Hi Yanche,, pretty frustrating for the scientific mind, huh? It's pretty interesting that here we have a really interesting combustion setup, big forced combustion fan, great efficiency, and.. then this trapped combustible gas situation shows up..
It's like we just can't win..
Thankfully for some reason,, my AA260 doesn't seem to have any puff-backs.. I have experienced one or two very minor puffs that just rattled the inspection port door. I experienced these only during initial fires.. but the volume of air in the 6" flue, and the larger heat exchanger area, larger inspection port size.. all seem to reduce the violence of the 'puff-back' to a meer... 'puff'.