It doesn’t make sense. Even though you keep a very clean home, you notice black staining above the hydronic (hot water) baseboard heating vent or around the discharge of the forced air heating system.
The surface of the furnace filter is clean. You vacuum routinely. You don’t have large pets, and the kids are long gone. Where is all the dirt coming from, and why is it above the heat sources? Is the furnace at fault?
A hydronic heating system provides some heat radiation, but much of the heat comes from convection and air flow. Hot water warms the pipes and attached fins. The fins warm the surrounding air, and it flows upward through convection – notice the slot above the fins to direct air flow. This slight air flow easily deposits any soot or dirt from the air onto the adjacent wall.
If you have a forced air heating system, it pushes warm air out of the grill. Warm air rises, flowing up and along walls – and depositing soot and dirt.
Don’t worry. This doesn’t mean your home is extremely dirty. Constant air flow deposits dirt, and there is little you can do about it.