Home » Do You Have Chlorine Lock? How to Unlock & Break It

Do You Have Chlorine Lock? How to Unlock & Break It

One of the few things that can ruin your swimming pool experience is chlorine lock. You may have added a heap of chlorine only to find your chlorine levels stay the same. But does chlorine lock exist? How do you break it? And how do you prevent it? 

This post reveals the answers to all these questions and more. 

Let’s dive in. 

rain water on pool

Is Pool Chlorine Lock A Myth? 

“Chlorine lock” is a term that describes a condition in the pool when the chlorine is rendered ineffective by the presence of too much chlorine stabilizer or Cyanuric acid (CYA). They often mistake this for high chlorine demand.

Chlorine lock is a term wrongly used by pool owners and manufacturers to indicate that the presence of increased CYA levels in the pool is causing the chlorine to be on ‘lock’ or ineffective even if the chlorine is at the right level. 

However, there is no evidence to back up the claim that too much CYA in the pool can lock the chlorine. 

CYA or cyanuric acid is a chlorine stabilizer that prevents chlorine depletion from the sun’s UV rays. This means the CYA helps the chlorine to work better since there is reduced chlorine loss. 

However, CYA that is beyond 100 ppm can mean the chlorine takes longer to work. You may think you have “chlorine lock” if this is the case. ” So if the pool’s CYA level is too high, you should lower it. You can learn more about lowering CYA levels by reading a previous article on this website. 

How to Unlock or Break Chlorine Lock

To unlock or break chlorine in a pool that has chlorine lock, in most situations more chlorine is needed. If you’ve added chlorine and have found that chlorine levels have stayed the same or even decreased, add 1lbs of calcium hypochlorite for every 10,000 gal of pool water.

People often mention chlorine lock when the chlorine in the pool isn’t sanitizing the water as it should or when there isn’t any free chlorine reading on the test strip. But as we said earlier, there isn’t such thing as “chlorine lock”.

So if there is no such thing as chlorine lock, then what is causing your chlorine not to work? Why does your test kit still say you have little or no chlorine in your pool after adding a heap?

There are two reasons:

1) Your test kit may read high enough.
Many pool test kits have a maximum reading and if your chlorine levels exceed this, then it may look like there’s no chlorine in your pool.

2) You have a chlorine demand problem.
It’s more likely a chlorine demand problem and a faulty test kit or chlorine lock. This brings us to the next question – what is chlorine demand?

Let’s take a closer look. 

What Is Chlorine Demand?

Chlorine demand describes a situation when there isn’t enough active or free chlorine in the pool. In most cases when people think they have chlorine lock, the true problem is actually high chlorine demand.

When the debris, contaminants, and scum in the pool is beyond the level that the active chlorine in the water can sanitize, then there will be chlorine demand. This indicates that the pool needs more chlorine to fight the contaminants and not that the chlorine is on lock. 

So what causes chlorine demand in a pool? Let’s find out. 

What Causes High Chlorine Demand? 

Chlorine demand is usually caused by several factors that reduce the active chlorine level in the pool. Here are the causes of chlorine demand:

  • Algae
  • High Swimmer Or Bather Load
  • Food And Drink Residue 
  • No or Not Enough Stabilizer
  • Untreated Pool Water
  • Rain or Filling The Pool With Dirty Water

Now let’s check out these factors in detail.

1. Algae

An outbreak of algae in your swimming pool will cause the chlorine demand to increase. In fact it’s not uncommon to add chlorine to your pool and find that you have no free chlorine in your pool overnight.

Getting rid of algae will require you to add lots of chlorine to kill it off over several days.

2. High Swimmer Or Bather Load

This is the primary cause of chlorine demand. The more the number of swimmers that use a pool, the higher the chances of chlorine demand. 

When you swim in the pool, you leave your bather load in the water. This includes waste like sweat, dead skin cells, wads of hair, and don’t even get me started on the drops of urine and feces that fall into the water as swimmers flap around.

You will need a high level of active chlorine to get rid of these contaminants. When more people use the pool, the higher the bather load and the more likely your pool is to demand more chlorine. 

3. Food And Drink Residue

If you are the type that eats in the pool, you are indirectly increasing your risk of chlorine demand. When food and drink residue falls into the pool, the chlorine in the pool treats the food the same way it treats ammonia. 

This is because ammonia, food, and drink are all contaminants. So if you test your chlorine levels after having a pool party, it’s very possible to not have a chlorine reading since the chlorine has been used up to destroy the food particles in the pool. This will cause an increased chlorine demand. 

