A tankless water heater system (100), with a heat exchanger device (20) comprising at least one hollow chamber (21, 22, 23, 24) and at least one electrical heating element (52, 53, 54), and a controller device (30) with a temperature control unit (35), a tap event counter unit (32), a down-time counter unit (33) and a time delay unit (34); an electrical switching element (41, 42, 43) for connecting or is connecting one or several heating elements (52, 53, 54) to/from a power supply; an outlet temperature sensor (27) linked with the temperature control unit (35); a flow rate sensor (29); wherein: the tap counter unit (32) is connected to the flow rate sensor (29) and is triggered when water flow rate exceeds a tap indication threshold the down-time counter unit (33) is triggered and retriggered by the tap counter unit (32) and both provide a down-time event signal after any inactivity period with no water flow and records the duration of inactivity; the time delay unit (34) is connected to and triggered by the tap counter unit (32) starting a delay period which duration is switched from a short default delay period to a long delay period by the down-time signal provided by the down-time counter unit (33); and the switching elements (41, 42, 53) are triggered by the time delay unit (34) only after the delay period has elapsed.