by Rob Cox January 07, 2014
A debacle of a debate has been brewing around the pool community regarding 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, while others state the opposite, boosting their chlorine levels even though the pool is not being used.
And some (like myself) say, It depends...
OPEN & OPERATIONAL POOLS - 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 pool cover that blocks the sun
3. Your level of combined chlorine (chloramines) is above 0.3 ppm
CLOSED & COVERED POOLS - you may want to Shock the Pool during Winter If:
1. You see visible algae or very poor water clarity
2. If you did not use enough winter algaecide during pool closing
3. When the pool has a poor fitting pool cover, with lots of sun and debris 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. You may 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 - temperature doesn't affect the chlorine residual (how much is Free and Available), only the strength or power of the HOCL increases. So, don't use 2 or 3 times less pool shock, but you could probably decrease your normal summer shock dose 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. According to a chlorine fact sheet produced by Oregon State, the optimum pH for chlorine disinfection is 4.5. This is the pH level at which your pool shock has the most efficacy, or killing power. However! If you lower your pH that much, the water becomes very aggressively acidic, and can damage your pool surfaces. I like to shock my pool with the pH at 7.2.
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, but strive to at least have 20 ppm of CYA (cyanuric acid) in the water, as measured by your local pool store, or by using your own cyanuric acid test.