The problem: After years of service, the adhesive on tape that seals joints between duct sections can dry out, allowing heated or cooled air to escape. Damage can also occur when homeowners or tradespeople access or work in areas where ducts are installed, such as crawl spaces, attics, and basements.
How to spot it: With the furnace or AC on, shine a high-powered flashlight on ducts, especially at junctions where they connect with registers. "If you see where dust on the exterior of the ducts has been blown away, that's usually the sign of a leak," says Meeks.
How to stop it: Patch small holes or misalignments with a water-based mastic sealant and mesh tape. Use HVAC foil tape to seal joints between sections.
The payoff: Cut your heating bill by 3 to 10 percent by reducing air leakage by up to 15 percent.