Abstract: A heat exchanger is formed of a pair of concentrically arranged open-ended cylindrical shells, defining a chamber therebetween, in through which water can pass. Each end of the pair of shells is closed by a removable cap. 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. A continuous helical wire is located in the chamber, between the inlet and the outlet and in contact with the opposing walls of the chamber to convert the chamber into an elongated helical passage.