Abstract: A solar heating control is adapted to be interconnected to a pre-existing auxiliary heating system connected to operate in response to a command signal issued by a pre-existing thermostat. The control operates in response to a first temperature sensed within the solar heating system to directly conduct the command signal from the thermostat to the auxiliary heating system to provide an auxiliary heating sequence. The control responds to a second sensed temperature at the solar heating system to respond to the command signal to operate the solar heating system. The control may be selectively deactivated to permit exclusive operation of the auxiliary heating system by the thermostat when desired. The control selectively connects an alternate impedance to the thermostat for operation in a solar heating sequence to provide similar cyclic operation in both the solar and auxiliary heating sequences.