PURPOSE: To obtain a reverse flow preventing device which will never generate reverse flow even when the level of water in a tank is high by a method wherein an air extracting pipe from the overflow port of a hopper to the inside of the heat exchanger of a bath boiler is made double and a plurality of small holes are provided on the inside pipe of the air extracting pipe.

CONSTITUTION: The water discharging port 11 of a funnel-shaped hopper 10 and the heat exchanger 2 of a bath boiler are connected through a communicating pipe 12. An air extracting pipe 25, from the overflow port 13 of the hopper 10 to the heat exchanger 2 of the bath boiler, is provided at the inside of the communicating pipe 12 and a plurality of small holes 26 is arranged between the water discharging port 11 and the heat exchanger 2 of the bath boiler. Water, supplied from a city water pipe passes through an inlet pipe 6, is heated by a hot-water supplying heat exchanger 1, is guided to an outlet pipe 7 and is supplied to a bathtub 4 through the hopper 10 and the communicating pipe 12. When the level of hot-water in the bathtub 4 becomes high, an air reservoir is generated in the communicating pipe 12. Reserved air enters into the air extracting pipe 25 through the small holes 26 and rises in the pipe 25, thereafter, is discharged into atmosphere through an overflow port 13. According to this method, the water below the hopper 10 will never flow reversely from a water pouring port 14 into the side of the city water pipe and is supplied into the bathtub 4.