Why is my AC leaking water inside?

A puddle under the indoor unit almost always means a clogged condensate drain line. The AC pulls water out of the air as it cools, and when the drain backs up, the pan overflows and drips onto the floor or through the ceiling. Shut the system off, clear the line with a wet/dry vac, then flush it with vinegar.

Where is the water coming from?

The water is condensation, not a leak in the plumbing sense. Warm house air passes over the cold evaporator coil inside the air handler, and the moisture in that air beads up on the coil the same way it beads on a glass of iced tea. On a humid day a working air conditioner pulls gallons of water off that coil.

That same condensation is how the AC pulls humidity out of the house. A leak is just that water ending up in the wrong place. It’s supposed to drip into a shallow pan under the coil and run out through the condensate drain, a white PVC line that empties outside near the foundation or into a floor drain. The whole system depends on that one drain staying clear. When it doesn’t, the water has nowhere to go but over the lip of the pan.

Where it lands depends on where the air handler sits. In a basement it puddles on the floor. In an attic it soaks through the ceiling below and shows up as a brown stain in an upstairs bedroom. In a closet or garage it pools under the unit. The stain is often the first thing a homeowner notices, days after the drain actually clogged.

Why is my AC leaking water inside?

A clogged condensate drain line is the cause about nine times out of ten. Algae and slime grow inside the damp PVC over a season, build into a plug, and stop the water from draining. The pan fills, then overflows. The Department of Energy is blunt about what that does:

Clogged drain channels prevent a unit from reducing humidity, and the resulting excess moisture may discolor walls or carpet.

A few other causes look the same from the floor.

A cracked or rusted drain pan leaks even when the line is clear. Pans on older systems rust through at the low corner, and the water drips straight down instead of running to the drain.

A disconnected or poorly sloped drain line spills at the joint. If the PVC came apart, or was never pitched downhill, water pools and runs out where the pipe sags instead of draining out the end.

A frozen coil that melts all at once overruns the pan. This one has a different fix, covered below, and it usually shows up as weak or warm airflow first.

How do I clear a clogged condensate drain line?

Shut the AC off at the thermostat first, so it stops making water while you work. Then find the white PVC line at the indoor air handler and follow it to where it drains outside or into a floor drain.

Put a wet/dry vac on the outdoor end. Hold the hose over the open pipe, seal the gap around it with a rag or your hand, and run it for a minute or two. A clog of algae and water pulls out with a wet slap you can hear in the hose. That clears the blockage.

Then kill what caused it. Near the air handler there’s a capped access tee on the drain line, a T-shaped fitting with a screw cap or a rubber plug. Pull the cap, pour in a cup of distilled white vinegar, and let it sit half an hour. The vinegar breaks down the algae film so the line doesn’t plug again in three weeks. ENERGY STAR lists inspecting the condensate drain among its routine cooling-season checks for exactly this reason.

Last, sponge the drain pan dry and find the float switch, a small plastic float that shuts the system off when the pan fills. Make sure it moves freely. That switch is what stops the next clog from becoming a ceiling repair, so a stuck one is worth fixing.

What if the leak is from a frozen coil?

Check whether the air from the vents went weak or warm before the water showed up. A frozen evaporator coil turns into a block of ice, then melts and dumps more water than the pan can hold once the system shuts off. The usual cause is airflow: a dirty filter, closed vents, or a weak blower.

Turn the system off and set the fan to On so the ice thaws, usually 2 to 4 hours, then change the air filter and open any closed vents. A coil that ices up is the same airflow problem behind a system that’s not cooling the house, so the filter is the first thing to check either way.

If it ices back up within a day of a fresh filter, the airflow isn’t the whole story, and the cause is inside the refrigerant system. That’s a pro’s call, not a filter change.

When should I call a professional?

Call one if the drain pan is cracked or rusted through, since a new pan means pulling the coil. Call one if the line disappears into a slab or a wall where a shop vac can’t reach it, or if it clogs again within a few weeks of a good flush, which points to a slope or install problem. Clearing a drain a homeowner can’t reach runs $75 to $250 for a service call.

Standing water also grows mold fast. The EPA notes that mold takes hold on damp surfaces within a day or two, so a leak that soaked drywall or carpet needs drying out, not just a cleared drain. And if the coil keeps freezing after a fresh filter, the refrigerant side needs a certified tech, which is work federal rules keep out of homeowner hands.

A note from Kempt

A clogged condensate drain is the kind of thing nobody thinks about until there’s a stain on the ceiling. Flushing the line with vinegar once a season is a ten-minute job that keeps the drain from ever plugging, but it’s easy to let slide when the AC is running fine and nothing looks wrong. Kempt keeps the house’s cooling-season upkeep on file, the filter size, the last change date, the drain flush, so the small maintenance that quietly prevents water damage gets handled before August, not after the drywall comes down.