Condensate drain pan

The condensate drain pan (or drip tray) is the tray directly under an AC's indoor evaporator coil that catches the water condensing out of the humid air as the coil cools it, and channels that water to a drain line — usually through a P-trap. It is the No.1 cause of 'my AC is leaking water': in the warm, damp inside of the unit, algae, biofilm and dust build into a slurry that clogs the pan's outlet or the drain line, so the condensate backs up and overflows the pan into the ceiling or wall instead of draining away. (A secondary/auxiliary pan is sometimes fitted below to catch that overflow before it reaches your plaster.) Prevention and cure are the same thing: keep the pan and drain line clear, which is a standard part of a proper AC coil and filter cleaning — flushing the drain and treating the pan, not just rinsing the filter. A pan that keeps overflowing after cleaning usually means a blocked or wrongly-sloped drain line.