Water leaking from an indoor air handler or mini-split head is a clear sign that the normal condensate management process has broken down. Air conditioners are also dehumidifiers; as warm air passes over the freezing cold evaporator coil, moisture condenses out of the air, much like on a cold glass of iced tea on a humid day. This water must be collected and safely channeled away. When it ends up on your floor or wall, three broad categories of problems are usually to blame: clogs, breaks, or air leaks.
The most overwhelmingly common cause is a clogged condensate drain line. This PVC pipe drains the pan under the indoor cooling coil. It’s a dark, damp environment where algae, mold, and bacterial sludge can flourish. Over time, this biofilm accumulates, restricting the flow until a total blockage occurs. The water then backs up and overflows the drain pan. A common solution for a slow-draining line is to use a wet/dry vacuum on the termination point of the drain line outside your home to suck the blockage out. Pouring a cup of vinegar or a mild bleach solution down the line a few times per year can prevent this growth. A failure of the condensate pump, if your system uses one (common when the unit is in a basement), will also lead to a full pan and an overflow. The pump in the reservoir can burn out, or the float switch that activates it can stick.
Physical damage to the drain system is another culprit. A plastic drain pan can develop cracks over decades of thermal expansion and contraction, especially in older units. The PVC joints in the drain line can also separate if they were improperly glued or if the unit has settled. Moving an air handler in an attic can easily crack the drain pan or disconnect the pipe. These physical failures require replacement of the pan or repair of the piping.
Finally, a water leak can be caused not by the drainage system at all, but by an airflow or refrigerant issue causing the evaporator coil to freeze. When you see a block of ice on the coil, the water is currently frozen. The real leak becomes a flood when the unit shuts off or goes into a defrost cycle, and all that ice melts rapidly. This sudden volume of water can overwhelm a drain pan that is perfectly functional under normal conditions. The root cause of the freezing—be it a dirty filter, low refrigerant, or a blocked return vent—must be fixed. An uninsulated or poorly sealed suction line can also sweat profusely, dripping small amounts of water, but a true leak is almost always related to the main condensate system.