much will likely still need ….
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.