How To Shock a Pool
Before we discuss how to shock a pool, let’s explain why and when to do it. Shocking a pool is part of regular maintenance during the warmer months. It is also done throughout the entire year, just less often during cooler months. Pool owners and pool service professionals shock a pool to “super-chlorinate” the water. Shocking a swimming pool will destroy bacteria, algae and organic waste that regular chlorination may have missed.
Why Shock a Pool?
Chlorine levels will fluctuate depending on the bather load and contaminants in your pool water. Pool chlorine is engineered to attack and kill bacteria so the pool is safe to swim, but will also bind to ammonia in water, forming chloramines. This type of wastes come from swimmers, fertilizers, bird droppings, sunscreen lotions, etc. The only way to release the bound chlorine (chloramines) is by shocking the pool.
When to Shock a Pool?
A regular weekly maintenance schedule of shocking and oxidizing would be to alternate between a chlorine shock and non-chlorine shock each week, depending on the tested levels of chlorine. For example, use a chlorine-based shock one week, and chlorine-free shock the next week to oxidize contaminants out of the water. This will help reduce the amount of chlorine needed and increase the overall effectiveness of the chlorine.
So, when should you shock the pool? By testing the water each week (during the swimming season), you will see the difference between Free Available Chlorine (FAC) and Total Available Chlorine (CAC) levels. We’ll explain the Chlorine definitions in the next section.
Rain & Thunderstorms: During storms, your pool is likely to be contaminated by rain water, dust, and other debris. Shocking after severe weather will help clean up after Mother Nature.
Opening & Closing: When opening your pool, it's recommended that you shock it after balancing the other chemicals. This will oxidize the particles, kill bacteria, and clarify the water. When it's time to close your pool, shocking will help fight off contaminants over the long winter months.
Algae Outbreaks: When algae appears in your pool, super-chlorination with powerful types of shock like Power Powder Pro or other variations of Cal-Hypo can often eliminate the problem.
Heavy Pool Usage: When pools are used by a large number of people in a short amount of time, the water can become contaminated in a variety of ways. Shocking after parties or other unexpected contaminations is highly recommended.
Low/No Chlorine: If there's not enough chlorine in your pool, superchlorinate the water to get it back on track. Make sure Free Available Chlorine levels stay within the ideal range of 2.0-4.0 ppm at all times.
Remove Chloramines: If your Total Available Chlorine level is 0.3 ppm or higher above your Free Available Chlorine level, it's time to shock the pool. This will break down the inactive Combined Chlorine in the water.
Fecal Incident: If there's been a fecal accident or other unsanitary incident, close the pool for swimming. Remove any debris, and superchlorinate the pool to kill off the germs and break down organic contaminants.
When your chlorine is in the proper range between 2-4ppm, we recommend using a non-chlorine shock like Leslie’s Fresh 'N Clear to oxidize and burn off the ammonia. The goal is for your Free Chlorine and Total Chlorine to be the same level at all times.
Types of Chlorine
Before shocking the pool, we need to understand the two types of chlorine that impact pool water chemistry.
Free Chlorine - Free Available Chlorine (FC or FAC) is the effective chlorine available to sanitize the pool water. It quickly kills contaminants and disinfects the water.
Combined Chlorine - (CAC) Have you ever smelled that “chlorine smell” while visiting a public pool or water park? This memorable scent is actually from the non-effective (dead) chloramines that form when not enough Free Chlorine is present.
Total Chlorine - Total Available Chlorine (TAC) The total amount of Free and Combined Chlorine. (FAC + CAC = TAC)
Types of Pool Shock
Now, let’s discuss the three types of pool shock.
Cal-Hypo - Calcium Hypochlorite pool shock is a powerful and effective form of chlorine. It quickly kills algae and contaminants and is ideal for pools with a normal-to-low pH level. This is because Cal-Hypo contains a high pH level (10.8) and contains calcium, which can contribute to the Calcium Hardness level.
Di-Chlor - Dichloro-S-Triazinetrione pool shock is a fast dissolving solution and will not raise the pH level. Di-Chlor also will not cloud the pool water. Just keep in mind that this type of chlorine pool shock contains CYA, which will increase the residual CYA level over time.
Chlorine-Free - Chlorine-free pool shock is great for quickly oxidizing contaminants, but will not kills germs or algae. This type of shock will improve water clarity, while complimenting enzyme treatments.
How to Shock a Pool
Step 1. Test the Water
Step 2. Circulate the Water
Turn on your pump and filter system to circulate the water. Now, pour the shock/water mixture around the inside perimeter of your pool. The best time to shock is at dusk or night, and then let your pump filter system run overnight.
Step 3. Wait for FAC Level to Drop
Continue letting the pump to run for about 6 hours and then re-test the water. The Free Available Chlorine (FAC) level may be very high, making it unsafe to swim.
Step 4. Enjoy Clean, Safe Water
Wait until the FAC level falls to 1 – 4 ppm before jumping in. Now, go enjoy your clear, clean and safe swimming pool!
Learn more about the importance of shocking a swimming pool in this helpful video: