Understanding chlorine lock for pool maintenance

Water chemistry is something that all pool owners have to master to keep their backyard swimming pools crystal clear and sanitary for swimmers. Pool water maintenance involves an understanding of water pH, sanitizer demand, sunlight, additional weather conditions, and various other factors. It’s not just a matter of dumping in chemicals and hoping for the best.
According to Pool magazine, chemical overdosing is one of the most overlooked cost drivers in DIY pool care and a big contributor to maintenance frustration. When homeowners do not test consistently or misinterpret test results, they often add more chemicals than the pool actually needs. Overdosing can cause secondary problems that may turn out to be even bigger headaches. Chlorine lock can be one of those problems.
Chlorine lock occurs when the sanitizer no longer cleans the pool properly. Chlorine testing will result in different numbers: free chlorine, combined chlorine and total chlorine. The pool care resource Mr. Pool Man says free chlorine is the amount of chlorine that is actually working to sanitize the pool. Combined chlorine is the used-up chlorine bound to contaminants, known as chloramines. Total chlorine is the total amount of free chlorine and combined chlorine in the pool. When total chlorine is higher than free chlorine, it means that there is a larger amount of “used up” chlorine in the pool and the sanitizer is no longer effective.
While this can occur due to demand on the chlorine, such as after a pool party with a lot of swimmers or after a particularly long run of rainy weather that leads to an algae bloom, sometimes chlorine fails to be effective due to the stabilizer in the product having built up. This can render the chlorine useless and no matter how much chlorine is added it won’t work.
Cyanuric acid (CYA), also known as stabilizer, is added to many chlorine formulations to protect chlorine from UV rays. Without stabilizer, the chlorine breaks down much too quickly and it will have to be added at a higher rate. However, when homeowners use multiple chlorine products, such as chlorine tablets and “shock” containing CYA throughout the season, they may inadvertently add much more stabilizer to the water than is warranted. This over-accumulation of stabilizer will then make the chlorine stop working, and the pool will become fouled. Individuals may compound the problem by adding more and more chlorine (containing CYA) to the water, further raising stabilizer levels.
It is very important to invest in water test kits that also test for CYA. This way the proper levels can be maintained. Also, some chlorine products, such as liquid sodium hypochlorite and calcium hypochlorite, are sanitizers that tend to be stabilizer-free. Products marketed as “trichlor” and “dichlor” contain cyanauric acid. 
Addressing chlorine lock often comes down to partially draining the pool as the most cost-effective way to do it, as CYA doesn’t dissipate. Global Pool Products reports another option is to add a certain chemical additive depending on the cause of chlorine lock. Those unsure should ask a pool professional for advice.
Switching to unstabilized chlorine and manually adding stabilizer to achieve the proper level may help prevent chlorine lock in the future; otherwise, pool owners have to keep a careful watch on stabilizer levels if they are using stabilized chlorine products.