The top of the hopper is gasketed but more for dust than anything else, however where the hopper slides into the stove isn't sealed (it is a serious wriggle in fit tho) and neither is the rear cover that the slide moves under....i was a bit concerned about the possibility of a burn back but i tested it extensively at the workshop to try to get it to burn back and could not get it to even start burning back towards the hopper....the coal has to move about 5 inches from the throat plate that adjusts the bed hight before it hits the first line of air holes.
even when i negative pressured the stove with a blower the same as the combustion blower (set up venturi style into the exhaust) it showed no tendancy to start burning back up the bed of coal....all that happened when i turned off the feed and combustion air was that the fire went out
i don't have any pics of the finished burner before i testfired it...i have a video of the burner running on the bench but lets just say that my best work isn't done with a camera.
The US manufacturers really haven't done anything new in coal burner design in many years......adoption of the coaltrol type controls was really the last 'big thing' that happened as far as i can see from a position many miles away....altho there is probably things going on that never make it to the internet
Polish manufacturers however seem to have been flat out coming up with several new designs including a flatbed stoker that has everything i built into my one, plus a few more things that i would build into it if i was to do it again. http://youtu.be/vkzm_BuOkAI
i'm seriously thinking about just buying an underfeed burner from there when i get to building my boiler.....