Abstract: A system for controlling the flow of a liquid through a solar heating system of the type having the output of a solar energy collector coupled to the input of a storage tank and having a circulating pump interposed along a return liquid pipe coupled between the recirculating output of the storage tank and the input of the solar energy collector. A first element senses the temperature of the liquid adjacent to the output of the solar energy collector and generates a first signal responsive thereto. A second element senses the temperature of the liquid at a second location within the solar heating system and generates a second signal responsive thereto. A first comparator is coupled between the first and second elements for generating a run signal responsive to the difference between the first and second signals exceeding a predetermined level.