Winter Pool Shocking Controversy

shocking a pool during winter
by Rob Cox, October 22, 2018

Shocking a Pool During Winter

winter pool shock


 A debate has been steadily brewing around the pool community on the topic of shocking a pool during winter.

Some say there is no need to do a full shocking of the pool during the off-season winter months. Others insist on doing the opposite, boosting chlorine levels even though the pool is not being used.

And some (like myself) say, "It depends."




winter pool shock


You may want to shock the pool during winter if:

1. The water temperature is above 60 degrees.

2. The pool is not covered with a winter cover or safety cover that blocks the sun.

3. Your level of combined chlorine (chloramines) is above 0.3 ppm.


You may want to shock the pool during winter if:

1. You see visible algae or very poor water clarity.

2. You did not use enough winter algaecide during pool closing.

3. The pool has a poorly fitting pool cover, or if lots of sun and debris are getting through.


If your pool is open and operational, shock the pool using your normal procedure. Keep the pump running for several hours after shocking to distribute the chlorine.

If your pool is closed and covered, pull back the pool cover along one side to add diluted pool shock. Then use your pool brush vigorously, to create circulating currents. This circulation is exremely important in distributing the shock throughout the pool and preventing damage to pool surfaces. Without the help of a pool pump or a brush to distribute the shock, damage can be caused by concentrated chlorine settling in one area of the pool, such as the steps or floor. For this reason, non-chlorine shock is a popular choice for pool owners during the cooler months - it won't stain or damage vinyl and plaster. After shocking and circulating the water, you may also want to keep your pool cover open for several hours to give your pool some air and allow some gassing-off to occur.

You may not need as much pool shock during winter as you normally would to achieve the same effect. The National Institutes of Health found that at normal summer water temperatures, the efficacy of chlorine was 2-3 times less than for water temperatures around 50 degrees. You won't notice this in your chlorine test since emperature doesn't affect the chlorine residual (how much chlorine is free and available). Only the strength or power of the HOCL increases. That said, don't use 2-3 times less pool shock than your normal dosage. However, you can definitely decrease the typical summertime amount by 10-20%.

Your pool water pH level is important. As always, you should do a pH test before shocking, and add a pH decreaser if the reading is above 7.5. Chlorine pool shock has more efficient disinfecting properties in water with a lower pH reading, and it loses effectiveness as the pH scale increases. Bear in mind that if the pH gets too low, the water becomes acidic and will damage your pool surfaces. I like to shock my pool when the pH is at 7.2-7.4.

For uncovered pools, you may also want to check your level of cyanuric acid before shocking a winter pool. True, the sun is not nearly as strong during winter, so a 50 ppm residual of stabilizer or conditioner is not needed. However, aim to have at least 20 ppm of CYA (cyanuric acid) in the water, as measured by your local pool store, or by using your own cyanuric acid test. High CYA content in the water can significantly decrease the effectiveness of chlorine, so be careful not to let the concentration get too high.