PURPOSE: To contrive to effectively utilize the heat by a method wherein various kinds of heat sources such as oil, wood, garbage and the like are allowed to be burnt in a single boiler.

CONSTITUTION: Pipe is wound spirally so as to form a heating pipe 1, at the lower end of which a water lead-in port is provided and at the upper end of which a hot water outlet port is provided, and the interior of which is rendered to be a combustion chamber. The heating pipe 1, the periphery of which is surrounded with a heat absorbing plate 3 fixed with respect to the heating pipe 1 by means of wedges 2, is installed within an outer wall 5. The heat absorbed and collected by the heat absorbing plate 3 is led through the wedges 2 to the heating pipe 1. A furnace can be disassembled by pulling off the wedges 2 so as to replace the heating pipe 1 with another combustion cylinder for exclusively burning garbage.