Both wood burning and pellet inserts are very much the same in the fact that they should use a 304 or 316 stainless steel liner system, flexible or rigid, that is sized to the opening of the flue collar on the appliance. The liner can indeed be supported and terminated just below the existing chimney cap. It is not necessary to use a factory top plate and cap, if the existing cap is in good condition. Metal strapping or fabricated brackets can be used to support the liner.
Collin C. - NFI Master Hearth Professional
on October 22, 2013