Our 1940s Colonial Revival has hot-water baseboard heaters. When we moved in a few years ago, the walls were pristine, but lately we’ve been plagued with ghosted jets of grime on the walls above the vent openings. It’s kind of creepy. —Gene Leigh
Your problem is a common one. It can happen whether the fuel source is gas or oil, and whether the heating system is a hot-water boiler or a hot-air furnace.
The ghostly stains, called thermal tracking, are actually deposits of soot or dust circulated in the air and deposited on the walls. They can also appear around electrical outlets and along studs or joists on the wall surface.
The reason the soot appears so prominently over the radiator vents has to do with temperature and humidity differences and properties of electrostatic attraction. Indoor air contains dust particles, which are usually invisible. A baseboard heater creates a convection current as it delivers warm air. As the air cools on the wall surface, dust or soot particles from the air coalesce on the wall. The stains may ghost above vents in your baseboard unit because of temperature differences between the wall and the radiator.
Provided the air is dirty enough, thermal tracking can also appear in streaks along wall studs. In a well-insulated house, the wall surface over a wall stud or joist may be slightly cooler than the areas backed by insulation. This causes the dirt in the air to condense along the length of the stud at a higher rate, producing a ghostly outline of the stud.
The solution, of course, is to improve indoor air quality. The pollution might be soot from a fireplace or furnace, cooking fumes, cigarettes, candles, pets, or a combination of factors. Have your furnace professionally serviced at least once a year and change any filters regularly. For baseboard radiators, turn off the furnace and clean the fins inside the unit through gentle vacuuming (inset). Clean fins equals less dust to be deposited. Last but not least, scrub or repaint the wall. With any luck, these ghosts will go away.