If you are wise you just get yourself a normal common Rheem power vented unit and stay away from those
other special units..... they cost about the same price on line but I think about the future servicability and I
have never actually stumbled onto a direct vented heater in our area.....
Just throw an extra anode rod into the hot side of the heater $25.00 and throw a water heater blanket on the unit $30.00
then throw an expansion tank on the heater $25, and you will probably get around12 ++ years of trouble free service
FYI you are never gonna figure out the difference you save by sucking air in from the outside of the home
I suggest that you just find something else to fret and worry about ......