PURPOSE: To unnecessitate the use of an expensive heat-resistant material as a partitioning plate and a heat insulating material by a method wherein the partitioning plate is provided at the upper end of a coil-type heat exchanger and a clearance between the partitioning plate and the upper plate of a boiler body is communicated with the discharging port of combustion gas to prevent the temperature rise of the upper plate of the boiler body as well as the partitioning plate.

CONSTITUTION: The partitioning plate 9 is provided at the upper end of the coil-type heat exchanger 4 and the clearance 6 between the upper plate 1A of the boiler body and the partitioning plate 9 is communicated with the discharging port 2 of the combustion gas. The high-temperature combustion gas is discharged out of the discharging port 2 through the clearances 7, 8, 6 under heating the liquid in the heat exchanger 4, however, the temperature thereof is reduced as it approaches the discharging port 2, therefore, the temperature of the upper plate 1A may be kept at 100W150°C, which is lower than the temperature of 150W200°C possessed by the combustion gas passing through the clearance 6. The partitioning plate 9 is cooled by the low-temperature combustion gas, therefore, the temperature of the partitioning plate 9 is reduced to 500W600°C. Thus, the temperature of the upper plate of the boiler body may be kept at a temperature lower than the same of the combustion gas at the exhaust port side, the expensive heat-resistant material is not necessitated to be used as the partitioning plate and the heat insulating material may also be not necessitated to be used for the same.