Why your AC drain line clogs every summer in Texas, and how to clear it

Cooling air also dries it. Moisture condenses on the cold indoor coil, drips into a pan, and runs out through a three-quarter-inch PVC pipe that ends outside near the foundation or into a plumbing drain. On a muggy July day a three-ton system can send out a bucket or more. Warm water sitting in a dark pipe grows algae and slime, and a Texas attic is the perfect incubator, so the line narrows through the summer until it stops.

Most systems in our area have a float switch in the primary pan or on the drain line. When water rises, it opens and shuts the system down. A house that suddenly has no cooling and a thermostat that looks normal is often just this switch, and clearing the line restarts everything.

Attic installations also have a secondary pan under the whole unit with its own drain that runs to a spot you cannot miss: over a window, a door or the edge of the roof. If water is dripping from a pipe over a window in July, that is not a roof leak. It is the overflow pan telling you the primary line is blocked. Act that day; the pan is the last defense before the ceiling.

You need a wet/dry vacuum, duct tape or a rag, and ten minutes.

1. 1Turn the system off at the thermostat.
2. 2Find the outside end of the drain line: a PVC pipe low on the wall near the outdoor unit, usually with a short elbow.
3. 3Seal the vacuum hose to the pipe with a rag or tape and run it for one to two minutes. You will hear the clog release and see gray slime in the tank.
4. 4Inside, find the access tee, a short capped PVC stub on the drain line near the indoor unit. Pull the cap and pour a cup of white vinegar in. Leave the cap off for 30 minutes.
5. 5Pour a quart of water into the tee and confirm it runs out the outside end.
6. 6If the pan under the unit holds water, soak it up with the vacuum so the float switch resets.
7. 7Replace the cap and turn the system back on.

A cup of white vinegar in the access tee on the first of every month from April through October. Not bleach, which attacks the metal pan and the glue joints over time. Some technicians also install a drain line treatment tablet or a clear trap that lets you see the water moving. If your line clogs anyway, the pitch of the pipe is often wrong, with a low spot where water sits, and that is a technician's fix.