An electronic circuit control arrangement to sustain water discharging from an instantaneous hot water heater at a set temperature or range having a proportional water temperature signal derived from a sensing arrangement in communication with water inlet and outlet ports so as to sense the respective temperatures at each port to provide a comparatively measurable proportional difference between the inlet and outlet temperatures set against referenced parameters, including a comparator that acts as an operable control of a switch adapted to couple and de-couple an alternating current power source to the heating element through a duty cycle of highs and lows to provide a rate to generate and maintain the appropriate coupling and/or de-coupling of the alternating current power source to and from the heating element to achieve the desired referenced temperature and/or range.