Furnace and AC replacement in Dunn Loring.

The weather station nearest Dunn Loring with thirty years of normals sits 3.6 miles west in Vienna, and it runs cooler in summer than the airports people tend to quote. July there averages a high of about 84 degrees, against nearly 88 at Dulles and about 90 at Reagan National, and it reaches 90 on about 13 days a year where Dulles sees about 33. Its winters, though, add up to roughly 4,900 heating degree days. A system here heats first and cools second.

That’s a reason to work the size out from the house rather than copy the old label. An air conditioner sized for an airport summer is too big for this one: it chills the rooms fast and shuts off before it has wrung the moisture out, leaving the upstairs cool but clammy on a muggy June night.

In 22027 the 1990s built 217 of the ZIP’s 659 homes, more than any other decade, and the 1980s another 125. The pair those builders installed ran on R-22, and many of those houses have already replaced it once, often in the R-410A years after 2010. That second pair is now in its teens, and a new outdoor unit can’t simply join half of it: today’s units run on the newer A2L refrigerants and need an indoor coil made for them, and that coil sits on top of the furnace.

Look up before the new outdoor unit goes down, too. The National Weather Service log for the June 29, 2012 derecho puts estimated gusts near 70 miles an hour in Dunn Loring, with three large branches across two lanes of Prosperity Avenue, and in July 2017 a thunderstorm dropped a tree onto a Metro car at the Dunn Loring station. If the old pad sits under a big limb, ask whether the new one can sit clear of it.