I'm continuing to think of ways to solve he puff back problem. I believe the consensus is that it occurs on combustion fan shut down. I'm not currently having problems but have in the past. I do find fly ash on the lower arc of the inspection door pipe, i.e. the bottom edge of what the flapper door seals against. This means there had to be a reversal of air flow to put it there. Some air, or the suction effect of the opening swinging inspection port door put it there. Or perhaps a very slight puff back, blowing fly ash out the inspection port.
There was a suggestion that a variable speed blower motor could be programmed to slowly transition to off. This would in theory continuously pull the combustion mix through the boiler tubes, even after the flapper door opened. Perhaps the existing motor could be used as a two speed motor. Suppose upon shutdown the dual voltage motor windings were put it series, just like you would if you were to wire it for 220 VAC, but instead powered it with 110 VAC. It would run much slower, but it likely would still run, especially since it had just been up to speed. Now hopefully the flapper door would slowly open and the lower induced draft would continue exhausting the combustion gases. Then after some time power would be completely turned off. Time delays could be controlled by an electric furnace control sequencer. What do you think?