It is generally recommended that flues that are no longer in use are ventilated, with a grille at the bottom opening into a room and a cowl of some sort at the top to stop rain entering, so that there is air movement up through the flue to prevent the build-up of damp within the flue and emerging on interior walls.
But what happens if there is a SS liner in the flue and the gas fire attached to that liner only gets occasional use? There's obviously a concentric space between the liner and the old flue wall; should this be ventilated? I can see how an entry into that space might be made at the lower end from a room, but if the termination of the liner at the top is sealed to the cowl, how will air circulate as it does in an unused flue? Or aren't SS liners actually sealed to the chimney at the top, but attached to a suitable cowl with a gap to allow air movement up through the concentric space?