Water heater flush in Vienna.

Inside the town, and on some streets outside it, the water comes from the Town of Vienna, which doesn’t treat its own. In 2024 it bought it from Fairfax Water’s Corbalis and Griffith plants and the Washington Aqueduct’s McMillan and Dalecarlia plants, and the Town’s 2025 report calls it moderately hard to hard: 5 to 10 grains per gallon, or 80 to 170 milligrams per liter. That’s past the 4 grain mark where Navien wants a tankless descaled every year when no softener is doing the work.

Where your bill comes from Fairfax Water instead, its 2025 figures ran 3 to 10.8 grains per gallon across its two plants. The advice doesn’t change across the town line: flush a tank every year, and descale a tankless on the same calendar.

Once a year the Town switches its disinfectant from chloramine to chlorine for a short stretch and flushes its fire hydrants while it does, more than 800 of them across its service area. Running the mains hard stirs up what has settled in them, and some of that can end up on the bottom of a tank. If your water ran cloudy or tinted that week, mention it when you book.

Where the heater sits depends on the house. In the town’s older homes, about two thirds of them built before 1980, it’s usually a gas tank in the basement beside the furnace, near a floor drain, and long past the house’s first one. In the houses built since 2000 it may be in a closet off an upstairs laundry, which means flushing into a bucket or the washer’s standpipe and checking the pan underneath.