A furnace system for suppressing process gas flow instabilities during drawing of an optical preform in the production of an optical fiber, the furnace system comprising: a muffle defining a furnace cavity; an upper muffle extension positioned above the furnace cavity; a downfeed handle for supporting the optical fiber preform as it is drawn into the furnace cavity; an upper heater thermally coupled to the upper muffle extension; a process gas supply positioned in the upper muffle extension; and a lower heater coupled to the muffle and configured to create a hot zone within the furnace cavity, which hot zone has a temperature of about 1700 °C or greater, for example of from about 1800 °C to about 2000 °C, for example of about 1700 °C, 1800 °C, 1900 °C, or about 2000 °C, or any range having any two of these values as endpoints, in which hot zone viscosity of the optical fiber preform is decreased, wherein the upper heater is configured to heat a furnace system area above the hot zone, to reduce the temperature difference between the furnace system area above the hot zone and the hot zone, wherein the downfeed handle is positioned within the upper muffle extension such that a gap is defined between an outer surface of the downfeed handle and an inner surface of the upper muffle extension, wherein the downfeed handle further comprises a boule, wherein the boule is configured to support the optical fiber preform and is drawable with the optical fiber perform into the furnace cavity and through the upper muffle extension, wherein an exterior surface of the boule is flush with the outer surface of the downfeed handle -, whereby the gap remains constant as the boule is drawn through the upper muffle extension into the furnace cavity. a muffle defining a furnace cavity; an upper muffle extension positioned above the furnace cavity; a downfeed handle for supporting the optical fiber preform as it is drawn into the furnace cavity; an upper heater thermally coupled to the upper muffle extension; a process gas supply positioned in the upper muffle extension; and a lower heater coupled to the muffle and configured to create a hot zone within the furnace cavity, which hot zone has a temperature of about 1700 °C or greater, for example of from about 1800 °C to about 2000 °C, for example of about 1700 °C, 1800 °C, 1900 °C, or about 2000 °C, or any range having any two of these values as endpoints, in which hot zone viscosity of the optical fiber preform is decreased, wherein the upper heater is configured to heat a furnace system area above the hot zone, to reduce the temperature difference between the furnace system area above the hot zone and the hot zone, wherein the downfeed handle is positioned within the upper muffle extension such that a gap is defined between an outer surface of the downfeed handle and an inner surface of the upper muffle extension, wherein the downfeed handle further comprises a boule, wherein the boule is configured to support the optical fiber preform and is drawable with the optical fiber perform into the furnace cavity and through the upper muffle extension, wherein an exterior surface of the boule is flush with the outer surface of the downfeed handle -, whereby the gap remains constant as the boule is drawn through the upper muffle extension into the furnace cavity.