To provide a bath hot water supply heating system capable of adding a heating function by post-mounting a heating device on an existing bath hot water supply heater.

A bathroom heater, which is selectively connected by means of three way valves 207 and 208, is provided in an existing system. Two differential water levels having high and low levels are detected with an H level water level sensor and an L level water level sensor. Hot water is filled up in a water storage tank when a heating operation command is received. The hot water fill-up operation is halted based on a signal of the H level water level sensor which detects a first water level and additional heating is started. In the case when the L level water level sensor fails to detect a second water level, a command is given to the bath hot water supply heater 3 so as to fill up hot water for replacement.