Calcium Hypochlorite: Winter Pool Shock

Dr. Pool of In The Swim

Hello class – welcome back from fall break. My lecture today looks at the ways to use calcium hypochlorite pool shock in your pool, and specifically for winter use in colder temperature water.

Calcium hypochlorite is a granular, powered chlorine product that is made by joining calcium and hypochlorite ions in an aqueous solution. Crystals of salts form, and are quickly separated and dried to produce a soluble form of powdered chlorine.

In our test labs, we have developed three different versions of calcium hypochlorite – Pool Shock, Super Pool Shock and Instant Liquid Chlorine.

regular-pool-shockPool Shock: Our regular strength calcium hypochlorite is 68% available chlorine. This means that 68% of each pound of pool shock is calcium hypochlorite ions, the remaining inert ingredients are binders and salts produced during the reaction with lime used in manufacture.

These binders will release and dissolve within 15 seconds, in 70° pool water that is circulating. In a winter pool, colder and uncirculated, it can take twice as long to dissolve. It’s always best to pre-dissolve calcium hypochlorite.

To dissolve calcium hypochlorite, pour 1-2 lbs of  the pool shock into a clean 5-gal bucket that is full of water. To prevent a volatile reaction, always add shock to water, not water to shock. Stir it with a suitable instrument for 15-30 seconds, depending on the water temperature. Pour it into the pool, but before it’s completely empty, stop pouring and fill the bucket with pool water. Stir again and then pour in the remaining solution.

Super Pool ShockSuper Pool Shock: Our enhanced strength form of calcium hypochlorite. By boosting the reaction time during manufacture, we were able to increase the strength of the product to 73% available chlorine, reducing the inert ingredients to 27%.

Super Pool Shock is not only a stronger form of calcium hypochlorite, but we were also able to reduce the size of the crystals, resulting in a granules of a smaller size which dissolve at a faster rate. Super pool shock will dissolve in 8 seconds, in a circulating pool of 70° water. The rate of dissolution doubles when water temperatures fall below 60 degrees.

With fewer binders to separate, Super Pool Shock also has the advantage of being cleaner to use. Less pool shock binders to separate and a lower contribution to calcium hardness levels in the pool water, as compared to our regular strength calcium hypochlorite products.

Instant Liquid ChlorineInstant Liquid Chlorine: By modifying our reaction time and drying process further, we were able to create a new product recently, which we call Instant Liquid Chlorine. Think of it as a form of dehydrated liquid chlorine salts. The granules formed are even smaller than our Super Pool Shock, and are programmed to rehydrate within 2 seconds, upon being added to water.

The best thing about this dry liquid chlorine formula is that it dissolves very rapidly, even in cold water temperatures. We’ve packaged it in an easy to pour bottle, just walk it around the pool – no need to pre-dissolve.

Winter Pool Shock: Any of these calcium hypochlorite products can be used for winter pool chlorination, by dosing at a rate of 1 oz. per 5,000 gallons of pool water. This should last for 3-4 weeks in a covered and closed pool.

Uncovered winter pools with water temperatures below 60° should add a double dose, and depending on the sun exposure and temperature, will lose their chlorine residual in half the time, or within 7-14 days.

There is usually no need to ‘shock’ a winter pool with large amounts of cal hypo if water temperatures are below 60° – just use enough to maintain a residual of 1-2 ppm of free chlorine.

Class Dismissed!


Dr. Pool



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