Due to high efficiency furnaceshaving their own fresh air intake no air is drawn from inside your home.
High efficiency furnace no outside air intake.
Since high efficiency furnaces draw air directly from outside the furnace itself does not require a fresh air intake in order to replace inside air that otherwise would have been drawn from the room the furnace is located in.
High efficiency furnace combustion intake from inside or outside.
Generally you can pull combustion air either from the interior or exterior.
I always tell people to pull from the exterior.
Install manuals typically show both that way and wit h an outside air intake.
A high efficiency condensing furnace if not properly ventilated can present two safety hazards.
First the gas byproducts can contain residual carbon monoxide that could leak back into your home unless the ventilation pipes are airtight.
Exterior combustion air draw eliminates the useless wasted time that you are now going through.