PURPOSE: To retrieve the waste heat of an engine efficiently and improve the operating efficiency of a system in case the engine-driven hot-water supplier is operated as an exclusive machine for supplying hot-water by a method wherein a feed water heating heat exchanger in the heat medium path of the main body of the engine is provided in series to a waste heat retrieving heat exchanger.

CONSTITUTION: In case the engine-driven hot-water supplier is operated as the exclusive machine for supplying hot-water, a three-way valve 19 is controlled so as to flow feed water to both of water feeding ports 17, 18 as shown by the broken line B in the diagram. In this case, hot-water in a hot-water reserving tank 13 is heated by the condensing heat retrieving heat exchanger 14 below the tank and the waste heat retrieving heat exchanger 15 above the tank. The feed water heating heat exchanger 22, exchanging the heat of the heat medium having higher temperature than the hot-water in the tank, whose heat is exchanged in the heat exchanger 15, with the heat of the feed water, is provided to retrieve the waste heat of the engine sufficiently. The heat exchanger 22 is provided in series to the heat exchanger 15 in the heat medium path 21 in such a manner, therefore, waste heat may be retrieved from the heat medium having higher temperature than the feed water heated in the heat exchanger 15 to a temperature near the temperature of the feed water.