Wet fines dry and clog the 7 holes, then the pressure of the fan pushes the smoke down the tube and into the bin/house.
The Kimmel's coal I get in bags are wet, and I try to dry about 300lbs of coal before putting it into my bigger bin. I drain water from the bin and pre-bin, and wet vac a couple gallons for every 300lbs or so. Unfortunately, there is no other source of coal up where I live. I tried to buy a different brand, but they could not deliver until December so I went back to Kimmel's coal.
The only other thing I haven't changed is the upper/lower auger. I will try this later in the fall since these are expensive. I can disconnect the air feed to the pipe easily. I'll buy a new upper pipe if I get new coal source and augers next season. I can easily plug the top of the blower with 3/4" MIP plug since its tapped to go back to normal.
Draft is good. Pipes are new. Pot rebuilt last season. Once I clear the 7 holes in the upper pipe it runs normal so I know the cause, but don't know how to stop the 7 holes from clogging. I am sure its from the wet coal once the fines dry. Hopefully, I will be able to buy a different brand next year and have new augers.
Unfortunately, there are no EFM service guys in MA. I am not sure they can provide any more input than what I have already tried. BTW, this has never happen to the other EFM DF520 I run at my parents in PA. However, they get coal delivered dry and fills up bin which has 4-5 ton capacity in the basement.