Q: My mom has a forced-air heating and cooling system, and her home always seems very dusty. Yet the filters never seem to get dirty. The air returns are working fine, pulling in enough air to hold a piece of paper in place. What's going on?

A: Richard Trethewey replies: I can't tell you exactly, but I know which suspects to round up: your filter and your ducts.

I'll bet you're using those cheap, disposable spun-fiberglass filters, which trap only the biggest chunks of dust and do almost nothing to improve indoor air quality. Replacing them with pleated filters rated for the blower capacity of your system should put a big dent in the problem, as long as you change them when they get dirty.

You probably also have leaky ducts. According to one estimate, about 20 percent of the air that goes through a typical forced-air system is lost through holes or gaps around fittings or from unsealed joints between duct sections. These leaks let heated or cooled air escape and draw dirty air into the system. A heating contractor can pressure-test your system. If more than 2 percent of the air is leaking out, it's worth sealing every accessible joint with HVAC mastic or foil tape. Don't use duct tape, which eventually dries out and fails. For those duct runs that aren't accessible, you can hire a company such as Aeroseal that seals ducts from the inside with an aerosol spray.
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