As most in the industry know, the buildup of scale in a hot tub or pool 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 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 percent 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.
When to drain and replace hot tub water
There is a formula for the number of daily users recommended by the National Swimming Pool Foundation (NSPF). The formula can be used to determine when hot tub water should be drained and replaced, which is as follows:
# of gallons in the hot tub ÷ 3 ÷ users per day = the number of days between draining
A hot tub in a semi-commercial or commercial facility needs to be maintained very differently and with much more attention than a residential unit. Only chemicals that are specifically designed for hot tubs are to be used. Chemicals should be used and added in moderation and dosing should be gradually broken up to avoid oversaturation and chemical reactions that can occur. Since a hot tub is a much smaller body of water, the saturation and water balance can be affected by chemical additions much faster than in a swimming pool. When using a sanitizer such as chorine (Cl), it is important to only use specific hot tub formulas. The use of calcium hypochlorite (Ca[ClO]2) should be avoided in hot tubs as the calcium byproduct can lead to cloudy water and increased potential for calcium carbonate scale. Non-petroleum natural-based clarifiers should be used to obtain clear water and avoid the buildup of oils and scum.
Soft water or softened water
“In hard water areas, can the hot tub be filled through a softener?” This question is frequently asked by hot tub owners. The answer is usually: “‘Soft’ water is corrosive and can damage equipment and surfaces, so one should never fill a hot tub through a water softener.” However, this is not true. There is a difference between naturally soft water and water from a softener.
Naturally soft water means the source water itself is lacking in sufficient minerals, such as calcium and magnesium. This water is largely present in regions where there is more rain and snow that gets into surface water, so there is very little mineral contact and absorption. Water that has been contained in the ground for longer periods has more time to absorb minerals in the earth and, therefore, will be hard.
When a water softener is used in hard water areas it displaces calcium ions with sodium ions to reduce the buildup of scale in the household. Naturally soft water is low in mineral content, pH, alkalinity, and total dissolved solids (TDS). However, water from a water softener still has sufficient levels of alkalinity and TDS, meaning it will not be corrosive. Filling a hot tub using a water softener can be beneficial as it reduces the calcium ions but still has enough buffer in the water to be safe.
| Is the water scale forming or corrosive? |
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Understanding the makeup of the source water can help in determining the best way to treat the hot tub from the first fill. The Langlier Saturation Index (LSI) is a tool professionals can use to determine if water has the potential to be scale forming (hard water) or corrosive (soft water).The LSI works by using the test results of water balance as factors to determine the saturation of the water. The balance factors for LSI are as follows: pH, total alkalinity, calcium hardness, temperature, and total dissolved solids (TDS). Most test kit manufacturers can provide a chart that gives the numbers that correlate with the test results. Numbers from the LSI chart are matched to test results and then added together. The number for TDS is always subtracted from the total and that number depends on whether the TDS is over or under 1000 ppm. This formula will result in a number that will be on the plus or minus end. Any number over +0.3 would mean the water has the potential to be scale forming. A number below -0.3 means the water has the potential to be corrosive. Using this method, an operator can make changes to the water balance and re-calculate the numbers until they fall between the -0.3 to +0.3 range. This would mean the water is balanced. For details on the LSI, check with a test kit manufacturer.
Know your sourceIt is vital to know the makeup of the source water. The most important factor is to know the total dissolved solids (TDS) of the source water. When hot tub water is 1500 ppm over the source water, then 50 percent of sanitizer effectiveness can be lost. Based on this scenario, water should be diluted or drained when TDS levels are too high. |
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 color 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 break down 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 build up 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.
This article was written by Terry Arko and originally appeared on Pool & Spa Marketing [link].