Abstract: A long-lasting hot water boiler. The boiler has a lower drum with a first pass fire-tube for generating combustion gases, an upper drum with a second pass fire-tube bundle for the gaseous products of combustion, and a lined fire reversing chamber for directing the gaseous products from the first pass to the second pass. The reversing chamber liner includes a saddle piece and a turnaround piece which cooperate to protect vulnerable weld joints adjacent the reversing chamber and which maintain proper positioning even after weld pins melt. The upper and lower drums are interconnected by an expansion joint which permits unequal thermal expansion of the drums, and a bypass circulation system prevents thermal shocking of the boiler.