Air duct cleaning in Fort Belvoir.

Off post, most of the houses with Fort Belvoir mail sit in one subdivision Fairfax County calls Cook Inlet. The county’s records list 148 detached houses and 108 townhouses there, nearly all finished in 2002, 2003 or 2004, each two or two and a half stories over a full basement, and each with central air. In a house laid out like that the furnace and its blower usually stand in the basement, the main supply trunk runs along the basement ceiling, and the bedrooms are fed by risers that climb inside the walls. The camera goes in at both ends: where the trunk leaves the furnace, and at an upstairs register.

Twenty-odd years of filter changes are the history in those ducts, plus anything that happened in the basement since. Basements from those years often get finished later, and when one does, its supply runs get boxed into soffits and new branches get tapped off the old trunk, usually with flexible duct. Sawing and sanding with the blower on sends that dust everywhere the system reaches. If you know the basement was finished after you moved in, tell us; that’s the first branch we’d want a picture of.

The blower here doesn’t get much time off. NOAA’s 1991 to 2020 normals for Reagan National Airport, about 13 miles up the Potomac, count 40 days a year at 90 degrees or hotter and about 60 nights at freezing or below, and NOAA’s storm records note a heat index near 110 at Fort Belvoir in July 2011, June 2012 and August 2016. Air crosses the returns all summer and again all winter, which is why the return grilles and the filter rack are where the camera usually finds the most grey.

Accotink is the exception. Fairfax County dates most of the houses on Backlick Road to 1925 or 1978, about half of them stand without a basement, and gas isn’t available to nearly any of those lots. A house from the 1920s went up long before anybody had central air, so its ductwork came later, most often through the attic or a crawlspace, feeding a heat pump’s air handler. Tell us where yours lives when you book. On post, The Villages at Belvoir keeps up the heating and cooling in its homes, so residents there put in a service request with The Villages instead.