PURPOSE: To permit the early discovery of faulty circulation of hot-water in a bathtub by a method wherein the circulation is decided that it is faulty when the reduction of detecting amount of a water level sensor upon starting a forced circulation pump is smaller than a predetermined amount and the operation of the circulation pump is stopped.

CONSTITUTION: A water level sensor 15, detecting a water level in a bathtub, is provided in a bath boiler 13, however, the sensor 15 can be provided either in an upper circulation pipe 10 or in a lower circulation pipe 12. The decision of circulation, which determines whether the hot-water in the bathtub is heated or not, is started by putting on an additional heating switch 23 whereby a forced circulation pump 18 is put on and the detecting value of the water level sensor 15 is decided whether it is lower than a given amount, stored in a controller 21 previously, or not. When the detecting value of the water level sensor 15 is lower than the given amount, it is decided that circulation is faulty and the forced circulation pump 18 is stopped. According to this method, the bath boiler will never be heated when the circulation is faulty.