Hot Tub vs Swimming Pool Maintenance
Author: Alan Schuster
I am a graduate of the City University of New York, with a B.S. Degree in Chemistry. My experience in the Swimming Pool and Spa Industry goes back over forty years.
During this period, I have helped tens of thousands of consumers and dealers solve pool and spa water chemistry and water quality problems.
It might seem logical to think of a spa as a small pool, but there are major differences, which necessitate different approaches. Spas are definitely not small pools!
The most obvious difference, between a pool and a spa, is size and the volume of water each contains. A typical spa might contain 375 gallons or 1500 liters. A modest pool might contain 15,000 gallons or 57,000 liters. This makes a pool 38 times larger, than a spa, based on these examples. Let’s see how this affects the sanitizer level. Assuming that both use a chlorine-based sanitizer and have a free chlorine level of 2 PPM, the spa contains 3 grams of chlorine, while the pool contains 114 grams.
The purpose of the chlorine is to prevent the growth of microorganisms and oxidize organic wastes. There is a great disparity between the amount of chlorine available, when comparing a spa to a pool. It is the presence of chlorine or other sanitizers, that helps maintain sanitary conditions. Once the sanitizer level is depleted, microorganisms will start to flourish and organic wastes will start to accumulate.
Assume that the spa and pool are being used, while the water temperature is the same. If the spa and pool are used by two adults, it would be expected that the amounts of microorganisms and organic waste added, to the water, would be about the same, over similar periods of time. If that is the case, the spa, with only a reservoir of 3 grams of chlorine, would quickly be impacted and the free chlorine would start dropping towards zero.
The pool, on the other hand, has 114 grams of chlorine available to help sanitize and destroy organic wastes. Over similar time spans, the pool would appear relatively unaffected. The conclusion to draw is the sanitizer levels, in a spa, drop rapidly, as compared to a pool, once the units are in use.
Spas and pools are not usually operated, within the same temperature ranges. Spas are typically maintained, at temperatures of 36-40 Celsius, while pools can be anywhere from about 68 – 86 F or 20-30 C, depending on location, Sun exposure and the use of a heater.
The higher the water temperature, the greater the amount of wastes, contributed by bathers, within the same time frame. In addition, the higher temperatures make chlorine more reactive, so that the rate of depletion increases, as the water temperature rises. When water temperatures rise towards 104 F or 40 C, the growth of microorganisms increases, as the warmer water starts to function as an incubator. The presence of warmer spa water adds more stress to the chlorine level and speeds its depletion.
Spas are more challenged to maintain proper levels of sanitizer and eliminate organic wastes, due to their smaller size, making the bather load function, as if much higher, when compared to a pool. The warmer spa water makes chlorine more reactive, which speeds up depletion. While it is hard to quantify, two bathers in a typical spa might equate to 200 bathers in a pool, when one factors in the difference in water volumes, introduction of wastes and operating temperatures.
Chlorine is not the only sanitizer/oxidizer affected by the smaller spa water volumes and higher operating temperatures. The same relationships apply, to other popular sanitizers. Spas are not small pools: their sanitizer usage can be more in line with a much larger pool and the changes in sanitizer levels can take place more rapidly. Spas benefit, greatly, when sanitizing and oxidation take place on an ongoing basis, such as when a salt chlorine generator is in use. More chlorine is being added, as it is being exhausted, due to bather usage.
Some advanced models of salt chlorine generators include sensors that detect chlorine, so they only produce chlorine, when conditions warrant. This helps avoid over-chlorination and helps assure proper sanitation and bather comfort. Chlorine or bromine feeders and floating dispensers do not provide the same degree of control and require more monitoring. They tend to work much better in a pool, with its larger water volume and greater amount of chlorine in solution, at any given time. A proper sanitizer level in a spa can deplete rapidly, even when care is taken. Levels in a pool remain more consistent over time, but are still in need of replenishment, on a regular basis.
The main water chemistry parameters for spas and pools are generally the same, but there are differences. Using a reliable tester is important, as it forms the basis of which chemicals to add, how much is needed and when it needs to be added. It is best to use a reliable tester, that provides the right kind of information. You can choose from a variety of electronic testers, as well as test strips and color comparators. It is all a matter of finding something, that you are comfortable using. You will want to test free and total chlorine, on a daily basis, both before and after spa use. pH should be tested daily. Total alkalinity weekly. Calcium hardness monthly.
Cyanuric acid is a chemical that helps protect chlorine, from being destroyed by the Sun’s UV rays. It is not necessary to use in a spa, that remains covered most of the time. However, if sodium dichlor is used as a source of chlorine or as a shock treatment, its level should be monitored. As the CYA level builds up, due to dichlor use, the efficiency of chlorine decreases. At levels of 50 PPM, you should maintain the free chlorine at 2-4 PPM and consider water replacement and cleaning, when the level reaches 100 PPM. Salt, borates, phosphates, copper and iron are other tests, that might need consideration, based on individual circumstances.
