Water dripping from your indoor air conditioning unit is almost always the condensate the system pulls out of the air failing to drain away: the drain line is clogged with algae or sludge, the drain pan is cracked or overflowing, a condensate pump has failed, or a frozen coil is thawing faster than the drain can handle. Turn the system off to stop adding water, contain what’s there, and then work through the causes below — the clogged drain line is by far the most common and is often clearable yourself.
Right now
Set the thermostat to Off. Put towels or a pan under the leak; in an attic or over a finished ceiling, get water off the drywall as fast as you can. Look at the indoor coil and the copper lines: if you see ice, the leak is meltwater — set the fan to On to thaw and read AC freezing up, then come back here for the drain.
Likely causes, ranked
Clogged condensate drain line. In our climate the drain runs wet for six months and grows algae and biofilm that eventually block it; the pan fills and overflows. Find the PVC drain line near the air handler (there’s usually a capped tee or access port). Common DIY: remove the cap and pour a cup of distilled vinegar down; or attach a wet/dry vac to the outside end of the drain line where it exits the house and pull the clog through. Many systems have a float switch in the pan that shuts the system off when water rises; if yours did, that’s why it won’t run.
Overflowing or cracked drain pan. Primary pans under the coil rust or crack over years; secondary (emergency) pans under attic units fill when the primary drain clogs and drain out a separate line, often to a spot you’ll notice, like above a window. Water in the secondary pan is a warning that the primary drain is blocked.
Failed condensate pump. Where gravity drainage isn’t possible (basements, some closets), a small pump lifts the water out. When it fails, the reservoir overflows. Listen for it running; check its float. Pumps are $150–$400 installed.
Frozen coil. See above; when it thaws, the volume can overwhelm the drain. Fix the cause of the freezing (usually filter or refrigerant), not just the drain.
Disconnected or badly sloped drain. Occasionally a drain line has come apart at a joint or was installed without enough slope. Visible on inspection.
Dirty filter / coil. Not a direct cause, but a filthy coil sheds debris into the drain and freezes more easily. Change the filter.
When to call
Call if you can’t clear the drain, if the pump has failed, if the pan is cracked, if the coil was frozen and you don’t know why, or if water has reached electronics or a ceiling. Ask the technician to clear and flush the drain, check the pan and float switch, and confirm the coil isn’t icing. Typical costs: drain clearing $100–$250; pump $150–$400; pan replacement $200–$600 (more if the coil must come out). Full ranges in the repair cost guide.
Preventing the next one
A drain flush is part of every spring tune-up; between visits, a cup of vinegar down the drain line every couple of months in cooling season keeps the biofilm down. If your air handler is in an attic, make sure it has a secondary pan with a float switch; it’s the difference between a nuisance and a ceiling repair. Every contractor on the North Carolina registry shows the license status we found.