When to replace spa water: TDS, CYA, Bathers
The determination of when to replace stand-alone spa water depends on whether the vessel sees a lot of use or if it is an infrequently used residential spa.
In a heavily used spa, we are concerned with dead skin cells, sweat, body oils, urine, fecal matter, and more, and it’s necessary to change the water regularly to assure public health.
For the more common, residential pool, it should not be necessary to change the water as frequently on the basis of bather waste.
But one under-appreciated aspect of disinfection in residential spas that use dichlor is the buildup of cyanuric acid. Hot tub itch or rash and hot tub lung have been correlated with the regular use of dichlor. It a good idea to remember that for every 10 ppm free chlorine (FC) added by dichlor, it also increases cyanuric acid (CYA) by 9 ppm. That means that the cyanuric acid concentration can rise beyond acceptable levels within a few months, rendering the chlorine not as effective as it otherwise can be.
This is because the active chlorine level drops (since the free chlorine target is generally not increased while CYA is increasing) and this builds up slower-to-oxidize organics that cloud the water and also increase the risk of infection.
For this reason, it’s a good idea to regularly check the cyanuric acid levels and replace the spa water when it exceeds industry accepted guidelines. The current Model Aquatic Health Code recommends maintaining aquatic venues at a CYA concentration at or lower than 90 ppm. Industry experts generally accept the ideal CYA concentration to be between 30 and 50 ppm.
In general, many technical guidelines, such as the APSP Service Tech Manual, recommend changing spa and hot tub water using Total Dissolved Solids (TDS) as a basis. The water change rule for spas based on TDS simply uses TDS as a proxy for bather load.
Total dissolved solids are the summation of all the materials dissolved in the water, and industry guidelines advise that water should be replaced when the TDS is 1,500 ppm greater than what it was when the water was originally added.
For infrequently used spas, changing the water based on the TDS rule would likely result in changing the water too frequently. Consider a spa maintained with chlorine and having no bather load at all. It will still build up TDS but obviously does not need a change of water. In practice, a spa that is not used
How often to replace water in a commercial spa much will likely still need the water changed once a year to prevent TDS buildup - not because of water quality but to prevent corrosion of metal components.
For commercial, or frequently used spas, it is really the nature of the TDS that is problematic. Such spas see heavy bather waste in a relatively small volume of water.
Additionally, the aeration and higher temperatures cause water to evaporate faster, raising the TDS levels. Unlike with swimming pools, for such spas, TDS is a more relevant parameter. For highly used or commercial spas, the hot, turbulent water exfoliates dead skin cells and facilitates the release of sweat and oils.
The industry rule-of-thumb for commercial spas is:
Water replacement interval (days) = Spa Capacity (gallons) ÷ 3 Cumulative Bathers per day Compare that number to the number of days since the last water change.
If the Difference (WRI - Days since last change) is less than or equal to zero, the spa should be drained and refilled.
Consider a spa that was last drained and refilled on Sunday evening. Monday, the spa had 85 bathers. The water replacement interval for Monday would then be two days. Because it has been one day since the last change, the difference is 1, so the water does not need to be replaced on Monday evening.
On Tuesday, the spa had 2 visitors. Adding Monday’s 85 bathers plus Tuesday’s 2 bathers makes 87 bathers, requiring a water replacement interval of 2.
Because it has been two days since the last change, the difference is zero, and the water should be replaced Tuesday evening.
See the accompanying table for this spa’s usage and water replacement requirements.