Pool Closing Chemicals: Guide to Winter Chemicals

Pool Closing Chemicals

If you live in a climate that gets frost and snow in the winter, you must protect your pool from the massive damage that can result from freezing water. You may wonder, are pool closing chemicals needed? The answer is yes. Pool winterizing chemicals adjust the pH, alkalinity and hardness and prevent algae to make sure your pool is in tip-top shape for next summer.

Before Adding Pool Closing Chemicals for Winterization

First, clean out your pool by removing solid waste, such as leaves, dirt and insects. It's also a good idea to clean any tile surrounding your pool before winter, as if you wait until spring, it will harden and be more to remove.

Once your pool is clean, test the water for pH, total alkalinity, calcium hardness and chlorine concentration.

How to Add Pool Closing Chemicals Safely

You may typically use certain chemicals in tablet form for regular pool maintenance. However, when it comes to winterizing, this should be avoided, as tablets can cause damage to the pool's fittings or vinyl liner during colder months. Instead, dissolve granular chemicals in water before adding them to the pool water. Liquid chemicals can also be dissolved in water or added directly to the pool's water.

pH Adjustment

Adjust your pool's pH to between 7.4 and 7.6 so the chemicals can work most effectively. If your pH tests lower than 7.4, add soda ash to increase it. If the pH is higher than 7.6, add muriatic or granular acid to bring it down to the right level.

Once you have the pH in the right spot, you can proceed to the next steps, but remember to retest pH levels after you add each chemical.

Total Alkalinity

Next, adjust your pool's alkalinity to between 80 and 120 ppm. This helps to keep the pH stable and within the right range. Increase alkalinity by adding sodium bicarbonate, and decrease it with muriatic or granular acid. Do this very gradually, as these chemicals can alter the pool's pH if added in large quantities.

Water Hardness

The hardness or softness of water refers to the amount of dissolved calcium it contains. High amounts of calcium cause limescale buildup, but low amounts may cause the water to leech this mineral from the pool surround.

Your ideal water hardness level depends on the kind of pool you own. To reduce water hardness, add muriatic acid or sodium bisulfate. To increase hardness, add calcium chloride.


The next step is to shock the pool by adding calcium hypochlorite, or chlorine, to a concentration of at least 3.0 ppm. If the product you use doesn't already contain cyanuric acid as a stabilizer, you can add this separately, if you wish, at a concentration of 40 ppm.

Add Algaecide

The last pool closing chemical needed is a dose of a strong algaecide. As with chlorine, there are winterizing kits available for this purpose, although you can also just add a larger dose of your regular algaecide to get the same effect.

Note that after shocking the water you'll need to wait until the chlorine level falls below 3.0 ppm before adding algaecide. Otherwise the chlorine will inactivate it.

Final Steps

Once you've added all the pool chemicals for winter preparation, you'll have a few additional tasks to prepare your pool for winter. These include removing fittings, such as skimmer baskets, ladders and pool toys; lowering the water level; and draining all water from the pump and any other fixtures and equipment. Depending on the kind of pool you have, you may need to take additional steps as well.

Once you complete these steps, cover the pool--you're ready for winter.

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