PURPOSE: To improve thermal efficiency by a method wherein the opening end of the shell plate of a baffle plate is sealed air-tightly with respect to a water can.

CONSTITUTION: Furl, injected from a burner 17, is burnt at the inside of the baffle plate 20. The shell plate 23 of the baffle plate 20 is opened at the opposite side of an exhaust port 18, therefore, water in the water can 9a, located at the opposite side of the exhaust port 18, receives the radiation heat of a combustion flame and is heated. A clearance between the opening ends 23a, 23b of the shell plate 23 and the water can 9 is sealed and the lower part of the burner 17 is closed by a bottom plate 24, therefore, combustion gas, discharged out of the opening part of the shell plate 23, descends along the water can 9a, enters into the lower part of the bottom plate 24, the combustion gas, turned here, spreads uniformly in a combustion gas path 21 formed between the baffle plate 20 and the water can 9b and is discharged from the exhaust port 18 under contacting with the water can. According to this method, the heat transfer of the whole of the water can is effected under a good condition and the thermal efficiency of the boiler may be improved.