Condensate drain pipes carry away the water that forms on the evaporator coil as it cools humid air, and blockages in this line are among the most common causes of indoor water leakage. Over time, algae, mold, and fine dust particles combine inside the drain line to form a biofilm that gradually narrows the passage until water can no longer flow freely.
When this happens, water backs up into the drain pan and eventually overflows, dripping from the indoor unit or, in some installations, running down through the ceiling in multi-story buildings. Clearing a blocked drain typically involves flushing the line with a wet-dry vacuum from the outdoor end or using a flexible brush to physically dislodge the buildup, followed by a cleaning solution to reduce future biofilm growth.
The slope of the drain pipe matters as much as its cleanliness; a pipe installed without sufficient downward gradient allows water to sit rather than flow out, encouraging further blockage even after cleaning. In humid climates or rooms with heavy AC usage, periodic drain line flushing, done every few months, prevents the gradual buildup that leads to overflow incidents.