Abstract: Each longitudinal section of a concentric direct vent duct structure for a fuel-fired appliance, such as a furnace, is formed using a tubular outer duct having an axially spaced pair of triangular stand-off frame members anchored therein, and a smaller diameter inner duct having an axially spaced pair of ramped external annular locking projections thereon. To assemble the concentric duct section, the inner duct is simply axially inserted into the outer duct. As the inner duct is inserted into the outer duct, a ramped annular side surface on the leading inner duct locking projection resiliently and radially outwardly deflects side wall portions of the stand-off frame members and then permits them to snap back to their original undeflected positions.