Seems to me what's needed is a different control system. The root cause of puff backs is a unique combination of coal gases, combustion air and an ignition source. Alter significantly any of those and puff back will be gone. What do we have to significantly alter things? Answer, a significant air sucker; the built in combustion blower. So, upon a turn off desire, first open the inspection port door, second allow the blower to run for some timed amount. This will remove the coal gases because you will have over the fire air; significant amounts. Enough to cool the coal if left to run long enough. Third, after the timed duration, shut the combustion blower off. The design right now depends on passive actions to achieve a change one of the three things need for a puff back to occur. Flapper spring door tension just right, coal not outgassing, a good idle chimney draft, maybe a coasting blower fan. It's just not enough for all cases. Opinions?