Enerally permissible to install a furnace using indoor air for combustion no outside air intake.
High efficiency furnace no outside air intake.
Generally you can pull combustion air either from the interior or exterior.
A high efficiency condensing furnace if not properly ventilated can present two safety hazards.
Due to high efficiency furnaceshaving their own fresh air intake no air is drawn from inside your home.
I always tell people to pull from the exterior.
Unfortunately the fresh air intake you have described is the second type and does not provide direct combustion air to the furnace room as it is directly connected to the return air plenum for.
See reference 2 in a sealed combustion system as high efficiency furnaces are designed to be outdoor air is essential since the system does not draw any air from inside of the house.
Exterior combustion air draw eliminates the useless wasted time that you are now going through.
Install manuals typically show both that way and wit h an outside air intake.