PURPOSE: To facilitate the installation and maintenance of a detector wiring in the water heater wherein a circulation pump is operated to accumulate heat based on a difference of the water temperature in a solar energy collector and a heat accumulator by a method wherein the water temperature at the inlet of the heat accumulator is detected as the water temperature of the solar energy collector part.

CONSTITUTION: In the water heater utilizing solar energy, the circulation pump 3 is connected in series between the solar energy collector 1 installed on a roof of a building or the like and the heat accumulator 2 installed on the ground. This circulation pump 3 is adapted to operate only when the water temperatre in the collector 1 part is higher than that within the heat accumulator 2. In this instance, as the water temperature of the collector 1 part, the water temperature t1 at the inlet of the heat accumulator 2 is detected by a thermistor 6, and compared with the water temperature t2 in the heat accumulator 2 detected by another thermistor 7 to provide a temperature difference, which is detected by a temperature difference thermostat 8. If the thermostat 8 detects that the water temperature t1 is higher than the water temperature t2 by a predetermined value Δt or more, the circulation pump 3 is operated. Thereafter, if the temperature difference becomes within the predetermined value Δt, the circulation pump 3 is subjected to the control of the controller 9 so that it contines to operate for a certain period of time and then stops.