Abstract: Heat exchange apparatus for raising the temperature of a subject fluid in which a fuel burner is located at least partially within a heat-exchange liquid and the subject fluid is passed through heat-exchange liquid, the combustion products of the fuel burner being contacted with the heat exchange fluid in the vicinity of the subject fluid. In this way the fuel burner may be constructed entirely of metal so that large units of low weight may be used. Preferably the fuel burner includes a tangential air inlet and a combustion chamber of frusto-conical form.