How does this conversation apply, if at all, to wood pellet and corn burning appliances? For reference I've been looking into the Selkirk Direct Temp system for combining the exhaust flue with preheated air intake for my pellet stove. I'd be interested in whether or not such a design might alleviate the problem with wind induced pressure differentials since both intake and exhaust are subject to the same pressure. Someone brought up the idea that oxygen concentrations are higher at low temperatures than at high temperatures. That's true, sorta. The amount of oxygen by volume is greater at low temperatures but not in relation to other gases (i.e. percentage). If outside air containing 4.5% oxygen is pre-heated by passing around the combustion chamber it is still 4.5% oxygen (assuming minimal loss to oxidation of the appliance) when it reaches the fuel. Combustion efficiency is improved by raising the temperature of all of the reactants and increasing the amount of time the reactants are in contact with one another. Air preheating is used in a variety of commercial furnaces to increase combustion efficiency and is one part (the other being oxygen enrichment) of the improved efficiency of modern blast furnaces. It seems reasonable that the indoor air would have a slightly lower oxygen concentration than the outdoor air for a given building with oxygen consuming occupants. However I don't know that the difference in oxygen concentration is great enough to adversely impact the appliance. Lower concentrations of oxygen should just mean increasing the air flow to the appliance but I doubt the difference between indoor and outdoor is enough to cause more than a point or two different in efficiency, if at all. Assuming the stack temperature can be kept high enough to efficiently exhaust flue gasses and prevent creosote condensation it would seem to me that a system that combines both into a single pipe, such as the Direct Temp product, would be ideal. I don't believe there is a similar product for wood burning appliances unfortunately. Someone please let me know if I'm completely off base here. It's been some time since I've taught chemistry and I haven't had opportunity to apply that knowledge to wood burning appliances.