Combustion air coming from outside is fine, and it allows the house to remain as sealed as you want... but I still can't comprehend a (efficient) system that expect fresh air on the intake side. It just defeats the entire purpose of a closed system. It would be like standing in front of a window-style A/C unit while you're in the middle of a park... you'll get cool, but only as long as you stand directly in front of the unit. It's not going to cool the park around you.