We have a 2 door BIS 1.2 ZC fireplace. When adding firewood, I usually only open one door. However, if I also open the second door (part way) a lot of smoke (and some ash) escapes into the room. What is causing this to happen? I ask because we are thinking of replacing our unit with a new single door fireplace and I fear I'll have constant problems with smoke whenever I add firewood. Do other people have this problem with single door units?