Maintaining free chlorine at 1-3 PPM or 2-4 PPM is a place to start. In a spa, with its small volume and higher bather load, a 2-4 PPM might prove more advantageous. Pools can use the entire range of chlorine products. Spas tend to use fewer products. Lithium, sodium and calcium hypochlorites are less popular, with spas, because they tend to make the pH rise. Trichlor tablets are popular in pools, in feeders and floaters, but are not recommended for spa use, simply because they dissolve too rapidly, at the temperature of a typical spa. Sodium dichlor is popular in spas, because it is quick-dissolving and has little effect on the pH. Overuse of stabilized chlorine can lead to high CYA levels and require maintaining a higher free chlorine level or water replacement.
pH testing is important in both pools and spas and the typical range is 7.2-7.8, in both cases. The pH of a spa can change rapidly, after the addition of many types of chemicals. In a pool, change will occur, but more slowly. A high pH can make chlorine less effective and might create water chemistry problems. A low pH will create corrosive conditions, that could cause heater damage. In addition, a low pH can lead to odor problems and cause bather discomfort. In a spa, it is best to test the pH, before and after each chemical addition.
Total alkalinity or TA is important, in both pools and spas, in order to help stabilize the pH, within the optimum range. A range of 80-150 PPM is typically recommended. pH and TA tend to move in the same direction. It is not always easy to get them both within the ideal range and, if that happens, please remember that pH is always the more important parameter.
Calcium hardness testing is important, in both pools and spas, but not necessarily for the same reason. In masonry pools and spas, a calcium hardness level of 150-250 PPM helps protect the underwater surfaces from etching and reduces the corrosive effects on equipment. In portable spas, there is no etching to be concerned about, but there is another important reason to maintain a calcium hardness level at around 200 PPM.
While you’re luxuriating and enjoying the hot water experience, body oils are entering the water. Slowly, they react with the natural alkalinity, driven by the higher temperature of the spa water, and form “soaps”, which leads to foam formation. The sanitizer level and microbial content of the foam can be very different, from that of the spa water. Proper spa maintenance mandates that the resultant foam be periodically eliminated, by the addition of anti-foam or an enzyme treatment. A calcium hardness level of 150-250 PPM makes the water harder and, in turn, depresses the foam. Foaming can occur in swimming pools, but for a different reason. The addition of algaecides containing dimethyl benzyl ammonium chloride, which is not recommended for use in spas, can cause foaming, when the pool water is agitated or aerated.
Both spas and pools need effective filtration in order to optimize water quality and clarity. Pools use a wide range of products, designed to meet varying needs and budgets. They tend to be larger, because of the larger volumes of water, and cleaning and maintenance can become an issue. Spa filters are generally much more compact and easier to service. Not all spa filters are the same. Some simple filters can fail to remove the finest particles. Better results are possible, when an in-depth approach to spa filtration is taken. This approach allows for finer and finer particles to be removed, as water passes through the depths of the filter media. When more wastes and debris are removed by good filtration, more free chlorine will be left available for sanitizing purposes.
Water quality issues in pools are most often in the form of cloudy water and algae blooms, which can be the result of pool sanitation, water chemistry and poor filtration. Algae is less of an issue in spas, due to the absence of light, for most of the day. An uncovered spa can experience algae problems, if conditions deteriorate.
Pools due to their size are prone to areas with poor circulation. This leads to dead zones, that promote algae growth. Pump size, filter pressure, number and location of returns and skimmers all play a role in a pool’s ability to maintain optimum conditions. Spas, on the other hand, have vigorous and effective circulation, which helps disperse chemicals rapidly and filters the water many times, during a normal filter cycle.
Chemicals build-up in spa and pool water over time. This build-up includes the by-products of the chemicals being added, as well as those occurring naturally in the source water. Minerals are also tracked in and delivered by the wind. Such build-ups, in the TDS (total dissolved solids), can lead to water quality issues. In a spa, you have the option of emptying and cleaning, which is something that is recommended every 3-4 months, based on individual circumstances. Pools, on the other hand, involve much more thought and draining can never be taken lightly. Spas are built for easier maintenance.
Spa are not small pools, but are used in the same way, especially in the case of swim-spas. The smaller water volume does factor into sanitizer depletion and usage. While the temperature of a swim-spa may not be set as high as a spa, the activity level of the user may be higher than in a spa or pool. This means more sweat and wastes are being introduced and this has a direct effect, on the sanitizer level and usage rate.
In conclusion spas require different maintenance, as compared to swimming pools.
The good news is that, today, spas are better equipped than ever, to deal with the maintenance issues and provide a more rewarding hot water experience.