Method of manufacturing a water heater

A method of constructing a double walled vessel, the walls of the double walled vessel comprising an inner shell and an outer shell, respectively, and wherein the outer shell comprises a channel structure formed around the circumference thereof, the channel structure comprising a inner surface that is inwardly and radially offset from the surface of the outer shell around the circumference of the outer shell, the inner surface being bounded by upper and lower circumferential transition areas which loin the inner surface of the channel structure to the outer surface of the shell, the method comprising the steps of:

• providing an inner shell having a predetermined size and shape;
• providing an outer shell having a predetermined size and shape sufficient to contain therein the inner shell such that the walls of the respective shells are spaced apart from one another to define an annular space therebetween;
• sectioning said outer shell so as to enable the insertion of the inner shell into the interior of the outer shell by removing the material of the outer shell comprising the upper transition area of the channel structure, thereby separating the upper portion of the shell from a base portion of the shell so as to enable the insertion of the inner shell into the interior of the outer shell;
• arranging the inner shell within the outer shell in a predetermined relation to the outer shell;
• reassembling and thereby closing the outer shell around the inner shell;
• and
• injecting an expandable foam into the annular space between the inner and outer shells, the expandable foam expanding to substantially fill the space between the inner and outer shells, the expandable foam becoming substantially rigid after a predetermined cure time so as to permanently locate the inner shell within the outer shell in its predetermined relationship to the outer shell.