Sounds like a he'll of a back buff... I usually get them if the air is turned way down and there's no active flames. The off gasing from the slow burning wood builds up in the firebox and once it reaches a combustion point (either hits a flash point) or more oxygen is added BOOM as sudden explosion of flames which causes the WOOOFH sound and flames. Every time I get it I smell smoke that has been pushed out of the stove and can usually tell what the problem is even without hearing or seeing it.
Ok so my procedure here on the IS has almost totally eliminated those back-puffs: Sled-open on reload, intake full open until magnetic thermo top of stove is 300•. Close sled (engage cat). Wait 5 minutes. After 5 minutes, slide intake lever to half-open. Wait 3 minutes. After those 3minutes, slide intake lever to 1/4 open. Wait 3 minutes then slide wherever you want it to cruise for the next 10 hours. The take-away from this is that closing her down slowly & gradually gives the burning splits time to that slow burn coaling phase.
How many "air options" does the IS have? I was surprised to look at my buddies fireview...tiny little air control compared to the AS. AS looks bout like a big rig stick shift.
Another variable is how you load your stove, how tight its packed, east west, north south, density of wood and of course moisture content. Chimney length, did i forget any?
how tight its packed: reasonably tight but noth a monolithic block of wood east west, north south: east west density of wood and of course moisture content: 3 year old CSS read oak. Need to get the moisture meter for an exact reading, however, this stuff is dry. Splits the size of a deck of card will light with a piece of Super Cedar, no kindling needed Chimney length: Need an exact length but it's probably too long. There are two 90-degree bends in the venting system, one in the stove pipe and then the though-wall thimble where the chimney pipe transitions from horizontal to vertical. I added extra chimney pipe to make up for the two 90 degree bends. This thing drafts hard when its extra cold out. A stove shop recommended replacing the 90-degree bend in the stove pipe with a combination of 45's, however, it never got done.
Looking at what u posted I'm kinda surprised it acted that bad, maybe strong draft can make it worse under some circumstances.
Sounds like your 90 ⁰bends might be an issue here. With those bends, the draw in your chimney not being a straight shot, air gets stuck in there. Any way to take out the bends? I know this isn't likely to be changed but could help greatly.
I’m wondering if it isn’t being caused from activating the CAT before all the wood has fully charred? Then again, the CAT should love the smoke. Might think about adding some weight to the top of the stove, or a latch of some sort that will only allow it to open so far. Don’t want it staying fully closed because that allows pressure to build up that could blow out the glass with a strong backpuff such as the OP had. This is always possible when burning the black rocks which why we like to have secondary air vents open until volatiles burn out of black rocks. Typically this doesn’t usually happen burning wood, but as you can see from the OP’s description, it is still possible. Just another reason to always open and close doors and vents slowly…when burning coal or wood.