Bob wrote:Steamup. I am not clear on what you are disagreeing with.
I said: "I interpret the reference to a "thick fire" as being when the fire is very low in the column."
What the Axeman manual says is: "If the fire is too thin (combustion tube 3/4 or more ashed up)...." and "If fire is too thick (combustion tube 1/4 or less ashed up)..."
I hope we can agree that the ash is BELOW the fire. If the ash is below the fire then a thin fire has more ash under it than a thick fire. To put it another way a thin fire is higher in the column than a thick fire.
We maybe looking at it two different ways. I may have have mis-spoke in the previous post. Once I have run my AA more I will have a better feel for it. The instructions state that the combustion tube should be filled up all the way except for the few days after starting a new fire.
Many members recommend priming the tube with ash before a new fire. This makes sense as it starts the fire off at the proper level. Especially true with ASH that gravity feeds coal.
I agree with your point on the height of the fire. I was previously thinking more in terms of coal thickness over the fire, not ash thickness under the fire.
So thinking about this more, with the fire too thick ie low in the combustion tube with 1/4 ash or less and lots of coal over the fire, there is lots of air between the spaces in the coal. This will allow more gas buildup as there is more fresh coal to burn off volatiles from and since heat is present, the right combo of gas and air results in ka-boom. We can call this the AA/AHS coal burners "big bang theory".
So to continue with this theory, to limit explosions, the ash sensor must be adjusted to permit the ash buildup in the combustion tube to limit the coal volume in the combustion tube but not so far as to cause a "thin fire" or high ash buildup that could result in poor performance and outfires.
I will test this theory in the days to come. I may fire this weekend just to get things going to experiment.
Those with puff backs need to note the height of the fire in the coal tube,(and ash level) and see if it fits the theory.
I expect the more experienced members can enlighten us. I have read several posts about puffbacks, but have not researched as to how they resolved their issues.