Why Is My AC Leaking Water Inside the House?

Root cause

The condensate drain line is a PVC pipe that carries water from the drain pan outside. Over time it can clog with mold, algae, dust, and debris. Once blocked, water has nowhere to go but back up into the pan and onto the floor.

30-second check

Find the PVC drain line's exit point outside the house (usually near the outdoor unit) and check whether water is actively dripping from it during a cooling cycle. If nothing's coming out there, it's likely clogged.

Tools & materials

Wet/dry shop vacuum, white vinegar, a funnel.

Step-by-step

1. 1 Turn off the AC at the thermostat and switch off its breaker before opening anything or working near the unit.
2. 2 Locate the drain line's outdoor exit point and attempt to suction any clog with a wet/dry shop vacuum held over the opening for 2–3 minutes.
3. 3 Find the drain line's access point indoors (often a T-shaped PVC fitting with a cap near the indoor unit) and pour a quarter cup of white vinegar in using a funnel.
4. 4 Let it sit for 30 minutes, then flush with a cup of warm water.
5. 5 Restart the AC and confirm water is now draining outside at the exit point.
6. 6 Repeat this vinegar flush monthly during cooling season to prevent regrowth.

Root cause

A clogged filter restricts airflow across the evaporator coil. With too little airflow, the coil can drop below freezing and ice over. When the system cycles off or the ice eventually melts, it produces far more water than the drain pan is designed to handle at once, overflowing onto the floor.

30-second check

Check the filter for visible dust buildup, and look at the indoor coil (if accessible) for visible frost or ice.

Tools & materials

Replacement air filter matching your system's size.

Step-by-step

1. 1 Turn the AC off completely (thermostat and the breaker) if you see ice on the coil, and let it fully thaw for several hours before restarting. Running it iced-over makes the problem worse.
2. 2 Replace the air filter with a fresh one matching your system's size.
3. 3 Once thawed, restart the system and monitor for the next hour to confirm no new ice forms.
4. 4 Set a recurring reminder to replace the filter every 30–90 days going forward.

Root cause

If the indoor air handler was installed slightly out of level, or the drain pan has shifted, water can miss the drain opening or pool at one corner instead of flowing out normally, producing a slow, chronic leak rather than a sudden flood.

30-second check

Place a small level on top of the indoor unit or drain pan, in both directions, to check it's sitting flat.

Tools & materials

A small level, shims if adjustment is needed.

Step-by-step

1. 1 Locate the indoor air handler and, if accessible, the drain pan underneath it.
2. 2 Check level in both directions with a small level.
3. 3 If it's off, carefully adjust with thin shims until level, or note this for a technician if the unit is difficult to access safely.
4. 4 Recheck for leaking over the next full cooling cycle.

Root cause

The drain pan sits under the indoor coil and collects condensation before it reaches the drain line. Metal pans can rust through and plastic ones can crack with age, and once that happens, water leaks directly through the pan itself rather than following the intended path, no clog required.

30-second check

With the drain line and filter both confirmed clear, look directly at the pan for visible cracks, rust-through spots, or water actively seeping from the pan itself rather than overflowing its rim.

What a professional actually does

Confirms the pan is cracked, shuts down and isolates the unit, fits the matching replacement pan, and tests the drain and the unit before leaving.

Root cause

Low refrigerant drops the evaporator coil's temperature further than normal, causing the same freeze-then-flood pattern as a dirty filter but from a different root cause: refrigerant is leaking somewhere in the sealed system. Separately, systems installed in a basement often use a condensate pump to push water uphill to the drain; if that pump's motor or float switch fails, water simply isn't removed at all.

30-second check

You've cleared the drain line, replaced the filter, confirmed the unit is level, and ruled out a cracked pan, but the coil keeps freezing, or you have a basement condensate pump that isn't audibly running during a cooling cycle.

What a professional actually does

A technician checks refrigerant charge and pressures with certified gauges, uses leak-detection equipment to find and repair a refrigerant leak if that's the cause, and tests or replaces a failed condensate pump's motor or float switch.