The swine flue is the most important part of your pig furnace. You have to have the damper open just the right amount to vent off the lighter pigs while retaining the larger ones in the pork tank.
Let's take a look here at Diagram One. Note call-out A. The pig damper. It's hard to remember whether the damn thing is open or closed. Do you pull it forward to open it, or is that closed? You could stick your head in there and look, but by the time your remember to check, you've usually got a nice, crackling fire going, and looking would roast your head.
You'll know soon enough. If the damper's closed, pretty soon a bunch of hydrogen-filled piglets comes pouring out of the front of the furnace, collecting up near the ceiling, running around upside-down, squealing. One little spark from their hooves on a light fixture, and kablam! It's raining bacon. That's not a bad thing, but it needs to happen in a controlled space. And the little piglets are cute. Nobody likes to see them get blown up.
If you get the swine flue adjusted correctly, the stream of piglets flies up the chimney and out into the wind. The larger swine come out the vents at the bottom, the swine heatelator, and you have to let them out the door.
If you'd burn something other than those acorns, you wouldn't have to worry about all those pigs. It's the combustion product. You combine acorns and oxygen in the presence of heat, and you get pigs, water, hydrogen, and extra heat. That's just the chemistry of it.