a building official once told me he saw a video of flames shooting out of the house though the "vent" on a direct vent hook-up installation.
here is the rub. one can see why one would want a sealed loop for combustion. lots of other appliances are set up that way, often with a tube within a tube type combination vent and exhaust (sometimes/often with a fan assist). however, the exhaust on a wood/coal stove is much hotter (fans not practical), and therefore more dangerous should you experience negative pressure in the home. always your home (boat ) must have positive pressure for a natural draft appliance. if it is negative and less than the draft of your flue you are in trouble.
