PURPOSE: To efficiently recover the waste heat and reduce the fuel consumption by a method wherein a funnel surmounting part is surrounded by a water tank and heat exchange pipes, the upper and lower ends of which are open in the water tank, are provided within the funnel along the surmounting part.

CONSTITUTION: The exhaust gas produced in a boiler 1 ascends the funnel 3. At this time, the heat held in the exhaust gas heats the water 11 in the heat exchange pipes 5. As a result, the water 11 in the water tank 4 flows in the currents as indicated with the arrows 12 to absorb the waste heat, resulting in raising the water temperature in the water tank 4. Said hot water 11 is fed to the boiler 1. In such a manner as mentioned above, the waste heat of the boiler 1 can be efficiently recovered and the reduction of the fuel consumption can be resulted.