Water heater flush in Fort Belvoir.

Fairfax Water supplies the houses off post, and by its own account homes toward the south of the county generally get water from its Griffith plant, which treats the Occoquan Reservoir. So a tank in Cook Inlet or on Backlick Road is usually refilling with Occoquan water. Griffith’s monthly lab results for 2025 put its hardness between 51 and 137 milligrams a liter, call it 3 to 8 grains a gallon: moderately hard, enough to leave a pale crust on a showerhead and a layer of grit on the floor of a tank.

That figure swings with the seasons. Griffith’s water tested softest in July and August, at 51, and hardest in January, at 137, with November close behind at 135. January is also when the water arriving from the street is at its coldest and the burner runs longest to bring it up to temperature, so the hardest water of the year usually reaches the tank in the same weeks it works hardest. NOAA’s normals for Reagan National Airport, about 13 miles north, put the average January low near 30, and the late January 2026 cold snap brought wind chills of zero to ten below from the Piedmont to I-95.

Fairfax County’s records show a gas connection at every Cook Inlet lot and a full basement under every house, built 2002 to 2004, so the usual setup is a gas tank standing in the basement, often beside the furnace. A tank still original to one of those houses is past twenty, well beyond what most tanks give, and a drain valve that has never been turned can start the very leak you were trying to prevent. That’s why the tech reads the date on the label first. A replacement from the last ten years or so is worth flushing; an original usually isn’t.

On Backlick Road, where the county shows no gas available to nearly every lot, the tank is most likely electric, or propane if there’s a tank in the yard. On an electric tank the grit piles up at the bottom element, and draining it out is how that element keeps working. If a Cook Inlet owner already traded the original tank for a tankless, its descale is $300, regularly $350, since hard water scales a heat exchanger the same way it crusts a tank bottom. On post, The Villages at Belvoir maintains the water heaters in its homes; residents put in a service request with The Villages.