openings 140 to allow a flow u201c d u201d of the combustion products to enter third annular chamber 94 a cylindrical chamber wall 142 forming the central axial chamber 104 and bounding the third annular chamber 94 directs the flow of combustion gases through the boiler feed water tubes 20 openings 144 located within the bottom of cylindrical baffle plate 142 cause the combustion products to reverse direction and then pass over the second set of heat exchange tubes 86 and into an exhaust provided by an open end of central axial chamber 104 cooler 42 is formed in housing 2 within an outer cylindrical wall 148 that encloses the third set of heat exchange tubes 89 between outer cylindrical wall 148 and cylindrical chamber wall 102 a cooling fluid stream 150 for instance a water glycol mixture is introduced into an inlet 151 of the cooler 42 and is discharged as a heated cooling fluid stream 152 from an outlet 153 after passing through the third set of heat exchange tubes 89 for starting purposes a burner fuel stream 154 can be introduced to burners 156 firing into the second annular chamber 92 at openings 118 within base wall 120 of first.