This has been hashed through pretty well, see (among others): Discussion 1, Discussion 2. Also see Energy in Wood. Energy in Wood calculates My improved data logging is producing repeatable efficiency results in weighed wood burns, based on the Energy in Wood assumptions. That calculation, adjusted for boiler water heated, is coming out at right around 83% net btu's delivered to storage during weighed wood burns. In Horizontal Tank, I calculated that "Net btu’s/lb [delivered to storage] of wood were 4,628," but I did not take into account the fact of about an additional 55 gallons of boiler water being heated. In a burn yesterday, the calculation was 4,680 net btu's/lb delivered to storage and also not taking into account heating of boiler water. These calculations resulted in 77% efficiency, but when 55 additional gallons of boiler water are entered into the calculation, the result is 83% efficiency. In making these calculations, I am 1) weighing the wood, including kindling; 2) firing the boiler at a cold start; 3) mixing the 1000 gal storage tank to a uniform temperature before starting the burn; 4) mixing the storage tank to a uniform temperature at the end of the burn; 5) adding 55 gallons of water to account for the boiler; 6) calculating the btu's based on the deltaT in the tank+boiler from beginning to end of burn; and 7) not drawing heat from the system during the burns. I believe my calculation remains conservative because it does not account for heat loss from the boiler water and from the tank during the burn period. It also is conservative because my boiler and tank are in the heated space, and during each burn the temperature of the heated space rises by several degrees, as heat from the boiler, flue stack, and tank are released into the heated space. The variables which remain assumptions in these calculations are the available energy in the wood of 6,050 btu/lb and flue gases at a constant 400F. My actual flue gas temp varies during the burn, but based on logging stack temp it appears that 400F is a good average.