Due to their higher temperatures and smaller volume, spas must be monitored and maintained differently than pools.
The higher temperatures will cause faster chemical reactions and evaporation rates, increased scale formation, more organic waste in the water, and accelerated bacteria growth.
On the other hand, smaller water volume will result in heavier bather loads with precise chemical dosages needed; higher filtration (turnover) rates; faster depletion of sanitizer residual and more abrupt changes in pH.
High water temperatures
Pools usually operate between 76- and 86 degrees Fahrenheit, while spas fall in a range of 96- to 104 degrees. This difference in temperature changes the water chemistry in important ways.
Chemical reactions occur much faster in hot tubs than in pools. For every 18-degree increase, the chemical reactions proceed twice as fast. For example, chemical reactions in a hot tub heated to 102 degrees occur in half the time of a pool heated to 84 degrees. Any chemical adjustments happen more quickly. The water comes to equilibrium sooner, and water treatment can be completed in a shorter period of time.
The water in a hot tub also evaporates at a higher rate due to high water temperature, rapid water circulation and aeration. As the water evaporates, the spa owner adds fill water to refresh the system. Any water that evaporates is pure, leaving behind everything else — the stuff that we call total dissolved solids (TDS). Makeup water also contains minerals, salts and other things that increase the TDS, so adding fresh water increases the TDS level as well. High levels of TDS decrease the effectiveness of some chemicals and may cause cloudy water.
Higher temperatures in spa water will cause most chemicals to dissolve faster than in lower temperatures, except for calcium carbonate. This form of hardness works in the opposite way: It’s actually more insoluble in hot water. Therefore, calcium carbonate scale is more likely to occur in hot spa water.
The hot water in spas also makes people sweat. The average bather sweats a pint in just 20 minutes. In addition, the jets in a hot tub can scrub off dirt and dead skin very quickly. All this means that the filter and chemical sanitizer in a spa have to process a high percentage of waste. Consequently, paying close attention to the sanitizer level is critical in a spa.
If not cared for properly, a hot tub creates a perfect incubator for bacteria. Hot water promotes the growth of most types of bacteria.
Smaller volume
Hot tubs contain a much smaller volume of water than pools. This glaring fact leads to some other differences that may not be as obvious.
For one thing, the bather load is heavier for spas because of its much smaller size. A common load in a hot tub would have one person in 100- to 400 gallons of water. Pools tend to have at least 10 times that amount of water for every swimmer.
Consider this: Two bathers in a 400-gallon spa are roughly equivalent to 150 people in a 30,000-gallon pool.
This significant bather load can decrease the sanitizer levels very quickly. Chemicals need to be measured precisely, and the water needs to be tested weekly. Misjudging the required dosage can drastically alter the chemistry in a small volume of water. For this reason, there are chemicals specifically designed and labeled for treating spas. These lower the risk of adding too much or not enough of a particular chemical.
Spas use up the sanitizer residual very quickly, which is why a higher level of sanitizer needs to be maintained in spa water than in pools. Industry standards reflect this fact, requiring sanitizer levels to be higher in hot tubs.
Lastly, the spa’s water volume means that even small additions of sanitizers and other chemicals can have an immediate effect on the pH of the water. If the wrong amount of a chemical is added to a pool, there’s a little time before the chemical circulates throughout the entire system. In the case of a spa, that reaction time is lost completely. To avoid equipment damage, the sanitizer should be measured carefully and the pH tested frequently.
Spa water-care checklist
Here’s a handy checklist of essentials that service pros say they need for the spas in their charge:
• Anti-foaming agents
• Bromine
• Clarifiers
• Filter cleaners and degreasers
• Scalers
• Sodium bisulfate
• Sodium carbonate
• Sodium dichlor-isocyanurate
• Sequestering agents (aka chelating agents)
• Potassium monopersulfate
Spa service to-do listTesting: Test the sanitizer (chlorine or bromine) and pH levels with every visit and adjust accordingly, so that levels fall within industry standards. Also test for total alkalinity, calcium hardness and stabilizer. Once you get consistent results for these variables after several visits, testing can be reduced to once a month.
Shocking: Because spas are often left covered when not in use, the right time to shock depends largely on frequency of operation and bather load — the higher the temperature and the larger the load, the more shocking will be necessary.
Most spa techs agree that at minimum, a spa should be shocked weekly. Ideally, the water should be shocked after each time the spa is used. The choice of a shocking chemical is a matter of preference. Some prefer mono-persulfate, because it destroys wastes but does not cause a large increase in chlorine or bromine levels. Others prefer chlorine, because it adds extra sanitizer; however, the type of chlorine you use is crucial. Sodium dichlor is a popular choice because it dissolves rapidly in warmer water and has a neutral pH. Some stay away from liquid bleach because of concerns that salt and hardness level build up too quickly, and that maintaining pH levels will be a challenge.
Algae control: If regular shock treatments don’t reduce the algae presence, veteran service techs recommend increasing the amount of chlorine or adding an algaecide to the water one hour after shocking. Follow product directions carefully because some algaecides, if not used properly, can cause foaming water in spas.
Avoid algaecides that contain ammonia because they can cause foaming, techs say. Remind spa owners that sanitizer and algaecides will take care of the issue, but if a cover is used, algae spores should never get in there in the first place.
Scale and stain control: Most sequestering agents can control metal staining and scaling in spas. If you’re worried about the impact such chemicals could have on the spa shell, call the spa maker and ask what’s recommended to remove stains.
Foaming: Several things can cause sudsy or foamy water in a spa: algaecides, high TDS levels, soap residue (from clothing or hair) and improper water balance.
The first step is to make sure the spa water is balanced. Take the appropriate tests (pH, TA, calcium) and use the Saturation Index. Make adjustments if the water is unbalanced and see if the foaming subsides.
If using an algaecide, be sure it is of the nonfoaming variety.
Assuming that the water is balanced, the filter is working efficiently, and the bromine levels are at least 3 ppm, you can also try shocking with chlorine at 20 ppm. Chlorine will recharge the combined bromine back into active bromine.
Draining and refilling is another option. If the water is balanced and no algaecide has been used, it could be a matter of increasing draining frequencies. Re-examine the customer’s bather load patterns and use the industry formula to determine how often the water should be changed (see sidebar below).
Or, take a preventive approach: In addition to a defoamer, use a clarifying agent on a weekly basis; that should keep the foam down.
To drain or not to drain?For spa-service techs who want to follow the industry’s standard formula to determine when to change spa water, here it is:
Gallons in spa ÷ 3 ÷ average daily bathers = number of days between drainings.
For example, if you have a 500-gallon spa with an average daily bather load of two, the formula would work this way:
500 ÷ 3 = 166.66 / 2 = 83 days
According to the formula, this particular spa’s water should be drained and changed approximately every three months.
