Abstract: A water-tube boiler (1) of the "D" type is disclosed which includes a water-cooled rear wall comprising a plurality of vertically disposed riser tubes (14a, b) located at the rear end of the furnace chamber (5). Novel sealing means are disclosed for sealing the aforesaid rear wall to its adjacent vertically extending walls of the furnace chamber (5) and convection section (7). A further aspect of the invention lies in the particular feeder conduit means (9, 10, 11, 12, 13) provided for supplying water to the riser tubes of the water-cooled rear wall and also in the particular return conduit means (15a, b; 16a, b) provided for connecting the riser tubes (14a, b) to the steam drum (3). In yet another aspect of the invention, novel construction is provided associated with the convection section for discharging flue gases from the boiler.