Im looking for some pointers on installing a direct vent gas cabinet fireplace using an existing fireplace for venting. From reading the manual and what I envision I plan on using a co-axial to co-linear connector to break the exhaust and combustion into 4" pipes. Extend the exhaust all the way up the chimney to a cap. My question is what to do with the combustion air pipe. Do I need to extend it up to the top of the chimney? Or can I have it draw air from inside the chimney space? I would put in a blockoff plate or stuff insulation in the chimney so the combustion air is drawn from the outside down the chimney. Sound like a plan?