A
control unit (1, 1a) for a
softening device (2), comprising a primary inlet (4) for untreated water, a primary outlet (7) for blended water, a sensor for determining the water
hardness of the untreated water WHraw or the blended water WHblend, a secondary outlet (5), a secondary inlet (6), a bypass line (9), a blending means which can be automatically adjusted for mixing a blended
water flow Vblend(t) from a first partial flow (V(t)part1soft of the secondary inlet (6) and a second partial flow V(t)part2raw of the bypass line (9), and an electronic control means (11, 11a), wherein the control means (11, 11a) is designed to readjust the adjustment position of the blending means by means of the determined water
hardness in such a fashion that the water
hardness in the blended
water flow Vblend(t) is adjusted to a predetermined desired value, is characterized in that the
control unit (1, 1a) is designed in the form of an external
control unit (1, 1a) for the
softening device (2), wherein the control unit (1, 1a) has a control
unit housing (3), the outer side of which is provided with the primary inlet (4), the primary outlet (7), the secondary outlet (5), and the secondary inlet (6), and wherein the control
unit housing (3) contains the sensor, the bypass line (9), the blending means and the electronic control means (11, 11a). The inventive control unit enables partial
softening of water on the basis of
fully automatic blending, thereby utilizing already existing softening devices.