In the spring we are going to add more soil, seeds and mulch. It will be a couple feet taller.
This is an excellent example of a stratification chamber: made of several barrels welded together. The rocket exhaust is pushed into the far end of the stratification chamber. And then exhaust is pulled from the bottom of the stratification chamber near the vertical barrels. The vertical exhaust actually passes through the inside of the vertical barrels. And the vertical barrels are insulated on the inside, so the heat is pushed into the stratification chamber (rather than radiating out the barrel (for quick heat) like most rocket mass heaters).
This was really fun to build. We really stretched the limit with six drums in the underground bell (stratification chamber). If I did it again I would cut it down to 5 barrels so there is more heat left in the chimney at the end. The chimney going up through the vertical barrel and being heated really makes a difference in pulling that big plug of cold air out of the bell. I envision this being used in a greenhouse embedded in the floor or in a pebble mass bench so you can water seedlings on it. That doesn't work with a cob bench.
I have built a similar one in a cob bench in a straw bale schoolhouse near Cleveland Ohio and a smaller one in my house. The bell provides a nice even heat and costs a lot less time, money and hassle to build than the pipe in the cob bench thermal mass.--Uncle Mud