Downtown is mill country: the Boott and Massachusetts mills, the canals that powered them, Market Street and the blocks around Lowell National Historical Park. A large share of the housing is in converted mill space, where the mechanical equipment sits in a closet, on a roof or at the end of a corridor rather than in a basement.
That changes the work. Vent runs have to find a path through brick and granite, sidewall terminations may not be possible, and exterior changes in the Downtown Historic District go through the Historic Board. A furnace that would take an afternoon in a Belvidere basement can take planning in a loft with a two-foot mechanical closet.