How to Tell Soot, Dust and Mold Apart
| Clue | Soot | Dust | Mold |
|---|---|---|---|
| Look | Fine, oily black film | Gray to dark streaks | Fuzzy or blotchy spots |
| Wipe test | Smears black | Wipes off | Stains remain |
| Where else | Near candles, fireplace, walls above heat sources | On furniture too | Near damp areas, the coil, wet insulation |
| Smell | Sometimes smoky | None | Musty |
The EPA says only an expert can confirm mold, sometimes with lab analysis. A tape sample sent to a lab costs about $50.
When It's Soot: Find the Flame
Candles, oil lamps and a fireplace put fine soot into the air, and the heating system spreads it to every register. That kind is a cleaning problem. Soot with no candle or fireplace in the house is different: it can point to a fuel-burning furnace or water heater that isn't burning cleanly, and that is a job for an HVAC contractor, not a duct cleaner.
Carbon monoxide has no smell. The Consumer Product Safety Commission advises CO alarms on every level of the home and outside sleeping areas. If one sounds, leave and call 911 from outside. Burning smells from the vents are covered in the burning smell guide.
When It's Mold: Fix the Moisture
Mold around a register usually means moist air is condensing there, often on a cold supply register in a humid room or at a boot with missing insulation. The EPA recommends indoor humidity between 30 and 50 percent. Mold confirmed inside the ducts or on the coil is a removal job; the air duct mold removal page explains the steps and costs.
When It's Dust: Look for Leaks
Dark streaks along the edge of a ceiling register often come from air leaking through the gap between the register and the drywall, which leaves particles on the ceiling. Sealing the boot to the drywall stops it. A camera inspection shows whether dust inside the ducts is also part of the problem, and the dust guide covers the other sources.