HORIZONTAL FLUE PART

PROBLEM TO BE SOLVED: To provide a horizontal flue part solving such a disadvantage that a ratio of the combustion gas flow rate flowing through a gap having a small passage resistance increases compared with the combustion gas flowing through heat exchanger tubes and hence the heat transfer efficiency of the heat exchanging part of the horizontal flue part gets worse because the weir of the horizontal flue part is positioned at the opening to the inlet of the vertical flue part of the gap. SOLUTION: A weir 1 installed at the bottom face of a horizontal flue part 05 which allows the combustion gas 016 generated in a furnace to nearly horizontally pass through the inside of heat exchanger tubes of a plurality of heat exchangers 011, 012 extended down from the ceiling firnace wall 09 and the inside of a gap formed at the lower part of the heat exchangers 011, 012 and enter a vertical flue part, is erected from the bottom face of the further upstream side than the middle part of the heat exchangers 011, 012 adjacently extended down from the ceiling furnace wall 09 or the heat exchanger part 011 installed at the uppermost upstram side of the horizontal flue part 05.