Since I have the furnace pretty tight, my thought is that turning on the inducer is absolutely no different than opening combustion air feed on the ash pan door OR turning on a combustion blower.. It doesn't matter how hard the chimney is pulling. If a gallon of air goes in the furnace, then only a gallon of air plus/with coal exhaust will exit the furnace. NOW, if I have the load door cracked OR the gasket around it is letting air in, that would be a different ball game. Then, air sneaking in this way would serve no combustion purpose, and would be carrying heat with it up the chimney.
You are absolutely right, running the inducer is pulling air out of the furnace, which is causing more air to come in at the ash pan door without changing my combustion air feeding the fire. Since there is more negative pressure in the fire box with the inducer on, more air will get thru the same size hole.
This is my logic on the whole system..
My point is, I don't think it matters if combustion air is being pulled in OR if its being pushed in..
Maybe its wrong, but so far I don't think it is
Its been working pretty dam good!