Abstract: A condensing heating module for heating a fluid and adapted for use with a radiant infrared burner. The heat exchanger within the module is spirally disposed about a central burner cavity and has a fluid flow passage of serpentine shape formed within its wall. The upper and lower surfaces of the module, together with the spiral wall of the heat exchanger, form a spiral condensing flue leading from the central burner cavity to an external flue. Those portions of the spiral heat exchanger wall and the module upper and lower surfaces which are exposed to direct radiation from the burner are formed of a corrosion resistant steel, while the remainder of the surfaces and wall are formed of carbon steel to which has been laminated a layer of polypropylene such that the polypropylene layer will be exposed to the flue gas and condensate environment and provide corrosion protection to the carbon steel.