7 Ways to Fix Chlorine Lock and Chlorine Demand in Your Pool

If the pool cannot maintain the chlorine, and you can’t get the chlorine level up fast enough, there is a risk that the water will quickly become cloudy.

But why does the chlorine content in the pool water decrease, and how to fix this?

In this article, I’ll explain why your pool can’t maintain chlorine levels and how to solve the problem.

What is chlorine demand and chlorine lock?

Often beginners have a problem with the chlorine demand and the chlorine lock, but what exactly is this?

Sometimes the term “chlorine lock” or “chlorine lockout” is used. This cannot be very clear because it does not mean that hydrogen (H) and oxygen (O) are locked in. Instead of “lock,” sometimes other terms like “surge” are also used.

This is because the pool’s chlorine level suddenly rises and then falls again rapidly.

What is chlorine demand?

Chlorine demand in a pool means that the chlorine in your pool is consumed faster than if it was added.

Chlorine demand describes the loss of free chlorine from a pool due to sun exposure, rainwater, bather load, and water evaporation.

The amount of chlorine lost in a day is usually 2 to 4 parts per million (ppm) or 1 to 2 ppm depending on whether there are many bathers or not.

A higher number of bathers in a pool corresponds to more water loss due to more frequent swimming and unforeseen showers (rain).

Showers, rain, and solar radiation are the most common causes of chlorine demand.

The lost chlorine with these means must be replaced by adding chlorinating products like a chlorine tablet.

Short answer: Chlorine demand is a measurement of the amount of chlorine required to keep a certain water supply free from bacteria. Some substances can bind with chlorine to form chloramines in pool water. The chlorine demand says how much the water uses up the chlorine. The lower the chlorine demand, the better.

What is chlorine lock?

Chlorine lock in a pool is when the chlorine is ‘locked’ in the water. This often happens when you shock or super chlorinate your pool, and it’s not removing all of the chloramines from the water.

Chlorine lock in a pool can be extremely frustrating because you will notice that there are no more active free chlorine molecules in your pool.

Yet your chemical levels like pH, alkalinity, the Total Dissolved Solids (TDS), and even your cyanuric acid may say otherwise.

The most common symptom of chlorine lock is a yeast-like smell to the water.

Let’s take a closer look at the most common causes of chlorine demand and chlorine lock and see how you can quickly fix the problem.

#1 Excessive level of contaminants in the pool

There are too many contaminants in the pool, which may cause the chlorine to break down quickly – the free chlorine is used much faster.

If there isn’t enough free chlorine in water, bacteria can proliferate and lead to cloudy water. This can be dangerous for your family, especially if you have small children.

Check if your pool has high amounts of contaminants. The most common are oils and scum from swimmer’s bodies, urine, sweat, dirt, or mud.

A method you can use to check the level of contaminants in your pool is by using a TDS (total dissolved solids) meter.

This handy device lets you measure the number of contaminants to identify what could cause excessive chlorine loss in your pool.

You can buy one at any pool store, and they are cheap and easy to use. Just dip it into the pool water for a few seconds until it starts to beep. The more contaminants, the faster it will beep.

If you notice that the pool has high levels of algae or bacteria, you need to shock your pool water with chlorine before using it again – this is needed to clean the pool and boost the level of available chlorine in the water.

#2 Too much cyanuric acid in the pool water

Cyanuric acid is a chlorine stabilizer that reserves chlorine. If the cyanuric acid level is too high, it becomes difficult for the chlorine to act effectively against dirt and algae in the water.

You have to test the cyanuric level to know if it’s time to add a stabilizer or chlorine.

The ideal range for cyanuric acid is between 30 to 50 ppm.

  • If the pool water has a level of more than 50 ppm, make sure to get rid of 1/5 of the pool water to lower the stabilizer.
  • If it is below 30 ppm, then you have to add chlorine to compensate for the lack of cyanuric acid in the pool.

Read more about cyanuric acid in my beginner’s guide. Just follow the link to read the post.

#3 The pH level in your pool is too low

Remember, if the pH level in your pool is too low, then the water is more acidic. The chlorine consumption will be much higher, and therefore your pool can’t hold the chlorine level.

If your pool’s pH level is too low, it will be necessary to raise the value with a pH increaser product before the chlorine content starts to rise again. Otherwise, the chlorine level will start to decrease rapidly.

Test the pH level daily and ensure it is between 7.2 – 7.6.

Make sure that you use the right chemistry product for your pool water. There are different products for high or low pH levels.

Read in my guide what causes a low pH and learn how to raise the pH level in a pool.

#4 The pool water is heated too much

If the water in your pool is too hot, then the chlorine will lose some of its ability to kill fungus and bacteria.

This means that the water in the pool becomes cloudy because these microorganisms multiply rapidly.

When the water is too warm, it’s recommended that you turn off the heating, so the chlorine levels increase and maintain a uniform concentration throughout your pool.

