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 .