Bumps, Puff Backs, Superior Coal, AA & AHS

I was just reading past posts about puff backs. Trying to learn how to avoid them. One post said that burning buckwheat in the Axeman was a cause of puff backs. I plan on burning pea. It also mentioned that Superior pea coal was a bit smaller than other pea coal and might lead to puff issues. Any truth to that? I have friend that's reseaching coal and is looking hard at the AHS. From the posts on the Forum it seems the AHS has more puff backs than the AA, but they can be an issue with any coal unit, it's hard to know! I'd like to have zero puff backs. Any input is appreciated!
When I ordered my AA, I was told that you should have zero. Running proper sized coal, if you have one, open the little hole and you should have no more.

I've burned nothing but Nut sized coal for 25 years & I've had puff backs in all 3 stoves I've owned if I don't burn off the volatiles while loading. Look for some How to shake down threads. Puff backs are covered well there. (Some setups don't get them but others (like mine) do if you I don't burn off the volatiles while loading) It's really just a question of leaving a small part of the burning coal bed uncovered by fresh coal until you start to get the blue flames dancing in the freshly covered areas. Then you can cover the area left untouched. Pretty simple really.

I've had puff backs with my AHS several times, some more robust than others. The real cause remains elusive. Clearly it's just the right combination of coal gases, ignition source and combustion air. The two times I observed them it occurred when the combustion blower had be running and then stopped. Clearly there was a good ignition source then. The Bureau of Mines report describes puff backs occurring with buckwheat coal and a fix was to provide additional air by drilling a hole in the damper door. I have observed more puff backs seem to occur with fresh coal. Coal that was mined in the last few weeks. I've read reports that all coal, including Anthracite oxidizes, i.e. it combines with oxygen in the air reducing it's BTU content. For Anthracite coal it is a small effect but it does occur. The effect would be greatest shortly after the coal is mined. So, it kind of explains my observation, I can get puff backs with fresh coal. I don't seem to get them with coal that's been uncovered and sits outside in the weather.

My AHS coalgun has a flapper door with a bar across it. I suspect the bar was an improvement to restrict the flapper door movement during a puff back. I've see photos of newer coalguns which have sort of a hood over the flapper door with an apparent opening on the bottom the hood. I guess it's an attempt to get a chimney effect and clear out any combustion gases.

My general recommendation is to join the flue pipe securely with three screws, tape the joints with high temperature aluminum faced tape and securely support the pipes. Same for the barometric damper. For me the cause still is not fully explained and I can not cause it to happen on purpose. I've tried. My frequency of occurrence has been 3-4 times a heating season. It doesn't seem to occur during my summertime heating of domestic hot water only.

I've only burned my stove for six months but haven't had any puffs Scott