The controller for solar thermal heating systems manage the interaction between the solar collectors, boilers, buffer vessels, pumps, valves and electric heaters. It helps for increasing the comfort during operation by combining high efficiency with autonomy of the system. Joint work on functional differential temperature range of flexible boundary temperatures and internal clock further helps to increase energy savings in long terms. The build-in eight functional circuits serve more than enough for the the established practice conversions. Application of the device expands the thermosyphon water heaters to more complex solar systems with a combination of additional buffer, boiler and heater.