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Here is a problem that exists in a lot of homes and I have yet to find the solution.
In most homes including the last two I lived in, the carpet where it terminates at walls will show a black dirt line that is almost impossible to clean. What I have noticed, is it mostly occurs on the second floor and almost always where the carpet is terminated at interior walls. Most exterior perimeter areas are clean but the dirt occurs at the locations where the carpet meets some interior partitions. Carpet installers will tell you that this is caused by the return air using the wall cavities to get return air back to the forced air furnace. That is not the case in the home I live in now, because I installed the ductwork, all hard duct, no flex and I caulked all the duct joints and finished them with foil tape. The system is tight, and where some of the black that is on the carpet on the floor is located, is right where the ceiling return is above. The return is definably the least path of resistance. I might add that this house has 2 forced air gas condensing furnaces, one for the 1st floor and the 2nd located in the unheated attic to condition the 2nd floor. All the ductwork for the second floor is finished with duct wrap.
My thoughts are this is caused by the changes in the Barometric pressure using the 2nd floor partitions with the least resistance to relieve the excess pressure to the exterior of attic which is equal to the exterior Barometric pressure due to gable vents, perforated soffit and ridge vents when the pressure drops below that which is in the home. When the exterior pressure increase I think it equalizes through the first floor areas.
I have removed the carpet with the worst case of this black and really there is no noticeable void for the air to have a path thought the partition to the attic, but the reason it might uses this path is because the larger void is there at the top of the partition in the attic where I cannot view it.
Anyone have an ideas on this ongoing problem?