AC replacement in Dunn Loring.

Gas heats about seven in ten owner homes in 22027, and the county’s records put it in 326 of the 334 detached houses built here from 1990 through 2009, with full basements under them. That’s the classic AC-only house: the furnace stands in the basement, the cooling coil rides on top of it, and the condenser sits on a pad outside. A new AC means a new condenser and a coil listed for the same refrigerant, since today’s units run on A2L refrigerants. The permit comes from Fairfax County, the only building office Dunn Loring has, with the county’s certification for a system on that kind of refrigerant.

Plenty of those houses are big. Fifty-seven in 22027 top 4,000 square feet above ground, and a house that size often splits the cooling: one system for the main floors and basement, a second air handler in the attic for the bedrooms, each with its own condenser outside. They wear out on their own schedules, so you can replace the one that’s failing and keep the other. If both went in with the house, they’re the same age, and pricing both at once tells you what next summer might bring.

The townhouses tell the age story plainly. The 60 at Regal Oaks on Journet Drive went up in 1994 and 1995, so a condenser out back that’s still the builder’s has cooled about thirty summers, on R-22. The 42 at Laing at Governors Square on Harithy Drive came in 2000 to 2002, and their builder units, past twenty now, most likely hold R-22 as well. Both rows keep the furnace and coil over a full basement, and the new condenser has to fit the same small pad behind the unit.

Size it for Dunn Loring’s summer, not an airport’s. Vienna’s weather station, 3.6 miles west, averages a July high near 84 and sees about 13 days a year at 90 or hotter, cooler than the airport figures sizing often leans on. An air conditioner picked for a hotter summer chills the rooms fast and shuts off before it pulls the moisture out. Look up before the new pad goes down, too: the June 2012 derecho brought gusts near 70 miles an hour here, and a condenser under a big limb can move clear while it’s being replaced anyway.