A steam accumulator has an upright vertically elongated vessel provided internally with a guide that subdivides a body of hot water in the vessel into an outer upflow column and a central downflow column. This guide is at least partially formed as a downwardly tapering frustocone and may have an upper portion carried on a float so as to maintain the upper edge of its upper portion a predetermined distance below the surface of a body of water within the vessel. A discharge conduit opens at the extreme upper end of the vessel and may have a section extending downwardly through the liquid body in the vessel.