A condensate drain line is the pipe, usually PVC, that carries water from an air conditioner's evaporator coil drain pan, or from a condensing furnace, to a floor drain, the outdoors or a condensate pump. Many systems add a float safety switch that shuts cooling off when the line backs up, protecting ceilings and floors from overflow.
Why it matters to you
Algae and biofilm clog the line over time; when it backs up, water can overflow into ceilings, floors or the furnace, or the safety switch shuts cooling off on a hot day.
Practical guidance
Ask for the drain to be cleared at every cooling tune-up, and find the line's cleanout so it can be flushed. Some equipment manuals allow a periodic flush with distilled vinegar through the cleanout, so check yours before trying it.