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.

Abstract: A sheet metal colinear-to-coaxial duct transition assembly for a fuel-fired, direct vent heating appliance is fabricated by providing a one-piece rectangular sheet metal plenum box having a rear wall with first and second circular openings therein, and an open front side. A separate rectangular front wall of the plenum box is also formed and has a circular opening therein. Prior to assembling the plenum box, a rearwardly projecting combustion air stub duct is secured to the rear plenum box wall at the first opening therein, and a forwardly projecting combustion air intake duct is secured to the front plenum box wall at the opening therein. Cooperating side edge portions and flange pockets on the front wall and the open side of the plenum box and then interlocked and crimped to complete the plenum box.