PURPOSE: To enable perfect combustion to be made by a method wherein in the bath heater using the waste material such as chaff as a fuel, air chambers are formed in the fuel by agitating the fuel by utilizing the vibrations of an automatic feed plate driven by a motor and further, burned ashes are removed.

CONSTITUTION: The bath heater 1 is provided with a fuel chamber 6 into which the waste material such as the chaff or sawdust used as the fuel is thrown from an inlet port 42, a fuel chamber 6 and a combustion chamber 7. Further, the chambers 6 and 7 are partitioned with a partition plate 9 having a water chamber 8 while they are made to communicate with each other through a communication passage 10 and heat receiving drums 13 and 14 having water chambers 11 and 12 communicating with a water chamber 4 therearound are arranged in two stages above the combustion chamber 7. A transfer section 26 provided below the fuel chamber 6 comprises a frame 32 provided with vibratory roosters 29 and 30 and the feed plate 31 supported by a shaft so as to oscillate through a rotary shaft 33 linked to a drive mechanism such as a motor arranged outside the bath heater. In short, the fuel is transferred toward the combustion chamber in sequence by the alternative vibrations of the roosters and the feed plate arranged below the combustion chamber while the fuel is agitated by the vibrations of the feed plate with the formation of the air chambers or gaps therein and transferred further. Moreover, since the burned ashes are dropped down through the vibrating roosters, a sufficient quantity of air is supplied to the fuel so that the fuel is burned completely.