The invention provides an ion exchange cartridge (13), in particular for a domestic appliance that comprises between a water inlet (1A) and a water outlet (2A) a weak acid ion exchanger in the H<+> form (5), followed by a strong acid ion exchanger in the Na<+> form (7). This cartridge may be compact because it removes hard scale forming ions only. The cartridge may further comprise a first (16) and a second probe (17) to measure hard scale causing ions by a differential measurement of the conductivity of incoming water and water that has passed one or both ion exchangers. Herewith the appliance can be switched off when the cartridge is not functioning anymore, thus forcing a user to replace the cartridge, or to regenerate the ion exchangers before hard scale blocking of the appliance.
