Water Hardness in Spas & Hot Tubs

WATER Hardness

Hard water is water that contains dissolved chalk, lime and other minerals. Rainwater is naturally soft, but as it percolates through chalk and limestone it dissolves and collects these minerals. Rainwater which falls on hard rock remains naturally soft. The hardness of the supply of mains water to your home is dependant on where you live and the source (river or ground water) of your mains water supply.

Calcium and magnesium are the two primary minerals that make up hardness in water. Like alkalinity and pH, hardness affects the tendency of the water to be corrosive or scale-forming. (Scale is a deposit that forms on pool walls and equipment when the mineral content of the water is too high.) By maintaining the ideal ranges for hardness and alkalinity, you can keep scale formation to a minimum.

Low hardness levels require immediate attention! They can be very dangerous to your system. Water that is not properly saturated with hardness—water in which the hardness level is too low—will be very corrosive, causing significant damage to metal pipes and fixtures as well as plaster. You must be sure to balance hardness before adding any sanitizer to the water. Otherwise, the water will become even more aggressive (corrosive); it can cause severe damage in a short period of time.

When the hardness level is low, increase the hardness immediately to limit the damage of corrosive water. When the hardness level is too high, excessive scale formation occurs, and the water may become cloudy or discoloured. High pH and warmer temperatures will increase scale build-up too.

If the hardness level is too high, you can partially drain and refill with fresh water.

Increasing the Hardness

Most test strips test for the Total Hardness of your water, that is the concentration of all dissolved mineral salts.  The Calcium Hardness is the element of the Total Hardness attributable directly to Calcium Carbonate.

Pristine Blue Users Beware: If you use PristineBlue at a sanitiser it is the Calcium Hardness, not the same as the Total Hardness, that you must measure. Calcium ions bind to the copper ions and reduce the capability of the sanitiser to work properly. To test for Calcium Hardness you should use a calcium hardness test kit, not test strips.  The PristineBlue system includes a product called PristineCheck ro reduce the Calcium Hardness of the water.

What ever sanitizer you use, the Total Hardness should be tested on first filling your spa and thereafter at monthly intervals. As water evaporates from the spa the minerals get left behind and become more and more concentrated, consequently the hardness of the water in your tub will increase over time.  The recommended range for all sanitizers other than PristineBlue is 100 - 500 ppm (mg/litre).  It is very unlikely that your water will rise above that range, but if it does topping up the spa with tap water at a lower hardness will reduce the level.

If the Total Hardness in your tub is too low (soft water), etching can begin to occur on the tub surfaces which will become abrasive and uncomfortable for bathers.  [Rough surfaces are not good because they also increase the likelihood of problems with algae growth and make cleaning more difficult.]