My boiler burns absolutely clean most of the time. People, including me, can't believe that a firebox full of burning wood produces nothing but heat waves. I can't believe you can get that much heat with so little environmental impact. But occasionally, it will smoke. I've tried to figure this out without much luck. We're not talking much smoke--just a little white wispy trail that has very little smell (and not a bad smell, at that) and dissipates quickly once it gets into the air. But it bugs me. At first I thought it was steam, but I believe it's smoke. I've entertained a number of theories about this, most of which have been documented here in one place or another, and most of which have been discounted either through logic or observation. Well, my theory de jour is that there is a direct relationship between smoke and ash buildup in the gasification chamber. When the boiler is smoking, in other words, it's time to clean out the ashes. Tried it yesterday and it worked. Prior to this, I've waited until the firebrick labyrinths below the nozzles basically filled with ash and coals before doing the cleanout, which is to say every 3 or 4 days. Now I'm inclined to start doing it more often.