Abstract: A vertical watertube boiler in which a bank of vertically oriented uniformly spaced watertubes, located in an upright cylindrical shell that has enlarged diameter upper and lower end portions, connect paired headers mounted on and protruding into diametrically opposite sides of the enlarged diameter end portions of the shell, the end portions of the tubes being bent laterally outward and joined to wall portions common to the headers and the gas pass formed by the shell; the headers associated with the upper enlarged diameter end portion of the shell being connected by risers with a steam drum and the headers at the lower enlarged end portion of the shell being connected with the steam drum by downcomers.