I was helping some friends move to a new house.
Much to my amazement I found a Gas Furnance in a closet in the Masterbed room. The door didn’t have any vents for draft\combustion air what-so-ever, the flexible gas line passed through the furnace housing (some genuis had that black foam pumbing insulation installed as a bushing) and they were using metalic dryer vent to vent the unit.
I’ll get some pics next time I’m over there.