All About the Water: Spa Water Know-How

Technical Bulletin: Hot Water Chemistry

Big and small, hot and cool. What’s the difference? Can’t you just fill them, test them, and treat them the same? No!

Pool and spa water must be cared for in different ways. Spas or hot tubs operate at a higher temperature, and they are much smaller. These two facts change everything. That is why NSPI (the National Spa and Pool Institute in the U.S.) has established separate standards for spas and pools.

The two big differences—higher temperature and smaller volume—cause a number of other differences that need to be taken into consideration.

Higher temperature will cause…

1) Faster chemical reactions 2) Faster evaporation rate 3) Increased scale formation 4) More organic waste in the water 5) Accelerated bacteria growth

Smaller water volume will cause…

1) Heavier bather load 2) Exact chemical dosages needed 3) Higher filtration (turnover) rate 4) Faster depletion of sanitizer residual 5) More abrupt changes in pH

Higher Water Temperatures

Pools usually operate between 76 - 86°F (25 – 30°C), while spas fall in a range of 96 – 104°F (36 - 40°C). This difference in temperature changes the water chemistry in important ways.

Chemical reactions take place much faster in spas than in pools. For every 10°C (18°F) increase, the chemical reactions proceed twice as fast. For instance, a spa at 102°F allows chemical reactions to happen in half the time of a pool at 84°F. Any chemical adjustments occur 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 high rate due to high water temperature, rapid water circulation and aeration. As the water evaporates, the hot tub owner adds make-up (fill) water to refresh the system. Any water that evaporates is pure; basically pure water leaves behind everything else—the stuff that we call TDS (total dissolved solids). Make-up water also contains minerals, salts and other things that increase the total dissolved solids, so adding fresh water is increasing 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 calcium carbonate. This form of hardness works in the opposite way: it actually is 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 (about half a liter) in just twenty minutes. Plus, the power jets in a spa 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.

A spa makes a perfect incubator for bacteria, if not cared for properly. Hot water promotes the growth of most types of bacteria. Susceptible bathers can acquire serious illnesses, if water is chemically imbalanced.

Smaller Volume

Clearly, spas have a much smaller volume of water than pools. This glaringly obvious difference leads to some other differences that may not be as obvious.

Spas experience a much heavier bather load because they are so much smaller. While two people in a spa might feel cozy, being in a pool where the swimmers are elbow-to-elbow is not. A common load in a spa would have one person in 100 to 400 gallons of water. Pools tend to have at least ten 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. As a result, many places set regulatory limits on the number of bathers in spas. A common standard for public spas is one bather per one square meter of surface area.

Because of the lower water volume in a spa, chemicals need to be measured precisely and the water tested more often. 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.

The water in a hot tub or spa should turn over every 25 to 45 minutes when the system is running with the proper filtration, due to the smaller volume and increased wastes. The water in a pool might turn over in 6 to 12 hours. The faster turnover rate can take a toll on the spa ’s filtration system, particularly if the chemicals are not in proper balance. As a result maintenance on the filter will need to be performed more frequently.

A hot tub or spa, with its small water volume and higher water temperature, uses up the sanitizer residual very quickly. This is why you need to maintain a higher level of sanitizer in hot tub or spa water than you do in your pool. The NSPI standards reflect this fact, requiring sanitizer levels to be higher in spas and hot tubs.

Last, the spa ’s water volume means that even small additions of sanitizer 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 is a little time before the chemical circulates throughout the entire system. In the case of a spa, that reaction time is lost. To avoid equipment damage the sanitizer should be measured carefully and the pH tested frequently to avoid equipment damage.

Treating a spa the same way you treat a pool will not work. Because of the higher temperatures and smaller volumes, a new set of rules comes into play. Understand the unique features of spa water chemistry to ensure bather safety and lower risk of damaging expensive equipment.