Harold Kestenholz
You have the old, classic problem called “streaking.” When high water temperatures (over 180F) are run through the baseboard elements, the air speed over the fins is high enough to pick up minute soot and dust particles and throw them to the wall as the heat rises to warm the walls.
You will also see this happen around air registers in restaurants and other public places. It is not that there is not enough cleaning in the room, it just happens when there is any available and the air speed is fast enough to move it.
This can be eliminated or reduced by adding longer lengths of baseboard in the room, as the resulting water temperatures are lowered, reducing air speeds over the fins.
