The Highlands is the densest part of Lowell's south side, a grid of two- and three-family houses built between the 1880s and the 1920s for people who worked in the mills and the shops along the river. The streets are short and narrow, the lots are tight, and almost every building holds more than one household.
Heating here is stacked. Each flat has its own thermostat and usually its own furnace or air handler, the basement is shared, and the chimney often serves more than one appliance. A repair in one unit has to account for the other two, which is why the shared flue and the basement air are checked before the work is signed off.