Abstract: Apparatus for using exhaust heat from burning refuse is described wherein a heat exchanger constructed as a self-supporting unit is mounted, inside an enclosure, directly on the combustion chamber. The heat exchanger consists of at least two co-axial, vertical, cylindrical, single-layer packs of helical tubes with their convolutions in close contact with one another and located so that they lie nearly perpendicular to the direction of flow of the smoke and gases of combustion. One or each of the packs consists of a plurality of tubes with the convolutions of the respective tubes arranged in repeated series along the length of the pack. The packs of helical tubes may be connected in series or in parallel for heating water. Alternatively, they may be divided individually into sections for any one of a number of alternative boiler configurations in which the sections can provide preheaters, evaporators and superheaters.