How to Cure Chlorine Demand and Chlorine Lock

Your swimming pool is a hungry monster and your job, as its caretaker, is to feed and calm the beast.

Sometimes, when you have a lot of debris and other organic material overtaking your pool, it will require a lot of chlorine to handle the situation. This is called chlorine demand.

You will notice this problem if you constantly have an issue getting a chlorine reading, even after adding plenty of chlorine or shocking the water with a lot of chlorine. This means your pool has a lot of work to do fighting its containment and needs all the chlorine you can feed it. So, feed the beast!

Common Causes

Chlorine demand is rare but not uncommon. Its known to occur at the beginning of the spring when you open your pool. When the water has been stagnate and untreated, it can gain a lot of organic and inorganic debris that will take a lot of chlorine to neutralize. This could also happen if the pool is open, but hasn’t been cared for in a long time.

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

Be sure to keep fertilizers or other non-pool chemicals away from your water as well.

How to Cure Chlorine Demand

Your pool demands chlorine, so feed it what it wants. I recommend using calcium hypochlorite pool shock and adding 3 pounds per 10,000 gallons of pool water. For example, if you have a 21,000 gallon pool, I would add 9 pounds of pool shock.

Before adding pool shock, make sure you cyanuric acid (chlorine stabilizer) levels are correct (30 to 80 ppm) and remember to shock at night because the sun eats up chlorine.

Is Chlorine Demand the same as Chlorine Lock?

Chlorine demand is sometimes referred to as “chlorine lock,” but these are not the same issues. Chlorine lock is when chlorine is not working because it’s being “locked up” or inhibited from doing its job by cyanuric acid (chlorine stabilizer). According to the experts over at, it’s safe to say that chlorine lock just isn’t a real issue. In fact, there is no proof that an excessive amount of cyanuric acid will cause the chlorine “lock.”

Don’t be confused by the two. Chlorine demand is real and solvable by adding enough chlorine to your pool until it starts to read on your liquid test kit or test strips. Chlorine lock is a false issue, but keep in mind that some pool companies and experts will use these two phrases interchangeably. Take comfort in the fact that whichever the phrase used, both are solvable by feeding the beast.

Happy Swimming!

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