The 2nd flue (fireplace) is about 1' above the central heat flue but the clay liners are touching at the top of the brick chimney. Am installing wood stove insert in one. Meanwhile the central heat (propane boiler) is running in the other. 6" flex liner in 12x12 clay masonry flue is not connected to stove (trying to connect and having difficulty, but that's a different problem) is open in fireplace. When boiler is firing can smell exhaust in fireplace -- not strong, but definite. CO detector is nearby and not going off. Not feeling light headed or anything, so doubt there's CO. Had an insert in there previously connected to ss liner and never noticed this before.