AC tune-up in Dunn Loring.

Fairfax County’s records count 732 homes in 22027, Dunn Loring’s own ZIP: 630 detached houses, 102 townhouses and not one condominium. The decade decides what’s out back. Of the 81 detached houses put up in the 1970s, 70 have no gas on the county’s books, so it’s usually a heat pump that heated the house all winter. Tysons Grove on Arden Court is the plainest case: 20 houses from 1978 and 1979, not one with gas.

From 1990 on it flips. The county lists gas at 326 of the 334 detached houses built here through 2009, every one over a full basement, where the indoor coil usually rides on the gas furnace and borrows its blower. Many are big: 57 top 4,000 square feet above ground, and a house that size often splits the cooling in two, with the bedroom air handler in the attic. Two systems book as two visits, $99 each.

The townhouses are gas over full basements too: 60 at the Regal Oaks on Journet Drive, from 1994 and 1995, where a condenser that’s still the original has cooled thirty summers, and 42 at Laing at Governors Square on Harithy Drive, from 2000 to 2002.

Vienna’s weather station, 3.6 miles west, logs about 13 days a year at 90 degrees or hotter. Wind does the local damage: the June 2012 derecho dropped three big branches across Prosperity Avenue, and a July 2017 thunderstorm put a tree on a Metro car at Dunn Loring. After a storm, look at the outdoor unit before you run it: sticks in the fan grille, flattened fins, a unit knocked off level, insulation torn off the copper lines. If anything looks wrong, switch the cooling off and call us. We’ll check it all over, and anything it needs gets a written price first.