Furnace replacement in Oakton.

About half of Oakton’s owner homes heat with gas, and about four in a hundred run on propane, delivered to a tank outside. The gas is almost always Washington Gas: Fairfax County names it as the supplier for most of the county, and the exceptions it lists, served by Columbia Gas, are pieces of Chantilly and Herndon. Nearly all the other Oakton homes heat with electricity, usually a heat pump, which is a different job.

Half of the owner homes in 22124 were built before 1984 and half after, and nearly half of all homes here date from 1980 through 1999. A furnace in a 1980s house is rarely the first one, and furnaces and air conditioners almost never quit in the same year. The outdoor unit is usually the half that dies in the hottest week of July and gets replaced in a hurry, while the furnace keeps lighting for a few more winters. So it’s common for the cooling half to be ten or more years newer than the furnace under its coil.

Keeping that newer half takes three checks before the price is written. The outdoor unit’s label names its refrigerant: on R-22 it’s near the end of its own run, and keeping it only buys time. The coil perched on the old furnace has to match the new cabinet’s width, or a sheet metal transition joins the two. And the new blower is set for the air conditioner’s size, so the house cools the way it did last summer.

In the townhouse clusters off Jermantown Road and Miller Road, like Oak Marr Courts and Oakborough Square, the furnace tends to stand in a closet or a utility room on the lowest level, so whatever replaces it must pass through that same door. An inside unit of a row has only two outside walls for a high-efficiency furnace’s plastic vent pipes. On the big lots north of Jermantown Road, a large house often has a second furnace up in the attic serving the bedrooms, and only the one that’s failing has to go.