Water heater flush in Haymarket.

Most of 20169 drinks the same water. Prince William Water counts the Town and Dominion Valley as one Haymarket system and fills it with Potomac River water treated by Fairfax Water at Corbalis, blended with what the City of Manassas draws and treats from Lake Manassas. It rates that supply moderately hard, 3.5 to 7 grains a gallon, though its 2025 tests across every point where water enters its systems ranged from 2.6 to 10.5.

Up on Bull Run Mountain and at Evergreen the utility pumps from six wells instead and rates the water hard: 7 to 10.5 grains a gallon, the top of its scale short of very hard. That number doesn’t swing with a river the way the town’s does; it’s what the rock gives the well. On a private well nobody publishes a figure at all, so the number comes from testing your own tap.

Navien wants its tankless heaters descaled every year once hardness passes 4 grains a gallon and there’s no softener. The Haymarket system is over that line for much of the year, and the mountain wells never drop under it. A tank on the harder water collects sediment faster as well, so up there once a year is the minimum, not the target.

Age decides what a flush can still do. In the Town, where the typical owner home was built in 1995, the heater is seldom the original, so we find out its age from the label before anything gets drained. In Dominion Valley and Piedmont, where most houses went up in the 2000s, a builder’s tank pushing twenty years likely has a spent anode and a drain valve that may not seal again, and you’ll hear plainly whether a flush or a new heater is the better spend.