Actually, they’re just one problem, because chlorine lock is misleading.

Before we dive in, you know that chlorine is the number one thing you need to keep your pool water clean and safe. If you don’t have enough chlorine in the pool, then it’s not going to be a fun time for anyone. If you want to make sure that your chemistry is perfect all season long, then we recommend getting our video course which teaches everything about how to take care of your pool so that it will always be safe for swimming.

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What is Chlorine Lock?

There’s a theory flying around out there that chlorine can be inhibited from doing its job by cyanuric acid. The theory goes that, when this happens, the chlorine won’t be able to function as a sanitizer because it’s “locked up.”

This is not a real thing. There’s a reason cyanuric acid is also known as chlorine stabilizer. It’s because it actually helps chlorine to do its job by preventing it from breaking down as quickly in sunlight as it would if it weren’t stabilized.

Can you have too much stabilizer in the pool? Of course. You can have too much of any chemical in your pool. Just follow the manufacturer’s instructions when you use it, perform regular pool water testing to see what’s going on and keep the water balanced.

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Cyanuric Acid (Chlorine Stabilizer) for Pools

Keep your chlorine from being burned off too quickly by the sun's UV rays. CYA will help you keep a better chlorine reading in your pool.

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What is Chlorine Demand?

OK, this is a real thing. Chlorine’s effectiveness is finite. If there’s a high bather load, or there’s just a lot of debris and other organic contaminants in the pool, more chlorine is required to properly sanitize the water.

In other words, fighting those materials in the water demands more chlorine. So this situation is known as chlorine demand. It’s rare, but not unheard of.

What Causes Chlorine Demand?

Aside from bather load and debris, you may also run into this issue in the spring when you open your pool.

During the winter, as the water sat stagnant and untreated, it may have accumulated quite a bit of organic contaminants that will take a lot of chlorine to neutralize. This could also happen if the pool is open, but hasn’t been cared for properly or in a long time.

Heavy rainfall can also be a factor depending on the chemical makeup of the rainwater. There’s no way to test this for sure, but if you have trouble maintaining a chlorine reading after a rainstorm, chances are you have chlorine demand.

Runoff can introduce contaminants into the water that will essentially use up all the chlorine. Be sure to keep fertilizers or other non-pool chemicals away from the pool.

How to Diagnose Chlorine Demand

If you’ve just added chlorine to the pool, or you’ve used chlorine shock, and you test the water, and it still shows a low level of chlorine, you’ve got a demand problem on your hands.

How to Cure Chlorine Demand

The good news is, this is a fairly easy problem to fix. All you need to do is super chlorinate the water. The fastest way to do this is by shocking your pool. But not the way you normally do. Your best bet is to triple shock it with calcium hypochlorite.

To triple shock, add 3 pounds of shock per 10,000 gallons of water. You’ll need to know your pool’s volume to do this correctly, which you can determine with a pool calculator.

Pool Volume Calculator Choose the shape of your pool first.
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Before adding the shock, make sure the chlorine stabilizer level is somewhere in the range of 30 ppm to 80 ppm, with 50 ppm being ideal. Also, remember to shock at night because sunlight reduces chlorine’s effectiveness—which is the very reason you’re using stabilizer.

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Calcium Hypochlorite Shock

Use the entire contents of the bag when opened. If any granules settle to the bottom of the pool use brush to disperse. Add the right dosage of this product during evening hours while the filter pump is running.

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Are Chlorine Lock and Chlorine Demand the Same Thing?

These terms are sometimes used interchangeably, but they’re not the same issue.

Chlorine demand is real and solvable by adding enough chlorine to your pool until it starts to register on your liquid test kit or test strips.

Chlorine lock is a false issue, a term that some pool companies and experts will use when they really mean chlorine lock (should say chlorine demand). So if a pool pro tells you your pool is suffering from chlorine lock, you’ll know the fix is still to add more chlorine to the pool.

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Test Strips for Pools and Hot Tubs

Tests for 7 important chemistries in seconds: Total Hardness, Total Chlorine, Total Bromine, Free Chlorine, pH, Total Alkalinity, and Cyanuric Acid.

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That show wouldn’t last long anyway. And neither will chlorine demand if you take swift action to correct it. And as for chlorine lock, don’t let that old wives’ tale divert you from addressing problems that may actually develop, or from enjoying your well-cared-for pool.

Happy Swimming!

Matt Giovanisci is the founder of Swim University® and has been in the pool and spa industry since 1995. Since then, his mission is to make pool and hot tub care easy for everyone. And each year, he continues to help more people with water chemistry, cleaning, and troubleshooting.
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