The York Shipley Global Series 400L and 500L three- and four-pass water-back design accommodates lower BTUH/CF (Mcal/m³) volumetric furnace heat release rates in comparison to competitive water-back configurations. The intermediate tubesheet design reduces vessel thermal stresses. York Shipley Global water-back boiler employ a low offset furnace with completely submerged self cleaning cylindrical reversing chamber, which permits for scale formation from water bound solids to drift to the bottom of the boiler for removal during blowdown procedures. Fire-side and water-side access is readily provided. Top longitudinal level service platform ensures safe and secure access, while bottom longitudinal inspection trough ensures unencumbered visual access.