Why do dishwashers need salt?

The addition of salt to a dishwasher softens hard water and prevents the mineral deposits that cause scaling. Scaling is the insoluble scummy ring in the bathtub, for instance. Scaling also clogs pipes, coats the inside walls of appliances such as coffeemakers and causes water heater failure.

Dishwasher manufacturers in geographical areas with hard water often equip machines with a warning light that signals low salt content, or the literature offers recommendations regarding the addition of salt to the detergent. Granular salt is the only safe type to use for water softening. The additives in other types of salts, such as cooking, rock and sea salts, may increase water hardness and cause clogging, because they swell when wet.