The Role of Calcium Hardness in Water Balance


While hardness in water consists of both calcium and magnesium salts (“total hardness”), only the calcium component is relevant in the water balance calculation for pools and spas. Called the Saturation Index (SI) formula, it considers the interrelationships of four chemical factors—calcium hardness; pH; total alkalinity, as corrected for the contribution of any cyanuric acid stabilizer in use; and, to a much lesser extent, the total dissolved solids level—plus one physical factor, water temperature. All but the mathletes among us use a water balance calculator like Taylor Technologies’ Watergram® to do the number crunching involved. The value one arrives at reflects the water’s degree of saturation with calcium carbonate.

When the SI value is zero, the water is properly balanced. Its calcium hardness, pH, and total alkalinity are acting in harmony with one another. When the SI is +0.5 or more, the unbalanced water is trending toward scaling, meaning conditions are right for calcium carbonate to come out of solution and deposit on surfaces as “scale.” When the SI is -0.5 or less, the unbalanced water is trending toward corrosivity. Corrosive water attacks plaster, concrete, grout, and metal, resulting in etching, pitting, and surface stains and/or colored water caused by metal pulled out of piping, fittings, and equipment.

Can damage from scaling water be reversed? By reducing the SI to around -1.0 for a short time (usually by lowering pH), some calcium deposits in the filter and circulation piping can be dissolved, and the water’s flow may even remove chunks of loosened scale. But this will come at a price. A thin layer of the concrete surface of the pool may dissolve as well, and copper can be lost from piping and/or heat exchangers.

Damage from corrosive water cannot be reversed, only repaired by resurfacing the concrete and replacing piping. Colored water can be cured with a “metal out” product, or chelating agent, and some of these are reportedly successful at removing certain surface stains.


The Effects of High and Low Calcium Hardness
Specifically, water with high calcium hardness gets cloudy unless the alkalinity and/or pH are low enough to compensate. As mentioned, the excess calcium carbonate will precipitate as crusty, grayish white scale on surfaces, piping, and equipment. It’s unsightly, can cause abrasions on users and snag bathing suits, and makes a good anchor for microorganisms. It will clog filters. When it builds up in piping, circulation is reduced and pressure increases. Scaling is an especially acute problem in heaters because calcium’s solubility is inversely proportional to temperature:  as temperature increases, less calcium is able to stay dissolved. Scale on the pipes or coils acts as an insulator, slowing heat transfer. This makes it more expensive to heat the water. Over time, thick scale will cause a heater to fail.

Water with low calcium hardness will seek more by dissolving it from surfaces it comes in contact with that contain calcium, such as plaster, grout, and concrete decking. The late Dr. Neil Lowry, a well respected instructor in our industry, preferred to call water with low calcium hardness “aggressive” rather than “corrosive” because the latter term implies the destruction of metals. He would point out that copper water pipes in homes equipped with water softeners last for decades! The corrosiveness of unbalanced water, he would tell his students, comes from poorly maintained alkalinity and pH.

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