PURPOSE: To contrive the improvement of usability, safety and saving of energy as a heat exchanger, by a method wherein a switch is turned ON after the presetting of an estimated time required for heating up condition, after certain time passing, an automatic notifying is performed at a alarm part and a display part, then a hot water circulating is stopped, in an indirect additional heating type bath heater.

CONSTITUTION: A heating up information of an indirect additional heating heat exchanger 15 is notified by an operating of a control circuit 7 with rectified direct electric power. When a signal is transmitted into an indirect follow burning ON-OFF detecting part 4, a timer part 5 generates a space of time from a frequency of an electric power source, continues time up. The timer part 5 is provided with a time adjusting variable resistor 6 which adjusts the difference of heating time varied in each season. When the timer part 5 becomes the setting time, a signal is transmitted from the timer part 5 to a control circuit 7, then a buzzer 8 and LED9 forming a notifying part are operated, simultaneously, a hot water circulating control circuit 10 is operated, a flow of a hot water circulating part 14 is stopped with solenoid valve 17, a hot water from the heat exchanger 15 is also cut off.