Preventing calcium scale in hot tubs
The hot tub should be treated with a stain and scale inhibitor each time it is refilled. This should be done prior to adding calcium chloride. In areas with extremely hard water, one should also treat the water with a stain and scale inhibitor on a regular maintenance basis. A mild form of acid such as sodium bisulfate (NaHSO4) may also need to be used regularly to control water balance in naturally high calcium areas.
A more critical issue can be scaling on the surface of the vessel below the water line, especially in hard water areas where the majority of inground hot tubs are finished in plaster. This also occurs with other concrete pool finishes. This type of scale occurs when hot tubs are originally built, refinished, or when the finish is neglected for a long period of time. The problem becomes more unsightly when leaves, dirt, or algae lay on the scale and it takes on the colour of the debris. This type of scale can be costly and time consuming to treat, but can be avoided.
Once scaling of the hot tub surface occurs, a descaler should be used to slowly breakdown and lift the scale/stain. Another option is to drain the hot tub and carefully acid wash the surface.
If the latter form of treatment is chosen, it is strongly recommended the hot tub manufacturer or surface supplier be contacted to ensure no damage occurs. Either process is expensive, time consuming, and causes some surface degradation. Again, it is much easier to avoid the scale than it is to treat it.
To ensure this unsightly problem is avoided, a metal and scale inhibitor can be added upon startup. However, many products used to inhibit metal and scale formation also introduce complex forms of phosphate (PO43−) to the hot tub water, which can be detrimental to overall water quality. Hot tub water should be tested regularly for phosphates, especially if they are tied into a salt system to generate either chlorine or bromine (Br). Test kits are available and hot tub water should be maintained no higher than 200 parts per billion (ppb). It is important to keep in mind phosphates are a major pollutant to the environment, so hot tubs should always be tested and treated for phosphate removal before each draining.
New hot tub surfaces should be brushed daily and existing surfaces weekly. A metal and scale sequestrant is also recommended when the hot tub is started up and a stain and scale inhibitor be used on a maintenance basis. Oil, bather waste, and scum can buildup above the waterline, attracting calcium as part of the scaling process. To avoid this from happening, a natural-based clarifier can be added weekly as this will trap and filter out contaminants that add to the unsightly scaling and will reduce the buildup above the waterline. As always, proper water chemistry is the key to preventing scaling above and below the waterline in a hot tub, especially at the recommended operating temperature of 40 C (104 F).
Hot tub professionals should start testing the calcium hardness level as part of a regular maintenance program along with all the elements of proper water chemistry: alkalinity, pH, calcium hardness, and TDS. Regular testing and treating for scale prevention will help ensure the customer’s spa experience is a pleasant one.
Terry Arko has more than 40 years of experience in the pool, spa and hot tub industry, working in service, repair, retail sales, chemical manufacturing, and product development. He is a certified pool operator (CPO) instructor through the National Swimming Pool Foundation (NSPF). He also serves as instructor for the Pool Chemistry Training Institute (PCTI) to certify residential pool techs. Arko is an active member on the Association of Pool and Spa Professionals (APSP) Recreational Water Quality Committee (RWQC). He is a member of Pool & Spa Marketing‘s Editorial Advisory Committee and currently serves as a water specialist for NC Brands, parent company of SeaKlear, Natural Chemistry and Coral Seas. He can be reached via e-mail at tarko@ncbrands.com.
