Abstract: A device constituting an improved boiler design. The heat exchanger, known in the art as a pancake, is composed of an asymmetrical grid of interconnected hollow eliptical conduits through which flow the heat transfer fluid. In one embodiment of the invention, two pancakes are connected in series in such a way that their asymmetry is complementary. Such an arrangement is claimed to increase the efficiency of heat transfer and reduce the frequency of cleaning of the boiler when compared to boilers in the known art.