Abstract: In a water tube boiler comprising a housing enclosing tubes bent to form vertical chambers successively traversed by rising hot combustion gases, the tubes being connected at the bottom to the interior of the jacket of a jacketed cylindrical combustion chamber, which jacket is supplied with cold water, and at the top to a steam manifold, the manifold and jacketed chamber project beyond the housing with a downcomer outside the housing connecting the top of the jacket and the lowest part of the manifold, thereby permitting the boiler to operate with a shallow level of water in the manifold, speeding up circulation of water and its heating, and permitting substantially dry steam to be discharged from the upper manifold. The chambers may include baffles which are angled so that the hot gas hits them at an angle less than 90.degree. so as to be deflected thereby in the direction of its advance, thereby avoiding hot spots.