A steam boiler (15) comprising a fuel burning device (16) disposed in a combustion chamber and adapted to burn a fuel to form combustion gases (19), a passage (30) extending between a water inlet (20) and a steam outlet (23), the passage having a water space (31) and a steam space (32), a flue passage (33) extending between the combustion chamber and a flue outlet (24) and having a gas heat transfer space (34), a heat exchange element (43) between the gas heat transfer space and the water and steam spaces, the heat exchange element having a gas-side surface (35) that absorbs heat from the combustion gases and an opposed water-side surface (38) that radiates heat, the water-side surface having a first portion (39) adjacent to the water space and a second portion (40) adjacent to the steam space, the gas-side surface having a first portion (36) opposite the water-side first portion and a second portion (37) opposite the water-side second portion, the water-side first portion having a surface area and the water side second portion having a surface area, the gas-side first portion having a surface area and the gas-side second portion having a surface area, and a heat shield (41, 42, 45, 48) covering at least some of the surface area of the gas-side second portion. The surface area of the gas-side first portion may be greater than the surface area of the water-side first portion and the surface area of the gas-side second portion may be less than or approximately equal to the surface area of the water-side second portion.