Gasifier
Minister of Fire
Some things in this wood heating thing I still have a hard time getting my head around. You just brought up one. If the draft fan in a lambda boiler is slowed down - based not on burn conditions or fuel load but rather rising water temps & closing delta T's - wouldn't that lower the combustion efficiency? It is cutting down the air supply while the fuel supply might be remaining constant - which seems to me at that point to be setting up for a less efficient burn.
EDIT: That was replying to jebatty's post - gasifier got in ahead of me on the reply.