Through-the-wall units are everywhere in Manhattan because they need no ductwork and no roof space: one sleeve per room, under the window, in postwar apartment towers, hotels and office-to-residential conversions. That also means a building can have hundreds of the same model, aging together, with the same weak points.

In a hotel or a rental building, the unit is rarely the only thing that matters. Work has to fit around guests, tenants and the building's access rules, and one bad unit often points to a pattern on the same riser or the same exposure. When we repair a unit we note the model and the fault, so a building manager can see whether it is a one-off or the first of many.

Manhattan's grime is hard on PTAC coils. Outdoor coils face the street or an air shaft and load up with soot and dust, which makes the unit run longer, ice up and leak. Cleaning the coils and checking the sleeve pitch and drain on every visit prevents many of the leak calls that come in once the first hot week arrives.

Where PTACs are an apartment's heat source, a unit blowing cool air in January is a heat complaint, not a comfort issue. We check the electric heater and its safety limit first on those calls, because that is the part that fails when it is coldest.