I’ll second Gerry comments,
This most likely is natural draft, cat 1 ,negative pressure furnace. Now the part about the return air ducts missing is defeating the effectiveness of the heating system.
You want a return air duct in the living space. Not within 10 feet of the FAU in the same room!
Negative room pressure , cold air volume, duct sizing ( cold air requires a larger duct) all plays a major role on how the living space is going to be heated effectively. Since cold air is at the floor level , you want that to be “pulled” back into the FAU to be heated and “pushed” out the supply register. (pressure in smaller ducts) Not having a return air duct in the living spaces “works” but the efficiency and effectiveness of the system is not there.
The other open ducts are to supply combustion air to the FAU, 1 square inch for every 1000 BTU’s is the general rule. These ducts are installed in several different ways. Some take it from the attic (one high duct , one low duct) never just one duct!! One is at least open 12" above the floor where the FAU is located and the other is within 12" from the ceiling of the FAU room. Some ducts are routed to the outside walls, some are installed to pull from the crawl space and attic, some are louvers to adjacent rooms that are much larger to fulfill the combustion air requirements.
This allows for proper movement of combustion air so the FAU has enough O2 to burn clean and have the Natural draft hood dilution air so the flue can “draw” that product of combustion outside …
When you see an old natural draft FAU in a closet and it’s tight…It’s a death machine.
If the current H/O remodeled a basement and “hide the old beast” it’s a possible death machine. People don’t realize a FAU can be a killer.
Keep em safe …