Why is there a black line on the carpet in this doorway?
This “ghosting” on carpets is pretty common in homes. It merely shows you where there is a great deal of air movement. It is typically worse where there is inadequate clearance under the door and the door has been kept closed a lot. It is also more noticeable in homes that are not cleaned regularly---but even in well maintained homes they can be difficult to eliminate entirely.
For the inspector it is a clue.
The inspector will want to know why the air is moving here and at the very least recommend that the door be cut off to allow for air flow to improve. Because, even with inadequate clearances, this is the path of least resistance, air will be sucked through the carpet and the carpet is acting like a filter.
In this case, the furnace was located in the room and this was the only means of providing combustion/dilution air to the furnace. More often it is air moving from the room as heated air comes into the room and must return to the air return that is on the other side of the door. When combustion air is involved it is a little bit more problematic because it could mean that the appliance is not getting enough air and could become and health and safety concern related to oxygen depletion.
In this case it most certainly was a safety issue because the room where the furnace was, was also being used as a bedroom. The sleeping room with the door closed could result in a lack of oxygen in the bedroom---let alone the risk of combustion by-products being introduced into the room.
Definitely crossing the line.
P.S. Crushing those wires repeatedly is likely not a good thing either.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board