Most common causes
- Algae and sludge in the drain line. The water your AC removes from the air is warm and damp, so algae and slime grow inside the drain pipe until it blocks.
- Dirt and dust. A dirty filter lets dust reach the wet coil, and it washes into the pan and the drain.
- Blocked drain outlet. The end of the drain line outside can be covered by dirt, mulch, or plants, or crushed by foot traffic.
- Failed condensate pump. Systems without a downhill drain use a small pump. If it fails, water backs up and trips the float switch.
- Drain line sagging or disconnected. A sagging line can hold water and sludge. A disconnected line sends water where it should not go.
- Float switch stuck. Sometimes the switch itself sticks in the up position even after the water drains.
What you can safely check
- Turn the system off at the thermostat.
- Look at the drain pan under the indoor unit. If it has water, soak it up with a wet vacuum or towels.
- Find where the drain line ends outside, often near the foundation, and clear any dirt or plants from the opening.
- If there is a cleanout cap or a T fitting on the drain line near the unit, remove the cap and pour in a cup of plain white vinegar. Wait about 30 minutes.
- Check the float switch on the pan or drain line. Once the water is gone, it should drop back down on its own. Do not cut or bypass it.
- Replace the air filter, then turn the AC back on and check that water drips from the outside end of the drain.
Call a technician if
- The system will not restart after the pan is dry
- The pan fills again within a day
- Water is leaking through a ceiling or wall
- The condensate pump is not running or runs without moving water
- The drain pan is rusted or cracked
What affects the repair cost
Clearing a clogged drain line is usually a simple service call. Replacing a float switch or condensate pump costs more, and a drain pan replacement depends on how hard it is to reach. Adding a drain cleanout or treating the line during yearly maintenance helps prevent repeat clogs.
