Furnace and AC replacement in Alexandria.

Inside the City of Alexandria the median owner’s house was built in 1970, and close to three in ten went up before 1960. Out in the county, Belle Haven (1957), Fort Hunt (1963) and Rose Hill (1969) are just as old. Then the clock jumps: Kingstowne’s median owner home is from 1994 and Franconia’s from 1986, and about three in four Kingstowne homes date from 1980 through 1999.

Age changes the job, not just the odds. A 1960s Rose Hill or Fort Hunt house is likely on its third or fourth pair, its furnace down in the basement with the ducts overhead, so there’s room to work and to change sizes if the house has grown. A 1990s Kingstowne townhouse is on its first or second, with the furnace squeezed into a hall closet or up in the attic and the condenser a few feet from the neighbor’s.

Inside the City, the furnace half is what brings in the permit. The state rule the City follows lets a like-for-like electric air conditioner go in without one, but not a gas appliance, so a gas furnace and its air conditioner swapped as a pair go through the City’s permit office. Across the county line, Fairfax County treats the whole swap as mechanical permit work, with its own certification for the newer refrigerants.

In the historic districts the outdoor half has its own rules. Parker-Gray’s residential guide leaves a ground unit at the side or back of the house to nobody’s review, and sends one on the street side to staff. Anywhere in the City, rooftop equipment has to be hidden by, or built of, materials like the building’s own walls, so an Old Town rowhouse with its condenser on the roof should plan the screen into the replacement.