PURPOSE: To maintain a high fuel cost efficiency of the titled element as a whole by constituting the element in such a manner that a cylindrical flue is provided in a substantially perpendicular direction, a heat exchange part filled with a heat medium around the periphery thereof, and cooled waste gas forming a downward flow flows out toward a lower outlet port together with liquid drops generated in the inside of a heat transfer wall of a flue.

CONSTITUTION: A boiler waste heat pipe is coupled and fixed to the flue inner cylinder 2 of a waste heat recovering element, and further the flue inner cylinder is fixed and connected to a discharge port 3 for waste gas and waste liquid. A space encircled by the flue inner cylinder 2 is filled with water as a heat medium 8 and constitutes a heat exchange member 12. A circulating port 9 of a heat exchange coil 7 is connected to a load 10. The waste gas from a boiler introduced from the top portion of the waste heat recovering element is robbed of its sensible heat and latent heat by low- temperature water at the heat exchange portion and is converted into a low- temperature waste gas. The low-temperature waste gas flows downward and simultaneously is robbed of its latent heat. Acidic water contents condensed on the inner wall surface of the flue inner cylinder flow down and are discharged out of the waste gas and waste liquid discharge port 3.