A tankless water heating auxiliary system for a solar water heating system, includes a solar collector; a tankless water heater auxiliary system; an insulated water storage tank storing the potable water; a heat exchange system for heating stored water; and piping for connecting the collector, the storage tank and the heat exchanger in fluid communication. A first sensor is connected to and located adjacent the storage tank for sensing the temperature of the stored water at an outlet of the tank. A method for controlling initiation of heating in a tankless water heater auxiliary system, includes monitoring operation of a tankless water heater; measuring water flow using a water flow sensor to determine if water flow rate exceeds a use determined flow rate; implementing a control time delay into the tankless water heater to purge water from the heater and sense the inlet water supply temperature; measuring the water temperature using a heat exchanger outgoing thermistor; comparing the temperature measured by the thermistor to a predetermined temperature; and initiating a combustion sequence if the temperature measured by the outgoing themistor is less than the predetermined temperature.