Leaks generally occur when the evaporator coil drain line becomes clogged, and moisture removed from the air during the cooling process backs up and overflows. Because the evaporator coil is normally located in a closet or attic, leaks aren’t always discovered right away. Drain line clogs can be caused by a variety of factors, including algae build-up, crimped drain lines, airborne debris, attic insulation, house settling, and the actions of rodents, snakes or other animals.