Furnace tune-ups in Paeonian Springs.

Nearly half of the owner homes in ZIP 20129 heat with propane, about 35 percent with electricity and nearly one in five with oil. Natural gas shows up at zero in the Census figures. So the furnace we tune here is most often a propane unit, and the gas pressure gets read against the propane figure on its label, along with the temperature rise, the safeties and the order it lights, heats and shuts down. Oil burners are the one thing we don’t service, and we say so before you book.

The median owner home here dates to 1997, and about one in four was built before 1980. A furnace that came with a 1997 house is nearly three decades old, well past the age most are retired, and one from the early 1980s is older still. Whatever the age, the tune-up tells you where it stands. A reading in range goes in your file for next fall. One outside the range gets a written price first, and that price is the number to hold up against a new furnace.

Winter runs long here. The Purcellville station counts more than a hundred nights a year at or below freezing, and its average January low is 24.5. A trained spotter measured 13.0 inches of snow a mile southeast of town on February 21, 2015. When snow brings the power down, a propane furnace goes quiet too, because its blower and igniter run on electricity. A furnace with clean readings is one less thing to chase when the lights come back.

Loudoun County Fire and Rescue says to have heating equipment and chimneys cleaned and inspected every year by a qualified professional, and to keep anything that can burn at least three feet from all of it. Our visit is that yearly look at the furnace itself. It is $69 a system, and that price also covers a heat pump, a boiler or a ductless mini split, which is how about 35 percent of owner homes in town are heated.