Water heater flush in Annandale.

Annandale’s houses are old enough that the water heater in yours isn’t the first. The median owner home went up in 1967, so the tank there now is probably the house’s third or fourth, and its label tells us its age before anything else. In a rambler or split-level of that era it usually stands on the lowest floor beside the furnace, a few steps from a floor drain, which keeps the flush simple.

The water comes from Fairfax Water, supplier to 97 percent of Fairfax County’s residents. Its two plants measured 3 to 10.8 grains per gallon in 2025. Fairfax Water says northern homes generally get the Potomac plant’s water and southern homes the Occoquan plant’s; Annandale sits in the middle, so we plan around the high number. The Potomac plant read 70 milligrams per liter in May and 185 by November, so a tank takes in its hardest water in the fall.

Each spring Fairfax Water opens hydrants to flush its mains, late March through June this year, and it warns that the flushing can bring brief discoloration and sediment into the house. Whatever reaches the water heater settles on the bottom of the tank, where the water sits stillest. If grit keeps turning up at your taps, Fairfax Water’s rules let it reimburse part of a sediment filter, once per house.

Condos are the exception. About one Annandale home in five is in an apartment or condo building, and there the heater usually lives in a closet with a pan under it and no floor drain, so we drain it into buckets or the washing machine’s drain line instead.