PURPOSE: To contrive to enhance a low-combustion operating efficiency of a burner, by a method wherein water temperature in a water heater is divided into three temperature zones, and multi-stage control of an input device is performed in accordance with the variation in the water temperature with time in the case of an intermediate temperature zone.

CONSTITUTION: An appropriate low temperature set point is set on an operation controller 6 comprising a temperature detector 5 for the water heater 1, and an intermediate temperature set point and a high temperature set point are set which are increasingly higher than the low set point by arbitrary temperature differences. When the water temperature in the heater 1 is in the low temperature zone, the burner 4 is operated at high combustion output, and when the water temperature is in the high temperature zone, the burner 4 is operated at low combustion output. In the case where the water temperature is in the intermediate temperature zone, the burner 4 is operated at low combustion output when the change of the temperature with time is in a rising tendency, and the burner 4 is operated at high combustion output when the change is in a falling tendency.