ewdudley said:
Only the latent heat of evaporation is unrecoverable during the entire process. That is because it can only be regained by condensation of those vapors, which just doesn't (or shouldn't, at any rate) occur inside the flue. As long as the flue walls are above 212ºF, all of that valuable heat is lost forever. Same thing for the water formed by combustion itself (which is a much higher amount), but that heat loss was covered in my calculations by using the low heating value of the wood instead of the high heating value.
All other energy put into the system will come back during the cooling of the gases as they make their way from the firebox and through the stack. In other words, it doesn't make any difference how high you need to get the gases up to temp or how much energy it takes to do so. As long as the flue gas temps are not excessively high, all of that heat will need to be transferred back into the living space somewhere in order to satisfy the law of conservation of energy. In short, you cannot claim an increase in energy input without accounting for it at the output end. That's why research and test labs use the "stack-loss" method to determine stove efficiency. What goes in must come out, and they count every blessed thing that comes out of that stack.... including the exact amount of flue gases, their rate of removal, and their temperature all along the stack.
With wood stoves, you have to separate what in going on combustion wise with what is going on heat transfer wise. On several occasions I have posted a simple chart that shows the interrelationship between these two things and their effects on overall efficiency. Without a doubt, there are other heat losses involved in burning wood at higher moisture content. Nowhere did I say that there are not significant heat losses involved, it's just a matter of pinning them all down accurately. It is overall system efficiency that we should be concerned with.
Wood actually burns slower, cleaner, more predictably, and more efficiently as moisture content rises up to a certain point, but the increased rate of air delivery needed to burn it passes more heat up the flue. Above 20% water by weight (that is 25% MC as read on a resistance meter), overall efficiency begins to drop, even as combustion efficiency continues to increase. So burn wood at or around 20% water content for the very best overall efficiency, but let's not exaggerate the energy losses. They are certainly there, but twice as much heat from wood that is just a few points lower in MC is a total fallacy, and a very disheartening bit of misinformation for a new burner sitting on four cord of wood that measures a few points higher than "ideal".
