Air duct cleaning in Round Hill.

Round Hill once drew summer people escaping Washington’s heat, and the weather still leans that way. The Purcellville co-op station, about three miles off, averages roughly 4,940 heating degree days a year against 1,050 cooling, and around 115 nights at or below freezing. The blower in a 20141 house spends far more hours pushing warm air than cool, and every hour pulls the house’s air back through the returns.

What heats the house sets the pattern. About half the owner homes here heat with electricity, which in a house this age usually means a heat pump, and on a night near the January average low of 24.5 degrees a heat pump may barely shut off. Another four in ten burn propane in a furnace whose blower serves the same ducts the air conditioner uses in summer.

The subdivisions around the town went up mostly between the mid 1990s and 2005, and the typical owner’s home dates from 2004. Plenty of those basements were finished later, and inside the town limits a finished basement, an addition or an interior remodel starts with a Town zoning permit. If the system ran while somebody sanded drywall downstairs, that dust went into the returns, and the camera photos will show it.

The old town is the exception. The Town’s plan says most houses inside its limits went up before the end of the 1920s, long before central air, so the ducts in a house on Loudoun Street or Main Street were fitted in later, through closets, chases and attic corners. Some of those houses may cool with ductless heads and have little to clean, which the camera settles first.