The air-handler portion of your air conditioner resides inside the furnace. Within the handler, a set of evaporator coils take in liquid chemical refrigerant and perform a phase change to a gas. The change causes the coils to become cold, and that provides your home's air a source of cooling but also produces condensate. Condensate drips down into the bottom of the handler, where a drain pan collects the moisture and disposes of the water in one of two ways down a drainpipe.
If you spot water leaking out of the bottom of your air handler, there are a couple of potential causes related to that condensate.
Broken Condensate Pump
Drain trays empty either due to gravity, if your system is located in a basement, or due to a condensate pump, which provides a motorized boost for the drainage. If your system has a condensate pump, and the drain pan doesn't seem to go down at a regular rate, you might need to replace the pump.
Call a heating and cooling services company or a plumber to examine, diagnose, and replace the condensate pump, if necessary. A broken condensate pump might still work from time to time but won't drain the water fast enough to prevent leaking, which can cause water damage to your home around the furnace.
Clogged Drain Pipe
Regardless of whether you have a gravity or condensate pump system, a clogged drain pipe can completely thwart drainage and leave you with a nasty and persistent drain-pan leak. If you suspect a drain leak, you can attempt to clear out the line yourself using an auger. But the condensate draining out of the pan shouldn't cause a clog on its own, so there is likely a different clog deeper down the line.
Call a plumber to examine the drainpipe with a small camera to locate the clog and determine its nature. The plumber can then clear out the clog in the most efficient way and have your air conditioner operating without leaks.
Improper Refrigerant Levels
If the refrigerant leaks out of the system, the evaporator coils can become overly chilled due to a phase-change imbalance. This overchilling can cause excess condensate to form and drop into the drain pan. The condensate production can become so fast that the drain pan can't keep up with the supply. Your pan will end up leaking even though the drainpipe is clear and the condensate pump is working.
Call in a professional technician to service the unit with more refrigerant to keep the leak from continuing and to ensure your AC system doesn't lose efficiency.