PURPOSE: To dispense with a bypass smoke duct and to make it possible to wash the titled heat exchanger in water completely even during the operation of the heat exchanger by a method wherein the heat exchanger provided in a duct is made rotatable within a predetermined angle range and a water washing device used for removing dust is provided above the heat exchanger.

CONSTITUTION: In case where the amount of dust adhered to the fins of the heat exchanger 6 in the duct is small, the front face of the heat exchanger is held normal to the flow of a gas. On the other hand, when the draft loss of the heat exchanger becomes great due to the accumulation of dust or the flow rate of the gas goes to an eccess, the heat exchanger is rotated through a certain angle so that it becomes inclined with respect to the gas flow and at the same time, a gas passage is provided in the wall of the duct so that a part of the gas is allowed to escape without being applied for a heat exchange operation and the draft loss between the upstream and the downstream side of the heat exchanger is reduced. The angle of rotation of the heat exchanger is controlled by a pressure or temperature detecting method such that a motor 12 of a driving device is controlled by a command signal from a control box 11 and the rotation of the heat exchanger is stopped by links 13aW13c. Further, in case where the heat exchanger 6 is washed in water, it is held horizontal and hot water from a washing water box 19 is supplied on it to thereby wash it entirely.