When you fill water into the whirlpool, heat it up and swirl it, the carbon dioxide is released.
Carbon dioxide dissolved in water is an acid. When it is outgassed, the water becomes more alkaline. The pH of the water rises.
Because the carbon dioxide is missing as a "calcium solvent", the water-soluble calcium ions are transformed into limestone.

 

“The rising pH value indicates that the carbonic acid is outgassing. The faster the carbonic acid outgases and the higher the pH value rises, the faster and stronger limestone can form.”

To prevent the formation of limestone, one usually adds an acid to the water. A pH-lowering or also called pH-minus. This acid takes over the function of carbon dioxide and leaves the calcium in dissolved form in the water.

 

Therefore, controlling and lowering the pH is a very important factor in preventing limestone formation.

The relationship between water hardness and carbon dioxide also explains why the pH value behaves differently in soft water than in hard water.

With soft water it may be that you only have to lower the pH-value for two to three days and it will then remain stable. It remains stable as soon as the carbon dioxide is outgassed.

With hard water it can be necessary to lower the pH value permanently for weeks. It rises again and again. With hard water you also need much more pH-lowering agents than with soft water.