If you live in a region where it is quite warm during summer, be sure to check the temperature of the water every day.

If it’s too warm, there are several things you can do to cool down your pool. Read my guide on how to cool a pool.

#5 Direct UV rays from sunlight destroy the chlorine

Direct sunlight is bad for the chlorine content.

The chlorines are sensitive towards UV-rays from sunlight and are quickly destroyed when irradiated by strong direct sunlight.

The chlorine won’t dissolve properly in water either since it’s being eaten away by the sunlight. It would help if you always tried to keep your pool shady during daylight hours to dissolve the chlorine properly.

You should also use some of the cyanuric acids to stabilize the chlorine if you don’t use already di-chlorine or tri-chlorine (already stabilized chlorine).

But remember, too much of the chlorine stabilizer is also bad!

#6 Combining chlorine with active oxygen and other disinfectants

When you combine chlorine with other disinfectants in the water, it is important to know that the chemicals cancel each other out.

For example, you use di-chlorine and add some bromine to the pool. Or you use chlorine tablets and pure some active oxygen in the water.

Stop it!

Choose a single agent for pool water disinfection and stick with it!

If you want to change the agent, the pool may need to be completely drained, scrubbed, and refilled.

I have written about this topic sufficiently in my article about the alternatives to chlorine. If you are interested in the topic, read the article at the link.

#7 More swimmers in the pool and not adjusting the chlorine level

Your pool can handle just a certain number of bathers. If you suddenly get more swimmers than normal, the chlorine level will decrease.

If too many people have been in the pool for several hours or some children play with water from the pool, this might also have consequences for the chlorine level.

In these cases, it’s important to add some chlorine to the pool.

Make sure you use a chlorine tablet in your floater or the skimmer basket. If necessary, add one more tablet to it.

Tip: You can keep the same amount of chlorine by simply forcing your guests to take a shower before going into the pool. This will lower contaminants and reduce your chlorine consumption in the pool water.

Why is there no free chlorine in the pool after shocking?

You shocked your pool, and you tested zero free chlorine. What is going on?

When you shock your pool, you add extra chlorine to burn off the organic contaminants. If the level of contaminants is too high, then the free chlorine will get used much faster.

Maybe you have used too little of the shock, or you shock your pool at the wrong time at noon.

The direct sunlight will dissolve the chlorine much faster because a pool shock is normally not stabilized with cyanuric acid.

In both cases, you should read my full instructions on how to shock your pool with bleach. I also recommend using chlorine tablets to ensure that there is always sufficient free chlorine in the pool.

How to raise the chlorine level of your pool?

You now know the most common reasons why your pool can’t maintain chlorine levels.

Let’s move on to raising the chlorine level in pool water properly.

There are several ways to raise your pool’s free chlorine, but if you have been battling this problem for some time, these methods won’t help as much as they would if you had never let the chlorine drop in the first place.

Here are some simple steps to get it back up:

  1. First, test the pH and total alkalinity in your pool water. If these levels are off, they can inhibit chlorine from working properly. Then check your stabilizer level.
  2. Test the water and determine the amount of chlorine needed to back up your level. You can do this by measuring the ppm (parts per million) of free chlorine with a test kit or electronic instrument.
  3. Once you know how much chlorine is needed, add it slowly, about an hour apart, until you achieve the desired level.
  4. Start your pool pump and filter system when you add the chlorine. This will allow it to circulate throughout your pool water more freely and help maintain its level.
  5. Add an extra chlorine tablet into the floater or the skimmer to produce extra chlorine for your pool.

Keeping your pool water balanced will help keep your chlorine level optimum working efficiency.

If you’ve battled to keep your chlorine levels up, you probably need to make some big changes. Consider adding an Ozonator or convert your pool to a saltwater pool.

Ozone generators are great for killing bacteria and viruses quickly.

You can’t stop mother nature, but you can use technology to help the environment and your health by killing contaminants in the water that you swim in.

Ozone generators work well with chlorine. The two combined provide a potent way to kill germs while reducing exposure to carcinogens, oxidants, and other problem compounds often contained in untreated water.

The germ-busting ozone will also break down organics and make your pool sparkle in a matter of hours, not days, as chlorine does.

Yes, ozonized water is more expensive than chlorine. Still, if you factor in the price of adding chemicals to your pool over time, it’ll be cheaper because you won’t be adding salt, clarifiers, and other remedies that need to be added regularly.

Do you have further questions about the chlorine level in your pool?

The best way to contact me is @contactswimfool on Twitter or Pinterest. I am happy to help you.

Have fun swimming!

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Hi. I'm Max Berg. I've been in the pool industry since 2015 and have always felt drawn to water. I'm the author behind swimfool.com, where I share my years of experience in pool maintenance and give helpful tips on keeping a swimming pool or hot tub clean. My tips reduce the costs of water treatment and protect the environment.

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