Heat pump replacement in Round Hill.

The Census counts 1,257 of the ZIP’s 2,362 owner homes heating with electricity, and in the houses built here since the 1990s, electric heat is mostly a heat pump outside, backed by resistance strips inside the air handler. A heat pump runs in every month of the year, so one from around 2000 has logged far more hours than an air conditioner its age.

The cold is what sets Round Hill apart. In NOAA’s 1991 to 2020 normals, the Purcellville co-op station, about three miles east-northeast, sees 20 degrees or less on close to 31 nights in a normal year and 10 or less on about 5; out at Dulles Airport it’s 25 nights at 20 or below. As the air gets colder, an older single-speed heat pump’s output sags and its strips pick up the slack. A replacement chosen for those nights, often a variable-speed or cold-climate model, keeps more of the heating on the heat pump itself.

Ice is the other test. Freezing rain left up to fifteen hundredths of an inch at Round Hill on February 7, 2018, and 0.13 inch a mile west-northwest of town on January 5, 2014. The defrost cycle melts frost that grows on the coil; it can’t shed a shell of freezing rain over the cabinet and fan. With 36.3 inches measured nearby in the January 2016 blizzard, a new outdoor unit here usually belongs on a raised stand, so the coil clears the drifts and defrost water drains away instead of freezing underneath.

Then there’s the backup. In a propane house a heat pump often runs in front of the furnace. Where the house is all-electric, the strips are the backup, and they’re a big electrical load: the February 2010 snowstorm brought outages throughout Loudoun County, and a generator that runs the rest of the house may not carry them. Tell us how yours is backed up, and the quote will say what the new system needs.