Inside the City of Manassas, about half the houses were built in the 1980s and 90s, the median owner-occupied one in 1985, and close to three in ten are townhouses. The usual City duct job, then, is a metal trunk line under the main floor or boxed into a basement soffit, flexible branches off it to each room, and a return or two in the hall. In most of them nobody has looked inside since the first owners moved in.
The City also heats about half its owner homes with electricity, which in 1980s houses mostly means heat pumps. A heat pump delivers air that’s warm rather than hot, so through a Manassas winter it runs longer cycles than a gas furnace would, and the return pulls house air through the ducts the whole time it’s on. A few winters of that puts a lot of air through one filter slot.
Just outside the city line the houses get older before they get newer. Sudley’s median home went up in 1977 and close to half are townhouses. Loch Lomond is almost entirely single-family, most of it built before 1980. Bull Run and Buckhall are newer, each with a median home built in 1995: Bull Run in condo and apartment buildings with small closet systems, Buckhall in single-family houses, some big enough to carry a second system for the upstairs.
Close to four in ten homes in Yorkshire and in Buckhall went up after 2000, and about the same share in Manassas Park, its own city inside 20111. Twenty-odd years in, a system like that has gone through a lot of filters, some of them late, and the flexible runs tend to sag where they cross the attic, which is exactly where the dust settles.