A fluid heater constructed of rectangular tubing through which fluid to be heated is circulated in a circuitous path. The floor and walls of a combustion chamber of the heater are constructed of the rectangular tubing. A tubular coil stack is positioned over the combustion chamber and surrounded by coil stack chamber walls constructed of the rectangular tubing. Consequently, all surfaces of the fluid heater exposed directly to flame are constantly cooled by the fluid to be heated. A combustion unit of the fluid heater is removable to facilitate maintenance.