front end of the outer shell 10 on which the mounting flange 20 is disposed is unsealed to allow the inner chamber 30 to be inserted within the outer shell 10 the back end of the outer shell 10 remote from the mounting flange 20 is sealed this closed end serves as the primary surface through which heat is exchanged between the heating unit 6 and the water in one embodiment of the invention the closed end of the outer shell 10 is planar in an alternative embodiment of the invention the closed end of the outer shell 10 is dome shaped with the closed end curving outward an outer shell 10 having a dome shaped closed end is particularly advantageous when the heating unit 6 is to be installed in a large capacity tank 3 since it will better withstand the increased static pressure exerted on the exterior surfaces of outer shell 10 than would a planar shaped closed end to prevent the outer shell 10 from being corroded by the water in the tank 3 the exterior surfaces of the shell 10 may be clad in any suitable coating such as a thin sheet of copper alternatively it may be coated with ceramite 34 glassguard 34 34 glassguard 34 is a trademark of england hughes bell amp co ltd of manchester england as seen in fig2 and 4 the hollow cylindrical inner chamber 30 which serves as a combustion chamber is inserted into the outer shell 10 to form an annular clearance space 22 therebetween the inner chamber 30 is open at both ends the front end.