Tankless water heaters in Annandale.

Annandale is a good town for tankless on paper. About 72 percent of owners heat with gas, more than anywhere else on our list, and Wakefield is nearly all single-family houses. The wrinkle is age. The median owner home went up in 1967 and four in five were built before 1980, so the gas piping inside was laid for a furnace, a tank and a range, in a time when no one expected a burner as big as a tankless draws at full fire.

So the job often has two parts: a larger line from the meter, and a vent through the wall. Many Annandale ramblers have the water heater in a basement or utility room on the lowest floor, close to the meter side. That helps, because the gas line is short and the vent can go out through the foundation wall at grade. We route it so it stays clear of windows and the neighbor’s side, which matters on lots as tight as these.

Fairfax County treats a gas appliance replacement as a permit job, and new gas piping gets a permit of its own, so the bigger line is permitted by us as part of the work. After the install the unit is on a different clock too: it needs a yearly descale, and we put the service valves where the descale is easy.

Plenty of Annandale families have grown into these houses. A 1960s rambler built for a family of four now holds a household of six with a finished basement bathroom, all on a tank installed for a smaller house. Those are the houses that feel a tankless the most, as a flow-limited unit sized for the busiest hour.