There are a whole mess of possible scenarios here. Aquatherm is probably the biggest, but there are several other OWB out there designed from the factory to run as a pressurized system at 12+ PSI. EconoBurn, Royall, Woodmaster makes one now, Jack, Greenwood, Tarm, not to mention a plethora of the small manufacturers and homemade units. I've also seen a few folks try to pressurize a regular, non-pressurized unit which we know is a bomb. Also saw a P&M BL2840 once that the float on top would get frosty and freeze in, then build a bunch of pressure and blow the float 100 feet in the air and shoot a bunch of water out the fill/vent hole. There are a lot of units out there building unnecessary pressure. A guy on the FB page a few weeks ago was having some issues with his Aquatherm operating at 45psi! 1 gallon of water makes something like 1600 gallons of steam, so something like that could easily level a house if it were inside and flashed to steam. This poor fella that got killed, I would sure think there was some sort of catalyst involved like gasoline or whatever. However the back of my mind wonders what if it was just the wood gas igniting? We've all lost an eyebrow from opening the boiler door too fast in the wrong part of the cycle, and I myself broke a finger last winter from one of these flareups. I was standing behind the door with my hand on the latch, and when it caught enough air it jumped into a fireball which in turn smashed the door open with enough force to break my middle finger and made my wrist sore for weeks. Maybe if the poor guy was off guard and something like that happened, it's entirely possible that the door could've torn off the hinges and smashed his head or whatever. It's all speculation at this point and nobody really knows for sure, but no matter what happened it's still an unfortunate scenario and should be a reminder to all of us that what we're calling a lax daily activity, can in fact be very dangerous if handled improperly. BE CAREFUL!