Abstract: A system for softening water comprising a regeneratable softener having an ion-exchange resin exposable to the water, a salt reservoir containing a supersaturated sodium-chloride solution for the regeneration of the resin, and a water hardness detector including a housing containing an ion-exchange shrink resin charge regeneratable by the solution, the shrink resin charge being exposable to water having passed through at least a part of the resin in the softener. The detector further includes a piston reciprocable within the tank, which piston contacts a single free surface of the shrink resin charge at least during detection of water hardness. The regeneration of the softener is initiated in response to the position of the piston.