Ran into a frightening situation that's convinced me that there are a couple of steps that would be prudent in any gasifier installation. Inexperienced operator was reloading the boiler. A small stick blocked the bypass damper open. Operator closed bypass damper as far as possible and turned on the controller. The 'puff' almost dislodged the stovepipe from the chimney, and blew open the cleanout door at the chimney bottom. Stovepipe temperatures then proceeded to get much higher than normal. My thoughts: There needs to be a 'fan disable' relay in series between the controller and the fan so that the fan is disabled unless the relay is energized. The relay power should come through (1) a limit switch that's closed only when the bypass damper is fully closed, and (2) a 'panic' button. If you have a fire and can't close (or haven't closed) the bypass damper, then you don't want the fan to run. You might want the controller to be on, though, so that the circ pump can operate. If things go wrong, there needs to be a 'panic button' - a clear and appropriate way for a civilian to put the system in a safe condition. My thought is that controller on, fan disabled, and doors closed is the safest state. Other thoughts? Do other gasifiers have interlocks or other features to deal with this class of issues?