I used 6 pounds of air per pound of bone dry wood. As kuribo pointed out, that's the amount you need for a perfect stoichiometric ratio, and our boilers certainly provide more air than that. 65 pounds of wood + 390 pounds of air gives 455 pounds of flue gas. At .24 BTU/lb, raising it from 60 degrees to 600 degrees would take about 59,000 BTU. In my case, 600 is a bit high, but perhaps I should be using outdoor temp as the starting point. In any event, I think the differences are pretty minor in the grand scheme of things. If I use air at 1.6 x stoichiometric, the loss jumps from 59,000 to 89,000.