Stove has been cruising all evening, a few hours into the latest reload, stovetop holding 350F and cat holding 1300F, both steady for the last 2+ hours. Noticed the smell of smoke in the room a half hour ago, but figured it was my imagination. Then, while sitting here checking emails, I heard and saw a big backpuff, with the classic smoke pushing out through every gasket. Criteria is a Jotul 12 cat stove, 60% load probably loaded around 2.5 - 3 hours ago, all Walnut. Been burning with primary air set to minimum for at least the last 1.5 hours, likely longer. Stove had a back-puffing problem last year, but we thought we had solved it with the installation of a 6" insulated liner inside our exterior masonry chimney (was previously running into 8" clay liner). Solution was to bump air control open a hair, and now the stove's starting to really take off. Apparently a large part of my load was just starting to ignite at the same time, and likely the stove was not moving the gasses thru quickly enough. Not sure what the cure is to that, though. Thinking maybe a stop or drilling a bypass hole near the primary air damper, that keeps the air control from being shut down quite so far, but I hate doing something like that. Makes me nervous to leave the stove burning unattended. I've heard of too many backpuffing stoves blowing the doors open and throwing firebox contents into the room. Had it happen to a close family friend several years back.