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Why checking calcium hardness levels is a maintenance must

One of the more critical issues can be scaling on the surface of the vessel below the waterline, especially in hard water areas where the majority of inground hot tubs are finished in plaster.

By Terry Arko

As most in the industry know, the buildup of scale in a hot tub/spa can be a pain. Literally. Once the scale has formed, the only thing left to do is grab a mild scale remover and start scrubbing. After draining and several hours of elbow grease, some progress may have been made and the pain will be over…until next time. Fortunately, scaling in most cases can be easily prevented.

After one good scrub-down, it is plain to see how much easier and more cost-effective it is to prevent water problems from the start than it is to treat them after they have occurred.


Unfortunately, in today’s fast-paced world, many key elements of proper water management are overlooked. In particular, checking the calcium hardness level is a step many operators tend to skip. This is a mistake for two important reasons: first, low calcium hardness can result in a corrosive environment that is harmful to bathers, equipment, and the surface itself. Second, high calcium hardness can result in cloudy water, stains, and scale.

Calcium carbonate build up can be especially difficult to treat once it occurs, and can even result in damage to the finish.

Calcium scale in hot tubs

Calcium scale is typically more of a problem in hot tubs due to their higher water temperature. Scale forms when calcium attaches to carbonate in the water. Calcium carbonate becomes more insoluble at higher water temperatures, especially at the heat exchanger. Also, because of increased temperatures and aeration, the evaporation rate of a hot tub is very rapid. This leads to increased calcium carbonate buildup. Thus, hot tub scale can be especially difficult to treat once it occurs, and can even result in damage to the finish. However, more importantly than the damage done to the hot tub is the fact high levels of calcium hardness can cause discomfort to bathers and can even cause dry itchy skin. The good news is that calcium hardness can be checked, monitored, and calcium scale, prevented.

Defining calcium hardness

Calcium hardness testing (a.k.a. total hardness) is a measurement of the mineral salts present in hot tub water. Some of the mineral salts measured include: calcium (Ca), magnesium (Mg), aluminum (Al), iron (Fe), and manganese (Mn). When it comes to hot tub water, 70 to 75 per cent of the total hardness is made up of calcium. According to the Association of Pool and Spa Professionals (APSP), the ideal range for calcium hardness in a hot tub is 150 to 250 parts per million (ppm). Calcium hardness is raised by adding a chemical known as calcium chloride (CaCl2). This mostly comes in a powder form which must be diluted in a bucket before adding to the hot tub. There is a liquid version available for hot tubs which is much more convenient to use and gets into solution (i.e. mixes) faster than the powder form. Lowering calcium hardness can only be done by draining and replacing water. However, once properly adjusted, calcium hardness levels should stay within range until the hot tub is drained and refilled, which should typically be done every six to 12 weeks.

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