Water heater flush in Arlington.

Most of Arlington drinks Potomac water treated by the Army Corps of Engineers’ Washington Aqueduct at its Dalecarlia plant, bought straight from the Aqueduct rather than through Fairfax Water like the county next door. The County’s 2025 average hardness was 142 milligrams per liter, which it prints as 8 grains per gallon, and the Aqueduct’s own monthly tests at Dalecarlia ran from 106 in June to 187 in December. The water a tank sees in winter is the hardest of the year.

Every spring the Aqueduct switches its disinfectant from chloramine to chlorine, and Arlington’s crews flush hydrants across the county through the end of May. The County says that can bring temporary discoloration and sediment, enough to stain a load of laundry. Whatever rides in on that water and settles ends up on the bottom of your tank, which makes June a sensible month to drain it.

Where the heater sits changes the visit. In the older houses, like the 1940s brick-faced houses in Westover, each one built over a basement, it’s usually downstairs near a floor drain and the flush runs out a hose. In the condo and apartment buildings that hold more than half of Arlington’s homes, it’s in a closet, so we drain it in batches into a bucket or the washer’s standpipe.

Close to six in ten Arlington homes are rented, and the County asks property managers of apartment buildings to post its water quality report where tenants can read it. If you manage units, book the flush for turnover week, when the closet is empty and no tenant is waiting on hot water.