Understanding the differences between pool and hot tub water quality

Smaller volumes

‘Teaspoon chemistry’ is a term some service technicians use to describe how hot tubs should 
be chemically treated. Due to the smaller water volumes, lesser amounts of treatment products are added as opposed to a pool—teaspoons of this or capfuls of that—as opposed to kilograms of this or litres of that. Many hot tub owners think that more 
is always better, so they add more treatment product(s) than necessary, which only creates more problems.

Hot tub volumes vary from as low as 567 L (150 gal) for residential hot tubs to as high as 9464 to 11,356 L (2500 to 3000 gal) or more for commercial units. No matter the volume, the turnover rate (the amount of water in gallons/litres that flows through the entire system once) 
is anywhere from 15 to 45 minutes for hot tubs compared to six to eight hours for pools.

A smaller volume of water plus heavy usage equals a quicker depletion of sanitizer. Four people in a 1514-L (400-gal) hot tub is roughly equivalent to 250 people in a 94,635-L (25,000-gal) pool. Using the same hot tub size as an example, four bathers will also consume 2 ppm free chlorine (Cl) in the first 15 minutes.

Aeration (jetted water) also causes pH to rise by introducing additional hydroxide ions (OH^–) into the water. Careful and consistent pH testing is needed to make sure the increased pH level does not affect water balance or cause cloudiness/scaling.

Water balance in hot tubs

The five parameters of water balance are pH, total alkalinity (TA), calcium hardness, temperature, and TDS. These factors are then used in the Langelier Saturation Index (LSI or SI) to obtain a number which represents whether the water in a hot tub has a tendency to be corrosive or scale-forming.

Langelier Saturation Index

SI = pH + TF + logCH + logALK – Constant

• 
SI = Saturation Index
• 
pH = measured pH
• 
TF = temperature factor
• 
CH = measured calcium hardness
• 
ALK = measured carbonate alkalinity
• 
CONSTANT = combined factor for temperature and ionic strength correction, and concentration conversions.

Ideal ranges recommended for hot tubs

• 
Free chlorine = 3 to 5 ppm
• 
Combined chlorine = 0 ppm
• 
Bromine (Br) 4 to 6 ppm
• 
pH 7.4 to 7.6
• 
TA 80 to 120 ppm
• 
Calcium hardness 150 to 250 ppm
• 
Maximum temperature 40 C (104 F)
• 
TDS no greater than 1500 ppm above source water TDS level

Hot tub water is considered ‘balanced’ if the result of the SI calculation falls between -0.3 and +0.5. Positive values greater than +0.5 can indicate the potential for cloudy/scaling water. Negative values less than -0.3 can indicate the potential for corrosive water.

If hot tub water (or pool water for that matter) has an SI lower than -0.3, it will naturally attempt to balance itself by seeking calcium carbonate (CaCO₃) from any source in its quest (concrete, plaster, and especially grout or gunite), leaving behind pits, holes, and other signs of deterioration. Although this is not as much an issue since most hot tubs have fibreglass/acrylic shells, the water will still behave aggressively and attack any metals in the system, such as heater tubes, causing corrosion and eventual failure of the metals.