Thanks for getting back to my questions. You explained things nicely.
You can close the closet door for safety. My WH is in the middle of the house in a small utility closet. The combustion air comes from the attic.
In the ceiling is an opening ruffly 60 sq. inches and is screened off using hardware cloth. Then there's a 6" dia vent pipe exposed to the attic air. This pipe stops about 12" from the floor and is capped off in the attic using hardware cloth. Now there's an air flow circulation for combustion air and the door is sealed.
This venting is about the same for the gas fired whole-house ducted heating system.
I'm sure the codes have changed over the years but my systems were updated as described above some 30 years ago or so.
The flu pipe for the WH and the heater are double walled sheet metal and sized to fit the outlet on the top of the top of the equipment.
ZIPPER 78212.... There is a great need for a sarcasm font.