When Do You Shock a Pool?
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When is the best time to shock your swimming pool? First we need to clarify something which is a lot of people misuse the term "shock." It’s not the best word because it gets thrown around too often and usually incorrectly. What you’re really talking about is adding chlorine to your pool with one exception: if you plan to use a non-chlorine shock, also called an oxidizer, it’s still technically a shock product. You would use that anytime you need a fast acting solution like if the water isn’t looking great, or you have a lot of swimmers coming in, but you still want people to be able to swim shortly after. Always follow the manufacturer’s instructions - many will say safe to swim within 30 minutes to an hour.
Shocking your pool really should refer to breakpoint chlorination which is the process of destroying built up combined chlorine in the water. However, pool owners tend to refer to shocking the pool any time they add chlorine (or an oxidizer) and that is what creates this confusion. What you need to know is that you need to maintain your chlorine levels at 2 to 4 parts per million of free chlorine, and maintain your combined chlorine level at zero, ideally. Use breakpoint chlorination when combined chlorine levels climb above zero. Add an oxidizer to the pool to help maintain clarity and chlorine reserve when you have a lot of swimmers but need to let everyone back in the pool soon. It really is shocking how often shock is used incorrectly in regards to pools. No wonder owning and maintaining a pool can be so overwhelming to pool owners. If you feel overwhelmed you can check out the new pool owners page. For chemistry help check out the pool chemistry crash course.
Breakpoint chlorination – This is the real definition of "shocking" your pool. Breakpoint chlorination is when you add enough chlorine to completely destroy combined chlorine (chloramines) that have built up in the water. You’re not just boosting chlorine levels...you are forcing a chemical reset so your free chlorine can work properly again. To achieve breakpoint chlorination and destroy built up combined chlorine you must increase free chlorine levels in the water to be ten times the value of the combined chlorine level, and then hold it there for 12 to 14 hours sustained. This destroys chloramines and when the chlorine levels drop back down your combined chlorine level should be zero again.
Non-chlorine shock – These products use oxidizers instead of chlorine to clean up organic contaminants. They don’t raise chlorine levels, but they do help restore clarity, improve sanitizer efficiency, and let swimmers back in the water sooner. Great when you want fast results without waiting for chlorine to drop. Potassium monopersulfate and hydrogen peroxide are two examples of these non-chlorine shock chemical options.
Chlorine shock – This is simply a form of chlorine which quickly raises sanitizer levels so technically any time you are adding chlorine to a pool you might say you are adding shock but it would be more accurate to simply state you are adding chlorine to increase the free chlorine level.
So why is shocking to commonly misused? Most pool owners say they “shocked the pool” any time they add chlorine, which isn’t accurate. Tossing in some extra chlorine is just adding chlorine. True shocking means reaching breakpoint chlorination or using a dedicated oxidizer. Using the wrong term leads to confusion about what problem you’re actually trying to solve clarity, contaminants, or combined chlorine.
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Swimming Pool Steve is an award winning, second generation swimming pool specialist from Ontario Canada and one of the most trusted voices in the swimming pool industry. With over 20,000,000 views on the Swimming Pool Steve YouTube Channel, winner of the Pleatco Pool & Spa Industry Leadership award and author of hundreds of pool and spa articles both online and in print. Steve is committed to helping pool and spa owners as well as pool and spa industry workers learn more about the technical side of building, renovating, repairing and maintaining all types of swimming pools and spas. Follow Swimming Pool Steve on Facebook, Twitter and YouTube.
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