Just something I found interesting, the first year using our wood furnace I was using a standard 6"double wall with a standard cap....the draft was ok but I did install a Draft Inducer and it did improve the draw....This year I finished up a custom concrete cap, it almost looks like a Chimney Pot Hybrid...four open sides with SS screening and maybe 5-6" to the underside of the top concrete cap. Since the new season I have not needed the draft inducer once....the draw is better, much better, wood is the same, everything is the same except the cap...I have no idea what the specific science is behind the cap I built, I certainly was not thinking about anything but the basics...just got lucky I guess..I have been reading and observing a lot about many different aspects of burning..on flue pipes and drafting i think the individual house, roof lines, Topography around the structure , orientation of the structure, neighboring structures all play a part in how well or poorly a stove or furnace will draft.....