Abstract: A tankless hot water heating coil assembly is releasably sealed in place with respect to the fire tube of a boiler: by extending a tubular tankless sleeve up from an opening through a top plate of the boiler; by positioning an O-Ring type seal atop an upper edge of the tankless sleeve by forming a groove in the O-Ring type seal and positioning the groove around the upper edge of the tankless sleeve; and by extending threaded fasteners through openings in a tankless coil plate which carries the tankless coil assembly and through tubular fastener positioning members secured about and to the outer wall of the tankless sleeve proximate the upper edge thereof.