Instantaneous water heater for water heating, comprising a channel arrangement with a water inlet, which is adapted for connection to a water supply pipe, and with a water outlet, which is adapted for connection to a water discharge pipe, an electrical heating device for heating water flowing through the channel arrangement in the direction of flow, an electronic control device configured to control the heat output of the heating device and a motor-driven valve arranged in the water inlet and adapted to limit the water flow rate and controllable by means of the control device, wherein the motor-driven valve comprises a valve body forming a flow path, an electric motor and a throttle member which can be adjusted in position between an open position and a closed position by means of the electric motor and is adapted for adjusting the volume flow, and wherein the motor valve is configured such that the position adjustment of the throttle member in the direction of the closed position takes place with the flow direction of the water, and characterised in that the control device has an undervoltage detection device which is adapted to detect an undervoltage or the failure of one or more phases of the mains voltage as an undervoltage incident and, in the event of a detected undervoltage incident, to actuate the motor valve in such a way that the throttle element is moved into the closed position, and the valve body comprises a counter-position element set up to mechanically limit the path of the throttle member in the open position, and in that the control device is further configured to detect the current consumption of the electric motor and, following a preceding undervoltage incident, to actuate the electric motor and move the throttle element into the open position, wherein reaching of the open position is determined by means of the control device when a predetermined first reference value of the detected current consumption of the electric motor is exceeded. a channel arrangement with a water inlet, which is adapted for connection to a water supply pipe, and with a water outlet, which is adapted for connection to a water discharge pipe, an electrical heating device for heating water flowing through the channel arrangement in the direction of flow, an electronic control device configured to control the heat output of the heating device and a motor-driven valve arranged in the water inlet and adapted to limit the water flow rate and controllable by means of the control device, wherein the motor-driven valve comprises a valve body forming a flow path, an electric motor and a throttle member which can be adjusted in position between an open position and a closed position by means of the electric motor and is adapted for adjusting the volume flow, and wherein the motor valve is configured such that the position adjustment of the throttle member in the direction of the closed position takes place with the flow direction of the water, and characterised in that the control device has an undervoltage detection device which is adapted to detect an undervoltage or the failure of one or more phases of the mains voltage as an undervoltage incident and, in the event of a detected undervoltage incident, to actuate the motor valve in such a way that the throttle element is moved into the closed position, and the valve body comprises a counter-position element set up to mechanically limit the path of the throttle member in the open position, and in that the control device is further configured to detect the current consumption of the electric motor and, following a preceding undervoltage incident, to actuate the electric motor and move the throttle element into the open position, wherein reaching of the open position is determined by means of the control device when a predetermined first reference value of the detected current consumption of the electric motor is exceeded.