I have a heat loss calculator from builditsolar.com. Today I came up with a calc of 36,000 btu per hr. So I went ahead and weighed 45 pounds of dry wood and put it in the eko at 1:30. There was only coals left so I figure there were no real btus in the boiler. It took 7 hours before the boiler could not keep water temps @ 140 degrees which is the min. for usefull heat in a hx. The temp before I put in the wood was also 140. I figure 45 lbs of dry wood @7000 btu per lb =315,000 btu to keep the house @ 68 degrees. So I needed 252k btu but the wood put out 315k. So 252,000 btu divided by 315,000 btu =80% efficiency. Does this make sense? Have you ever figured your % efficiency? If so how did you figure it? Also, does anyone else know where I can find a heat loss calc online?