Understanding and Managing Calcium Hardness in Pool & Spa Water

Calcium hardness must be actively managed — along with pH and total alkalinity — to keep pool water in proper chemical balance. Industry standards call for maintaining calcium hardness in the ideal range of 200–400 ppm in pools and 150–250 ppm in spas. 

Calcium Hardness and the Saturation Index 

While “total hardness” in water consists of both calcium and magnesium salts, only the calcium component is relevant in the water balance calculation for pools and spas. The Saturation Index (SI) formula takes into consideration 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 level of total dissolved solids, plus one physical factor — water temperature. Unless you are a math expert, it is best to use a water balance calculator like Taylor Watergram® to do the number crunching involved. The value arrived at reflects the water's degree of saturation with calcium carbonate. 

When the SI value is zero, the water is properly balanced. That means 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 trends toward scaling, meaning conditions are right for calcium carbonate to come out of solution and deposit on surfaces as "scale." When the SI is -0.3 or less, the unbalanced water trends 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 pipes, fittings, and equipment. 

Damage from scaling water can sometimes 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 reversal will come at a price. A thin layer of the concrete pool surface 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 pool 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 Calcium Hardness 

Specifically, water with high calcium hardness gets cloudy unless the alkalinity and/or pH are low enough to compensate. The excess calcium carbonate will cause crusty, grayish white scale on surfaces, piping, and equipment. Not only is it unsightly, it can also cause abrasions on bathers, snag swimsuits and it can become an “anchor” for microorganisms and clog filters. When scale builds up in piping, circulation is reduced and pressure increases. Scaling is an especially serious problem in heaters because as temperature increases, less calcium is able to stay dissolved. Scale on the heater’s pipes or coils acts as an insulator, slowing heat transfer which makes it more expensive to heat the water. Over time, thick scale can eventually cause a heater to fail. 

The Effects of Low Calcium Hardness  

 Water with low calcium hardness will seek out more by dissolving it from surfaces that contain calcium. Unfortunately, these include pool plaster, tile grout, and concrete decking. Water with low calcium hardness is referred to as "corrosive" or “aggressive” due to its destructiveness that comes from poorly maintained alkalinity and pH. 

Calcium Hardness Summary

Calcium Hardness Testing 

To avoid damage to pools and spas from unbalanced water, test calcium hardness at least monthly. Calcium hardness is best monitored with a drop-count titration. Test strips can only measure total hardness. In addition, test strips for total hardness have just four or five color blocks to cover a tremendously wide range—from 0 to 1,000 parts per million (ppm) with color blocks for 0, 100, 250, 500, & 1,000. A drop test will allow you to approach the true concentration of calcium hardness in 10 ppm increments. For example, if you're trying to balance water in a spa by increasing the calcium hardness from 60 ppm to 150 ppm, the strip can't help but the drop test can. 

To perform a drop test, simply fill a test cell with sample water, add buffer and indicator reagents, and swirl to mix. The sample will turn red if calcium hardness is present. Next, add the calcium hardness reagent, swirling and counting each drop until the color changes from red to blue. Finally, multiply the number of drops used by the equivalence factor in the test instructions to determine the calcium hardness level. The reading is stated as parts per million as calcium carbonate. 

NOTE: When testing calcium hardness, you may occasionally get a purple endpoint instead of blue. This is called a "fading endpoint". It is due to interference from metal ions (most likely copper from algaecides, pipes, or source water). When this happens, you should re-test, but prevent the interference by adding five or six drops of hardness reagent to the sample before adding the buffer and indicator. Then proceed as usual. Remember to count the drops of titrant added at the beginning when you calculate the total number of drops required to reach the endpoint. 

The color progression in a hardness test is from red, to a mixture of red and blue, to blue. To be certain the color change is permanent, indicating you have reached the true endpoint, add one more drop of titrant. If the blue color remains unchanged, do not count this drop.

Adjusting Calcium Hardness 

You can easily raise water's calcium hardness by adding calcium chloride (CaCl2). Two forms are sold: hydrated (77% strength) and anhydrous (100% strength). Each will generate heat when contact with water is made. Therefore, generally you are instructed NOT to pre-dissolve the calcium chloride in a bucket, but to broadcast it over the water's surface with the pump running. Note that adding calcium chloride in the hours before or after treating with soda ash (sodium carbonate) or baking soda (sodium bicarbonate) will result in cloudy water. 

There are two ways to decrease calcium hardness. You can partially drain the pool or spa and refill it with lower-hardness water. However, if replacing water is not a viable option as in areas where drought is causing limits on water usage — unless pH and alkalinity are already too low — it will often be more practical to adjust the Saturation Index by lowering pH and alkalinity through the addition of acid rather than partially draining and refilling the pool.