I've just moved into my townhouse, and, over the last month, have noticed black dust accumulating in the kitchen. It is particularly visible because the kitchen colour is off-white. Actually, it's quite shocking. There are two heating vents in the ceiling, so I'm thinking that it might be from the gas heater. I changed the filter before these symptoms occured. Should I use one of those $20 filters? Or worse, do the ventillation ducts need cleaning? The grills of the ceiling vents appear clean.