....Hello, again. Finally got the old 80%, 24 year old furnace replaced this winter with a 95%, 2 stage valve, outside air intake, non-variable speed blower. House heated just fine. Gas bills not too bad either. In learning some things from both the installer and here on this forum; a question came to mind: My old furnace got it's combustion air from the basement. The "shutters" were used to regulate air to the burners, adjusted for optimal burn, right? Well, in looking through the "window" of my new unit as it's running I ask: Since the combustion air is coming from outside, and there's no "shutter" on the PVC air intake pipe---how the heck does the right amount of air get to the burners? I mean, the only thing I can figure (now remember, I'm a home owner) is that the induced draft blower controls the intake air based on the amount of "outflow" air it lets out. ??? What don't I understand? I guess I could call the installer and ask---but I know I drove him nuts with some of my H.O. questions while he was here. Might one of you pro's enlighten me, please? Can't help my curiosity. Thank you.