Smoke "puffing" out somewhere on secondary burn...

My Harmon Oakwood was very hot this morning, stovetop thermo read 650-700 deg, so I closed down the damper to ignite the secondary burn and turned down the air lever to about half.

I was expecting the temp to drop after 10 min or so like it usually does when I close the air down so far, but it stayed hot, and I wasn't really concerned because I had a great coal bed going and a load of dry slab hardwood.

So I told the kids to watch the glass and see the flames occasionally WHOOSH and then die, WHOOSH and then die, etc. But this time, when it WHOOSHED, a puff of smoke came out the back, near the bottom.

What's up with that?! I asked my wife, and she said, "oh yeah, I've seen that before".

Now, I've also seen the "whooshing" during secondary burn, thinking it was a cyclical buildup of gasses that rythmically ignites then goes out. It's kind of beautiful to watch, but I've NEVER seen the smoke before.

My theory: This was a big, hot fire, so the "whooshing" produced sufficient internal force with the damper closed that it forced smoke back out the intake vents.

But then: I thought the intake on my Oakwood was near the FRONT, not the back where I think the smoke came from.

And no, it's NOT coming from the ashpan door, or front door, or top door.

Thoughts?

Makes perfect sense!

Thanks for the sanity check!

By "damper", I mean the flue butterfly downstream of the flame. By "air control", I mean the primary air upstream of the flame. The previous poster makes sense; if I have a roaring fire and slam closed the flue butterfly (and force the combustion thru the more restrictive secondary combustion process), I would imagine the instantaneous internal pressure would rise, and when the gas DOES ignite, there's enough pressure to force back out the air intakes. In the future, I'll close down the primary air FIRST, let the flame die down, THEN close the bypass damper.