When chimneys are getting a constant cooling from above, the gases meant to be leaving it don't so well. It's often obvious on a cold start, but while in operation, if the terminal isn't heated in equal or higher temperature than the gases going up it you get negative flue pressure. Down draft burners avoid the necessity of countering the problem but conventional burners can burn cleanly and consistently with the right flue terminal ventilation, especially designed to deflect the condition. Most cowls obsess themselves on deflecting wind but more unknown and most common, the issue is a cooling chimney. Some say burn a bright hot fire to avoid the issue, but no one wants to waste their fuel and their heat by doing so. Protect the terminal effectively with applied pressure differential, and terminal heating, and puff back symptoms will significantly decrease.