AC Leaking Water Inside? Causes and How to Stop It
Water dripping from your indoor AC unit almost always means a clogged condensate drain. Here's how to find the cause, stop the leak, and clear the line yourself before it damages your ceiling or floor.
Finding water pooling under your indoor AC unit — or worse, a stain spreading across the ceiling below it — is alarming, but the cause is usually simple and fixable. Here's how to stop it fast and prevent the damage that a slow drip can do.
Quick answer: Water under an indoor AC unit is normal condensation that isn't draining properly — and the #1 cause is a clogged condensate drain line. Clear the line (a wet/dry vac on the outdoor end usually does it), check the drain pan, and replace a clogged filter. Shut the system off until it's fixed to avoid ceiling and floor damage.
Why your AC makes water in the first place
This part is normal: as warm, humid air passes over the cold evaporator coil, moisture condenses out of it — just like a cold glass sweats on a summer day. That water collects in a drain pan and flows outside through a condensate drain line. A leak inside means that normal water isn't getting where it should.
The #1 cause: a clogged condensate drain
The condensate drain line is a small PVC pipe, and it's dark and damp inside — perfect for algae and slime to grow. When it clogs, the water has nowhere to go and the drain pan overflows into your home.
This is the cause in most indoor AC leaks, and it's the one you can fix yourself.
Other causes worth ruling out
- A frozen evaporator coil. A dirty filter or low refrigerant can freeze the coil into a block of ice; when it melts, it overflows the pan all at once.
- A cracked or rusted drain pan. Older pans corrode and leak even when the drain is clear.
- A disconnected or sloped-wrong drain line, so water doesn't flow out.
- A failed condensate pump, on systems that pump water up and out instead of draining by gravity.
How to stop the leak and clear the drain
- Turn the AC off at the thermostat and the breaker so it stops making water.
- Mop up and find the source — watch whether it's the pan overflowing or the line itself dripping.
- Check the air filter. If it's clogged, replace it and let any ice on the coil fully thaw before running the unit again.
- Clear the drain line. Find the PVC line's exterior end and hold a wet/dry vacuum to it for a minute or two to suck out the clog. (Seal the connection with a rag for better suction.)
- Flush and confirm flow. Pour a cup of distilled vinegar or warm water into the indoor drain access port and confirm it runs out the exterior end freely.
- Restore power and watch the pan to make sure it stays dry.
This is exactly the routine behind the clear the HVAC condensate drain line maintenance task — doing it a couple of times each cooling season prevents almost all of these leaks.
When to call a pro
Call an HVAC technician if the coil keeps freezing after a filter change (a sign of low refrigerant or an airflow problem), the drain pan is cracked or rusted through, or you can't locate or clear the blockage. Running the system while it leaks risks water damage and mold.
Catch it before it stains the ceiling
A two-minute drain flush each season is far cheaper than drywall repair. Build your free Owner Tools plan and we'll put the condensate-line flush and filter changes on your calendar at the right time of year. No login, no address required.