Abstract: A heat exchanger is formed of a pair of concentrically arranged cylindrical shells, closed at each end to define a chamber through which water can pass. A combustion element extends into the inner shell from one end along the axis thereof and is connected to a source of heat such as a gas/air mixture which ignites in the combustion element causing the radiation of heat outwardly from the combustion element. The inner shell is formed of a corrugated wall which may be formed as a single corrugation comprising a continuous helix along the length thereof or a plurality of undulations extending along the length of the cylinder.

Abstract: There is described a porous ceramic reactor which is used in a heating system. The combustion of fuel occurs on the outer surface of the reactor. The outer surface has a plurality of indentations that break up the surface continuity. As a consequence, the scaling and cracking that occurs due to differential expansion and contraction is minimized. Alternatively, the surface continuity can be broken up by means of a wide mesh metal screen that is in contact with the surface thereby creating thermal continuities that minimize scaling.