PURPOSE: To improve the total thermal efficiency without increasing the installation space by a method wherein the shape and arrangement of a combustion chamber and a heating chamber are changed and furthermore the flow pattern of the air forcibly blown on the surfaces of the chambers is also changed.

CONSTITUTION: Within a closed casing 1, a primary, a secondary and a tertiary heating chambers 2W4, which are respectively connected with one another in series, are provided in the configuration that the secondary and the tertiary heating chambers 3 and 4 arranged contiguous to each other in parallel are located above the primary heating chamber 2. The combustion gas burnt in the primary chamber 2 is discharged through the secondary and the tertiary heating chambers 3 and 4 to the outside. An open air suction port 5 and an open air exhaust port 6 are opened in the casing 1 respectively above the secondary and the tertiary heating chambers 3 and 4 and below the lower part of the primary heating chamber 2. The open air forcibly blown in from the suction port 5 is flowed through the casing 1, heated and discharged from the exhaust port 6 to the outside of the casing 1.