To add to the Big John's reply regarding ghosting, as most heating units are customarily located on an outside wall, when the unit turns on the cold temp will attract the heated dust like a magnet. I've seen this a lot in my church. Some of the outside walls in the nave area are cinder block with 1" poly-styrene insulation between 3" furring strips. Over 5 or 6 years, with candles burning and the smoke adding to the dust you can see grayish stripes on the walls where all the furring strips are located. If you look at the inside walls you cannot see where the studding is located.