Abstract: An electric continuous-flow water heater generates a basic power with heating resistors in heating passages and a controllable power with three heating resistors in heating passages. The controllable power is controlled by a control system through a triac. At low flow rates two phases and a low controllable basic power is connected to the heating resistors. At high flow rates the heating resistors are star-connected and the controllable heating-resistors are delta-connected. An electric power output stage comprises a mains rectifier bridge, a Schmitt-trigger connected to the output thereof and a frequency divider with two outputs.The signals at the outputs are set by an edge of each third pulse and are reset by the corresponding edge of the following or next but one pulse. A semiconductor relay is optionally by the signals at the outputs through a selector switch or a logic circuit and is rendered conductive for one, two or three half waves.