My gas heating unit is in a closet in my rec room elevated about two feet off the floor
Level of the house. A few years ago I bought a new unit. When they installed it they
Closed off a 12" Square vent at the top and a 12" vent at the bootom. Both open into
The room. They then installed 6" pipe vent from the attic. The problem is that cold
Air roars down this vent. My question is, can safely close off the duct vent and re-open
The other two vents?

Cold air is supposed to be supplied to the furnace. It is called combustion air and is required by code on many but not all furnace installs.