Determining the water hardness level in a hot tub gives you the knowledge to understand how to treat the water. Hot tubs require some level of hardness to operate properly. But there is a line between what is acceptable and beneficial to the hot tub and what level of hardness creates problems. Achieving the correct level of hardness with chemicals is one option to address the water quality, but it's also possible to alter the water composition with other methods.
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Calcium hardness (CH) measures the amount of dissolved calcium in the water. Most water tests appropriate for testing pool or spa water determine the calcium hardness level. A spa supply retailer may offer the water testing for free or purchase a water test that specifies it tests for calcium hardness.
Low CH, or soft water, can stain the hot tub and corrode parts of the tub. Spa manufacturers do not recommend using only softened water to fill a hot tub. But water with a high CH, or hard water, can cause cloudy or colored water, scale buildup and corrode the heating element. The ideal calcium hardness range to aim for is between 200 and 400 ppm.
If the hot tub water has low CH add a mineral or calcium supplement to harden the water. For water with high pH use a water softening chemical appropriate for hot tubs. If using hard water in the hot tub, a stain and scale inhibitor chemical product can prevent stains and corrosion from hard water minerals. Stain and scale inhibitors dissolves metals, such as iron, copper and magnesium and aid in removing existing scale buildup.
To balance water hardness without the use of chemicals, fill the hot tub with a mixture of hard and softened water. Most calcium hardness treatment products only require one application each time the hot tub is filled. Add it when the hot tub is first filled. While the products only requires one application per fill, for the best results in treating calcium drain the hot tub, re-fill it and add the product once a week. Most products are fine to use in conjunction with other water treatment chemicals, including chlorine and other sanitizers.