Abstract: Three elements are secured together in new and unique fashion to form a heating unit, movable and installable as an integral unit and capable of obtaining maximum BTU's from the fuel used. These elements are (1) an outer jacket, which defines the design and outline of the heater, (2) an inner jacket, forming with the outer jacket a residual burning chamber, narrow in cross section and relatively high in vertical extent to provide maximum heat transfer surface, and (3) an A- or tent-shaped fire box or combustion chamber, enclosed by and secured to the inner and outer jackets. The tent-shaped fire box extends from the extreme lower end of the heater to its upper limit. The edges of the walls of the fire box are secured to outer and inner jackets in sealed relationship, dividing the areas within the heater into heat producing and heat conducting areas and clean air passageways. The configuration of the inner jacket 14 is an incomplete cylinder.