We had an issue last night with our Harmen pellet boiler. The pellets we had in it had lots of saw dust in the bags that then got into the small holes in the burn pot causing it not to light. We emptied the old pellets out "they were a customer return from last year and we are trying to get rid of them" and filled it up with some Cubex. The air out side had that layered temp thing going on so the smoke from wood stoves and such stays low out side. Well after filling the boiler up and watching it so make sure it lit lots of smoke started rolling out of the burn pot and just as I said it seems like it has been smoking for way to long with no fire POOOOFFFF!! I thought the thing exploded and was looking for glass in my legs from the window on the door. Luckily no glass because it didn't break or come out of the boiler. It blew ash out of any crack in the doors and such and scared the s**t out of my father and I. My guess was because everything cooled down the cold air was creating a back pressure on the pipe and let the gasses build up and then created an explosion in the fire box. The boiler ran great after that with a nice blue flame. Any one have any other ideas on why that would happen? The boiler has been running for 3 years this will be 4 never had this happen till now.