Furnace and AC replacement in Clifton.

Clifton built most of its houses in one burst. Of the 4,871 homes in 20124, nearly eight in ten went up between 1980 and 1999, and the median owner’s house dates from 1988. Fewer than one in ten were built after 2000. So the furnace and air conditioner a builder set in those years are long gone in most houses, and the pair that took their place, if it went in during the 2000s, is the one coming due now.

That timing is the argument for doing both at once. A pair that went in together has had the same summers and winters since, and whichever half quits first is rarely far ahead of the other. Replacing both also means the sizing gets worked out once, for the house as it stands now: a basement finished since the 1980s, an addition or a sunroom all change the load the builder planned for.

Size is the Clifton question. Six in ten homes in 20124 have nine rooms or more, more than double the Fairfax County share, and a house that big often splits the load across two systems, one in the basement for the main floors and one in the attic or a closet for the bedrooms. Tell our estimator the house is over 3,000 square feet and it says so too: you may need two systems, and its range is for one.

Two corners of Clifton break the pattern. On the big lots off Clifton, Compton and Yates Ford Roads, about half the owner homes heat with electricity, so the system there is usually a heat pump and an air handler, $8,499 to $21,650 installed, not a furnace and AC. Inside the Town of Clifton, where the houses were built long before central air, the cooling came later, so a full replacement starts with whether the ducts can carry what the new system moves.