Furnace replacement in Dunn Loring.

The closest weather station with thirty years of records is a cooperative one in Vienna, 3.6 miles west of Dunn Loring, and its 1991 to 2020 normals describe a heating climate. In half of years the first freezing night comes by October 29 and the last by April 14. Between them fall about 99 nights at or below 32 degrees, and about 13 days when the high never climbs past freezing. The station tallies roughly 4,900 heating degree days a year against fewer than 1,000 cooling degree days, so by that yardstick a year here asks about five times as much of the heat as of the air conditioning.

Dunn Loring’s own ZIP, 22027, is the newer east end around Gallows Road. Of its 659 homes, the Census counts 217 from the 1990s, more than any other decade, and 125 from the 1980s; the median owner home was built in 1991. A builder’s furnace from that year has seen thirty-five winters, well past the fifteen to twenty most gas furnaces give, so plenty of these houses are on a second one already, often in its teens by now.

About 426 of the 627 owner homes in 22027 burn natural gas from Washington Gas, the supplier Fairfax County names for most of the county, and a dozen or so burn propane. In a 1980s or 90s house the furnace usually stands in the basement beside the gas water heater, often both venting up one metal flue. A 95 percent furnace moves its exhaust to two plastic pipes out a side wall, and those pipes have to clear the snow: the January 2016 blizzard left 23 to 31 inches across Fairfax County, and a furnace with a buried intake can shut itself off right when the house needs it.

The cold numbers matter for sizing too. Of those 99 freezing nights, only about four drop to 10 degrees or lower. A furnace sized for the rare single-digit night spends most of the winter in short bursts; a two-stage or modulating one runs on low fire through the ordinary cold and saves its full output for the handful of nights that need it.