I was just about to make entry into a hole in a wall durring a controled burn drill when the room I was about to enter unexpectedly flashed over. My head was in the opening when it happend and apparently my helmet had caught fire. I was completely uninjured. After I got out of the building and took my gear off I saw the singe mark on the right cheek near the opening. Not once when it was happening did it feel like I had that kind of heat near my face.