a steam generator apparatus which includes an enclosure comprising vertically extending front and back waterwalls joined by opposed spaced first and second waterwalls , the enclosure also includes a plurality of spaced vertically extending windboxes , each of the windboxes faces the interior of the enclosure . each of the windboxes comprises a plurality of modules that are configured for installation at respective elevations , each of the modules being dimensioned and configured for shipment to the installation site . some forms of the invention further includes a vertically extending truss assembly and the ignitor and auxiliary air compartments are supported on the truss assembly . other forms of the invention include air ducting supplying air to air compartments and the truss assembly extends within the ducting . still other forms of the invention may utilize the truss assembly without the modular windbox .