4. No or Not Enough Stabilizer

If the pool is left uncovered during the day, the chlorine will be depleted by the sun’s UV rays, unless you have a stabilizer present in the water.. This is even worse during the hot summer months. Since the chlorine is lost through the sun’s UV rays, the use of stabilizer (aka cyanuric acid or CKY) in the water will “protect” the chlorine from the sun’s UV somewhat and help to minimize the degradation.  Stabilizer can be added to your pool as a separate product (cyanuric acid) or you can use stabilized chlorine products such as dichlor or trichlor.

5. Untreated Pool Water

When the pool water is left untreated and uncirculated for a long time, debris and organic matter will accumulate in the pool. The presence of these contaminants will deplete the chlorine levels quickly causing an increased chlorine demand. 

Also, if the pool is left uncovered and unused for weeks, organic matter like grass clippings, twigs, bird droppings, and the likes will get into the pool water and cause chlorine depletion.

6. Rain or Filling The Pool With Dirty Water

The water that goes in the pool hugely determines how quickly the chlorine will be used up. If you fill the pool with dirty water or have experienced heavy rain, the chlorine will deplete quickly because the chlorine is being used to clean the water. This will result in an increased pool demand for chlorine. 

This is more common with pool owners that fill their pools with well water. Well water contains a host of metals, sand, and debris that causes the water to be dirty. 

Other causes of a chlorine demand include inadequate chlorine levels in the water and unbalanced pool pH. 

So how do you know if you have chlorine demand in the pool? Keep reading to find out. 

How Do You Know If There’s a Chlorine Demand Problem?

Here are the signs that indicate a chlorine demand in the pool:

  • Strong Offensive Smell Of Chloramines 
  • Algae, Murky Or Brown Water
  • Swimmer’s Eye

Now, let’s check out these signs in detail and how they relate to chlorine demand.

Strong Offensive Smell Of Chloramines 

If you pass by the pool and pick up a strong whiff of chlorine smell in the water, it’s not because of too much chlorine. On the contrary, it’s because there isn’t enough chlorine in the pool.

A well-chlorinated pool wouldn’t smell at all. The strong smell you pick up is the smell of chloramines or ammonia. Chloramine is the by-product of the reaction between free chlorine and ammonia compounds. Ammonia compounds are introduced into the water usually by sweat and urine.

If there isn’t enough free chlorine in the pool to deal with the ammonia compounds, more chloramines will be produced by combining with ammonia to give a strong offensive odor. The pool will demand more chlorine to destroy the chloramines.

Murky Water, Algae Or Brown Water

Murky water, algae or brown pool water is often a result of insufficient chlorine levels. When the active chlorine level isn’t enough in the pool, bacteria will roam free and accumulate in the water causing the pool water to become murky or brown. The water will eventually have a green tint when algae starts to grow in the pool.

Further Reading:

Green Pool – Guide to Easily Preventing & Removing Pool Algae
Milky Pool Water: Is it OK to Swim and How to Clear It

Swimmer’s Eye

Swimmer’s eye is a red and stinging sensation in the eyes after swimming. The redness, swelling, and stinging feel are caused by bacteria and viral infection. This happens when the pool is heavily affected by bacteria and we all know bacteria will grow in the water when there isn’t enough chlorine. 

So if you experience a burning sensation in your eyes after swimming, your pool is in demand for chlorine. Another explanation for sore eyes after swimming is an incorrect pH balance. The acceptable range is 7.4 to 7.6.

All of these signs indicate a low chlorine level in the pool but the best way to be sure your pool is in demand for chlorine is to test the water. So how do you do that? Let’s find out. 

How To Test For Chlorine Demand

To test for chlorine demand in a pool, you’ll need: 

Pool Water Testing Kit Or Pool Test Strips

You can use a liquid pool test kit like this:

Taylor Technologies Pool / Spa Test Kit K-2006
  • Tests total alkalinity, free/combined chlorine, cyanuric acid, calcium hardness & pH
  • Extremely accurate (better than test strips)
  • For pools & spas
We're industry experts and only recommend products we would use ourselves. If you click this link, we may earn a commision at no additional cost to you.

Or you can use pool test strips like this:

Leisure Time Test Strips
  • Tests pH, alkalinity, calcium hardness, and chlorine levels
  • Fast and simple testing process
  • For pools and spas

Here is how to test for chlorine demand in a pool using test strips:

  • Dip A Test Strip In The Water 
  • Match The Color On The Test Strip To The Chart. 

Let’s go into detail.

Dip A Test Strip In The Water

Take a test strip from the bottle and dip it in the water for a few seconds. Do not shake or flick the test strip. After removing the test strip from the water, wait a few seconds for colors to appear on the test strip.

