You guy's really need to find someone who owns a gasifier and go take a look at it under full gasification. They are so cool to watch operate. I was a little bored at work about 6 or 8 weeks ago and I took an hour or so and built a rocket stove. They are a natural updraft gasifier. You can't believe how hot they get. After about 15 minutes of operation my infrared thermometer won't even take a reading(907*F upper limit). If you have a good, modern, conventional boiler with a lot of heat transfer area, you will cut your wood consumption by real close to half(roughly 45% in my case) with a gasifier. I agree with NCredneck about the thermal efficiencies of the old and the new ones. They all make impossible claims. When mine is under full load, it is supposed to be operating at 2000*. I've checked my exhaust temperature and it's at just over 300*. That's already a 15 percent loss. There are also many more factors involved, and in my mind the most important one would be boiling out the moisture present in the wood. I don't remember the numbers, so I'll apologize in advance, but I think a pound of wood contains about 8500 BTU's(dry). At 20% moisture it takes around 17 or 18% of the available BTU's to boil out the water. Even if we say that number is 15% and you tack on another 15% going up the exhaust, in my book that puts you down to about 70% at best. That's a far more realistic as far as I'm concerned.