I've got a CB 5648 that I have had running 24/7 for over a year now, heating 4300 square feet plus domestic HW for my house. I have a family of 7, we use a lot of hot water. I haven't been really careful in figuring burn quantities, but I can tell you that from December 2007 through May 2008, I was burning an average of exactly one cord a month. Little more in winter, little less in spring. As soon as the heat circulators shut down for the summer, I got to the point where all it took to heat water for the day was a few sticks. I went through about a full heaping contractor's wheelbarrow full of junk wood (scraps, bark chips, stump cuttings, rot & punk wood, pine slabs) per week during June, July and August. Right now I'm burning about the same amount every three days. Most of what I've burned the last year has been pine slabs and semi-rotten stuff, off my sawmill and scrounging near the house. I haven't even gotten into any significant amount of hardwoods yet.
I also discovered that you have to tinker with the temperature controller settings as the year goes by. I have mine set very low in the summer, as the boiler literally will boil off steam if the temperature is set at or near the high limit. The residual heat is enough to bring the water temp well over the 185 degree set point to 210-212 degrees (and a big cloud of steam) if the fire is good and hot. In the winter I bring the set point up to about 180.