Match The Colors On The Test Strip To The Chart

After the colors have appeared on the test strip, match the resulting colors to the color chart on the test strip bottle. The color chart is usually labeled and measured in parts per million. 

By matching the colors on the test strip to the color chart, you will know if the pool is in demand for chlorine.

You can also know this by checking the combined chlorine levels. If the combined chlorine is more than 0.5ppm, then the pool needs more chlorine. The free chlorine level should be at least 3.0ppm. 

A free chlorine level below 3ppm and a high level of combined chlorine both tell you the pool is in high demand for chlorine. 

However, testing for chlorine demand is one thing, knowing how to correct chlorine demand is another. Let’s check out how to correct chlorine demand. 

How To Correct Chlorine Demand 

Since a low free chlorine level is what is causing chlorine demand, you need to up the free chlorine level to correct the chlorine demand. The best way to do this is to super-chlorinate or shock the pool.  

To super-chlorinate the pool, you will need:

  • Calcium Hypochlorite or Pool Shock
  • A Measuring Cup

Before you super-chlorinate the pool, you need to check the CYA (cyanuric acid) levels and the pool pH levels. The cyanuric acid level should be between 30 and 80 ppm (parts per million) and the pool pH should be 7.2 on the pH scale.

We recommend you use this pool test kit.

Any reading above or below these should be corrected as required before you super-chlorinate the pool. 

Now, let’s get to work.

Here is how to super-chlorinate the pool:

  • Turn On The Pump
  • Measure The Calcium Hypochlorite or Shock
  • Add the Shock to thePool
  • Re-Test The Water

Going into details…

1. Turn On The Pump

The pool pump should be turned on to ensure the water is running. When the water is running, there is increased water circulation and this helps the calcium hypochlorite to be fully absorbed in the water. 

2. Measure The Calcium Hypochlorite 

Usually you’ll need 1-3 pounds of cal-hypo per 10,000 gallons of water. Check the manufacturer’s instructions for the exact amount. Only measure the required amount of calcium hypochlorite using the measuring cup. 

3. Super-Chlorinate / Shock The Pool

You should add the required amount of calcium hypo to the pool while the pump is still turned on. Leave the pump on for a few hours after adding the calcium hypo to the water.

We recommend you use this pool shock:

Champion Pool Shock Liquid Chlorine
  • Commercial-strength liquid chlorine (aka bleach or sodium hypochlorite)
  • Rapidly eliminates algae, bacteria, and other contaminants, ensuring crystal-clear water
  • Safe for pools & spas (saltwater & chlorine systems)
  • Use as a shock or regular sanitizer

It is best to do this at night to prevent the sun’s UV rays from destroying chlorine. 

4. Re-Test The Water

In the morning, dip a test strip in the water to check the chlorine levels. There should be more free chlorine in the water now than there was before. 

If the test result shows an active chlorine level of 3ppm or above, you have successfully corrected the pool’s chlorine demand. If you have less than this amount, you can repeat this process until your chlorine levels don’t drop overnight.

But remember, you will only need to correct chlorine demand if you allow chlorine demand to happen. So how do you prevent chlorine demand? Keep reading to find out how.

Related Reading: How to use Pool Chlorine Test Kits

How To Prevent Excessive Chlorine Demand

1. Maintain chlorine levels between 3-5 ppm

This helps you to stay ahead of any potential chlorine demand problem.

2. Regularly brush your walls

Brushing the walls will ensure any potential algae growth and contaminants are removed.

3. Skim the pool to remove large debris

Removing leaves, sticks, insects and flowers is essential to keeping the chlorine demand down. The more organic matter in the water, the higher the chlorine demand.

4. Run the filter for at least 8 hours per day

Running the filter will help to remove debris and organics from the water.

5. Reduce bather load

Finally, you can prevent chlorine demand by reducing the bather load and contaminants that you leave in the water. 

This means no eating or drinking in the pool or peeing in the pool! But you wouldn’t do that right? 

Is It Safe To Swim In A Pool With High Chlorine Demand? 

It’s not safe to swim in a pool with a high chlorine demand. When the chlorine demand is high, you likely have a lot of bacteria or unwanted organic matter in the pool. Swimming in a pool with a high chlorine demand will likely lead to health problems including skin irritation and or swimmer’s eye. 

Final Thoughts 

Now you know that chlorine lock is a myth and if your chlorine doesn’t seem to be working or going up, it’s likely that you have a chlorine demand problem. Overall, chlorine demand can severely affect your ability to enjoy your swimming pool.

But you can prevent it by regularly testing and adjusting the chlorine level in the pool. When there is enough chlorine, your pool won’t demand more. 

Related Reading: How Many Chlorine Tablets Do You Get in a Bucket?