My wood fired heating boiler uses gasification to extract the heat normally lost up the chimney. The wood fire burns on top of a refractory section that has three passages through it, A down draft grate draws the exhaust from the fire down into the central chamber where the gases re-burn, it then loops back through the outer two passages before going to the fire tube.
The manufacturer claims 1800 degrees burn temperature in the return section. Don't doubt it, If the wood is down and you can see into that chamber its pure white.
In 16 years I've never cleaned the chimney, nothing ever builds up>
Greg
