The condensate drain line is a PVC pipe that carries water from the air handler to the outside of the house. In a Florida summer, your AC pulls 5 to 20 gallons of moisture out of the air per day. All of that water flows through this pipe.
When the pipe clogs, the water backs up. Here is how to tell.
Signs of a clogged drain line
Water near the indoor air handler. If you see water pooling on the floor around the base of the unit, in the ceiling below it (if it is in the attic), or dripping from the unit, the drain pan is overflowing because the line is blocked.
The system shuts itself off. Most Florida AC systems have a float switch on the drain line or in the drain pan. When water backs up, the float rises and cuts power to the system. If your AC stopped working and you see water in the drain pan, the float switch did its job.
Musty smell from the vents. Standing water in the drain pan and a wet, clogged drain line grow mold and algae. The blower pushes air past the pan, picking up the smell and distributing it through the house.
The outdoor drain end is dry. Find where the drain line exits the house — usually a PVC pipe sticking out of the exterior wall near the outdoor unit, or draining onto the ground near the foundation. If the AC is running and nothing is dripping from the end of that pipe, water is not making it through. The line is blocked somewhere between the air handler and the exit.
How to confirm
Look at the drain pan beneath the evaporator coil inside the air handler. If it has standing water in it, the line is clogged. On a working system, the pan should be empty or have only a thin film of water flowing toward the drain fitting.
What clogs it
Algae. In Florida's humidity, algae grows inside the drain line because the pipe is constantly wet and warm. The growth accumulates over months and eventually blocks the pipe. Less commonly, the line clogs with dirt, rust from the drain pan, or insects that have nested in the outdoor end of the pipe.
Monthly maintenance (a cup of vinegar down the cleanout) prevents most clogs. See our post on how to clear it.