Your Spa Is Not Just a Small Pool
Remember the old adage: A square is a rectangle but a rectangle is NOT a square? Well, the same is true in the pool/spa industry—both are recreational waters, but a spa is NOT a small pool. In fact, the chemistry in hot water environments is unique!
When establishing a testing and treatment regime, it’s important to understand how the higher temperatures, smaller volume, jetted water, shorter turnover rates, and the likelihood of a high bather load in a spa/hot tub can affect the chemistry of the water. In general, the two key differences between pools and spas are temperature and volume.
The hotter the water, the faster chemical reactions occur. For every 18°F (10°C) temperature increase, chemical reactions will increase twofold. This explains why sanitizers are said to be “short-lived” in hot water environments — the depletion rate is faster. And while most bacteria will not survive in the 104°F recommended maximum spa temperature, some bacteria (Pseudomonas aeruginosa) actually thrive in spa and hot tub water. The hotter water in spas can also increase corrosion rates and scale formation, depending on the pH of the water.
In addition, higher temperatures cause natural body oils, perspiration, etc., to expel at a faster rate than in the cooler water of a pool, leading to — you guessed it — a quicker depletion of sanitizers. Did you know that heated water encourages more urine/sweat production, to the tune of an average 50 mL urine and 200 mL sweat during an average 15─20 minute soak? As bacteria multiply exponentially, you can see how an undertreated spa can become a health concern.
The smaller volume of water in a spa also plays a part in the water’s chemistry, creating chemical dosage sensitivity and higher turnover rates. The faster flow rates equal very short turnover times — some as fast as 15 minutes! Addition of large amounts of treatment chemicals would be disastrous for a spa; precise measurements in small dosages do the job. We call this teaspoon chemistry. Also consider the effect of a higher bather-to-water ratio in a spa verses a pool. In terms of sanitizer, 4 people in a 400-gallon spa is roughly equivalent to 250 people in a 25,000-gallon pool. Two people in a 400-gallon spa will consume 1 ppm free chlorine in the first 15 minutes!
The ultimate solution to hot water problems is fresh water. Despite well-meaning attempts to properly treat spa/hot tub water, at some point (because of small volume) you’ll need to drain and refill. The formula for draining is 1/3 of total gallons of water in spa ÷ maximum number of daily users, based on a 20-minute soak. The resulting figure is the number of days between complete draining regardless of service regimen. So if you have a 450-gallon spa with 10 daily users (1/3 of 400 = 150, 150/10 = 15), you would need to drain and refill the spa every 15 days.