Furnace tune-ups in Nokesville.

Of the 3,076 owner homes across 20181, 597 heat with propane and 459 with natural gas, and between them they fuel nearly every furnace we’d tune up here. Nearly all that piped gas is out past the village: inside it, propane outnumbers natural gas about three to one. More than half the ZIP heats with electricity, usually heat pumps.

In a propane house the furnace often isn’t the only thing on the tank. The water heater, the range and sometimes fireplace logs draw from the same supply, and on a cold evening several can fire at once. With the burners lit, the tech puts a gauge on the furnace’s gas valve and compares it against the propane number the maker printed on the label. If it reads low, the write-up says so, a number you can hand to whoever fills the tank.

On a well, the furnace often shares its basement room with the pressure tank, a water softener and the water heater. Softeners are common on Prince William wells, and that corner tends to collect salt bags, pool chlorine, bleach and paint thinner. An older furnace burns air from the room it stands in, and chlorine or solvent fumes pulled through a burner can corrode it from the inside over the years. Store those elsewhere.

Condensing furnaces, the ones vented through the wall in white plastic pipe, wring water out of their own exhaust. Where there’s no floor drain close by, a small condensate pump usually lifts that water out through a hose, and many carry a switch that shuts the furnace off when the reservoir fills. A stuck float looks exactly like a dead furnace. The furnace runs a full cycle during the tune-up, so the tech sees whether the pump clears what it’s given.