An annular tube heat exchanger immersed in a medium, such as a fluid. The heat exchanger comprises a fluid-tight combustion tube including a gas-fired radiant burner for providing a source of heat, at least one heat exchange tube including an inner tube disposed within an outer tube, the outer tube in thermal contact with the combustion tube, the inner tube being open-ended to allow the fluid to pass therethrough, and a fluid-tight combustion tube in thermal contact with the outer tube of the at least one heat exchange tube. The combustion gas from the radiant burner passes through the outer tube of the at least one heat exchange tube to cause an increase in the temperature of the fluid. By providing the maximum amount of heat transfer surface area, the annular tube heat exchanger provides a highly efficient means of increasing the temperature of the fluid.