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How Do I Fix Chlorine Demand and Chlorine Lock?

Are you having problems getting a sustained level of chlorine in your pool, despite regular maintenance and shocking? If so, your pool may have a chlorine demand problem.

How Chlorine Demand Occurs

Chlorine demand problem occurs when Free Chlorine is not available to sanitize the pool. What happens is most of the chlorine added to the water reacts with organic matter or other chemicals to form compounds called chloramines. Even though the total chlorine content in the pool is high, the Free Chlorine is consistently too low to sanitize. The cure usually involves adding large amounts of shock chemicals to the water to remove the chloramines, and then following up with routine shocking.

So what are chloramines?

Chloramines are also known as “combined-chlorines” because they are molecules formed by combining chlorine and organic waste such as saliva, perspiration, and body soil in the form nitrogen or ammonia. Chlorine is a not really particular about what it combines with and it will react with almost any substance in the water. The more debris that is in the pool water, the more the chlorine is exhausted and the more chloramines form. Chloramines produce the strong chlorine smell that people complain about too. As a rule, what causes chlorine demand is an unattended pool or a pool closed at the end of the prior season without being thoroughly cleaned. When it comes to pool maintenance, cleaning and closing your pool properly actually saves you work in the long run. While your pool is in use, add adequate sanitizer on a daily basis. Use the correct number of chlorine sticks or tablets and make sure that they are eroding as they have in the past. If your pool temperature has dropped, the chlorine will erode more slowly. Shock your pool once a week too. This replenishes the Free Chlorine so it can break down whatever is in your pool. If you have a solar blanket on your pool, remove it. The chemical reaction needs to gas off. There are also times when there is a chlorine demand and you have done everything you can to take care of your pool but some things are beyond your control. Heavy rainfall can wash unwanted chemicals into the water such as fertilizers and other chemicals. Some household bleach products contain more substances such as phosphates or nitrates that will interfere with the chlorine’s ability to sanitize the water too. Another reason some pool owners are seeing the chlorine demand issue more and more is because their municipalities use chloramine and not chlorine to sanitize their drinking water. If you see pink rings around your toilet, tub or pool this is likely the cause. Chloramines are considered less "offensive" than chlorine. The problem is that every time you top off your pool with the garden hose, you're putting more chloramines into your pool so it compounds the problem without you even knowing it.

Fixing The Problem

How do you fix this problem? First, test and adjust the pH to 7.4 - 7.6 and make sure your cyanuric acid (stabilizer) is between 30 ppm and 80 ppm. Then shock the pool at night so your chlorine isn’t affected by the sun. Use 3 lbs. of shock per 10,000 gallons of water. If you have a 21,000 gallon pool, add 9 pounds of pool shock. Check the chlorine reading the next day and continue with this routine until your chlorine residual reaches 1.0 - 3.0 ppm. This is enough chlorine to do the job, but be sure that you test your water and shock regularly to prevent this from happening again. If you have ever seen the term “chlorine lock” ignore it. The term describes a state where an excessive amount of cyanuric acid is said to cause chlorine to stop working. There is no evidence to support this claim and the two terms, chlorine lock and chlorine demand are NOT interchangeable. Chlorine demand is science – chlorine lock is a sales pitch. Have a question about using the right chemicals for your pool?  Feel free to send us a message here. Discount Pool Supply is Canada's leading online store for all of your pol and hot tub needs.  We offer free and fast shipping anywhere in Canada.

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