AC leaking water inside: causes and fixes

Water at the indoor unit is condensate, the humidity your AC has removed from the air. On a humid day a central AC produces a steady trickle. It collects in a pan under the evaporator coil and leaves through a plastic drain line, usually white PVC.

A leak means that path is blocked, broken or overwhelmed. It is water, so the immediate risk is to ceilings, floors and the furnace below the coil. It is not refrigerant, which does not pool as a liquid in your house.

Water dripping from the drain line outside, or from a second pipe above a window, is a different story. A drip from the main drain outdoors is normal. A drip from the secondary line, which is often routed to a visible spot on purpose, means the main drain is clogged and the backup is doing its job.

1. Clogged drain line. Algae, slime and dust build up inside the pipe and the trap until water backs up into the pan. This is the cause in most cases.
2. Dirty filter or coil. Low airflow freezes the coil. When the ice melts, the pan gets more water than it can handle. See the guide to a frozen AC coil.
3. Rusted or cracked drain pan. Metal pans on older units rust through, and plastic pans can crack. Water then leaks no matter how clear the line is.
4. Failed condensate pump. Where the unit sits below the drain point, a small pump lifts the water out. When it fails or its float sticks, the reservoir overflows.
5. Disconnected or badly sloped drain line. A fitting that has come unglued or a line that runs uphill will leak at the unit.
6. Low refrigerant. A low charge can freeze the coil, with the same melt-and-overflow result. Only a technician can confirm it.

A technician clears the drain line with a vacuum, compressed nitrogen or a pump, flushes it, and cleans the trap and the pan. They check that the line slopes toward the outlet and that the float switch works. If your system has no float switch, ask for one. It shuts the AC off when the pan fills, which costs little compared with a ceiling repair.

If the pan is cracked or rusted through, it has to be replaced, and on many units that means pulling the evaporator coil to reach it. If the coil froze, the technician will look for the reason: airflow first, then refrigerant charge.

You can clear a simple clog yourself from the outdoor end of the line. You cannot inspect the primary pan or the coil on most systems without opening sealed panels, so a leak that returns after you clear the line is a service call.

Change the filter on schedule so dust does not wash into the pan. If your drain line has a capped access tee near the indoor unit, pouring distilled white vinegar into it a few times each cooling season helps keep slime from forming. Have the drain flushed as part of a yearly tune-up.

A water leak alone rarely justifies a new system. The exceptions are a rusted-out pan or a badly corroded coil on an old air handler, where the repair involves major disassembly, and repeated freezing caused by a refrigerant leak in the coil. The AC repair overview explains how to weigh a large repair against replacement.