A furnace assembly for manufacturing a hollow-core optical fiber from a hollow-core fiber preform includes a furnace having a body defining a body cavity extending along a longitudinal axis between a preform input port and a hollow-core fiber output port. The body cavity is configured to locate the hollow-core optical fiber preform and a process gas. At least one primary heating element is proximate a necking region of the hollow-core optical fiber preform and configured to maintain the necking region at a draw temperature, the draw temperature sufficient to soften the necking region. The process gas occupies a flow field surrounding a cladding outer surface. The flow field extends from the necking region to the preform input port and the process gas has a flow with an average Grashof number less than 1.610.sup.4 in the flow field.