Air duct cleaning in Amissville.

Two systems carry most of ZIP 20106. About four owner homes in ten heat with propane, which usually means a furnace and an air conditioner sharing one set of ducts, and a bit over a third heat with electricity, mostly heat pumps that use the same ducts in both seasons. NOAA’s normals from the Boston station, 10.3 miles south-southwest, count 4,836 heating degree days and 936 cooling degree days, so the blower runs through a long winter and a real summer.

Houses here run from old to new. About one in eight was built before 1960, about a third from 1980 through 1999, and about three in ten since 2000, with a median of 1993. Ducts that have worked for three decades have collected three decades of dust, and a newer house can still hold the builder’s drywall dust. The camera shows which you have.

The oldest houses deserve a close look. Ducts in a house from before 1960 were usually added later, squeezed through closets and crawl spaces wherever they fit, and those added runs bend sharply and are hard to reach. If a house heats with a wood stove or baseboard and has no ducts, there’s nothing to clean, and about one owner home in eighteen here heats with wood or another fuel.

Summer asks a lot of ducts too. The Boston station averages about 22 days a year at 90 degrees or higher and hit 100 on July 8 and 9, 2012, the week after the derecho. The return grilles are where a duct system breathes in, and gray dust on their slats a few weeks after you’ve vacuumed is a cue to look further up the run.