Furnace tune-ups in Fort Belvoir.

The two nearest long weather records bracket a Fort Belvoir winter. Reagan National, 13 miles up the river at the water’s edge, counts 3,769 heating degree days and 60 freezing nights a year in its 1991 to 2020 normals. The Oxon Hill station across the river, 11.5 miles northeast and 190 feet up, counts 4,244 and 80, with its first 32 degree night by November 8 in half of years. A furnace here should have its tune-up before then.

Those averages hide the hard nights. An Arctic front in December 2022 drove wind chills to about 5 below near the tidal Potomac, and late January 2026 brought Extreme Cold Warnings and wind chills of 0 to 10 below from the Piedmont to I-95. A furnace that ran nonstop through that, or kept relighting, left clues the fall readings catch, like a temperature rise outside its rating plate’s range.

Off post, Fairfax County’s records put all 256 Cook Inlet houses on a gas line and over a full basement, nearly all built from 2002 to 2004. The usual layout is a gas furnace in the basement with the gas water heater a few steps away, and the furnace is either the builder’s, past twenty winters, or its first replacement. On an original, inducer motors, igniters and control boards tend to give out around now, and the tune-up finds the tired one in October, not January.

Backlick Road in Accotink is the other side of 22060. The county shows gas unavailable to 18 of its 19 lots, on houses dated 1900 to 2010. A heat pump there is the same $69 a system, and a propane furnace gets its pressure checked against propane’s settings, higher than natural gas runs. Eight of those houses have no basement, so the equipment may sit in a crawlspace or closet.