No soot yet on the outside. I've been running diesel but changed over to Clean Heat the other day. It has been doing this for a month or so, but it didn't exactly "poof" yesterday after the fuel change over, it was more of a "POOF !" - actually bounced a plastic cup sitting on the cabinet.
We usually have the stove on high since we only use it to make coffee/boil water. I believe that my manual said to turn it up on high to start, but when shutting down it didn't matter - I'll check.
After the last, I'm getting some balls to fit the exhaust. "Prepare for broadside" !!