Why is my AC leaking water inside?

Updated August 2026

Water pooling under an indoor AC unit or dripping through a ceiling is a clogged condensate drain line overflowing the pan about nine times out of ten. The line fouls with algae sludge, the pan fills, and it spills. Clearing it is often free with a wet/dry vac, or $75–250 for a tech. Shut the AC off first so the coil stops making water, then find the drain line and work the causes below.

Safety first: condensate water is dirty and will warp drywall, stain ceilings, and grow mold within a day or two, so shut the AC off at the thermostat the moment you see a leak and mop up standing water. A $15–40 float switch that cuts the system off when the pan fills is the single cheapest defense against a ruined ceiling, and any tech can add one during a service visit.

Causes, most likely first

What makes a furnace do this, most to least common (Updated August 2026)
Cause	How likely	The fix	Typical cost
Clogged condensate drain line	Most common	Algae sludge blocks the line and the pan overflows. Suck the clog out with a wet/dry vac at the outdoor end, or have a tech flush it.	Free to $250
Rusted or cracked drain pan	Common	An old steel pan corrodes through and drips. The pan gets replaced.	$150–600
Frozen coil melting	Common	A coil that iced over from low airflow or charge dumps water past the pan as it thaws. Fix the freeze cause.	$0–1,500
Failed condensate pump	Occasional	When the AC sits below the drain, a pump lifts the water out. A dead pump backs up and overflows.	$50–150
Disconnected or poorly sloped line	Occasional	A line that came loose or lost its downhill slope pools and spills. Reseat and re-slope it.	$0–200

Check these first

Shut the AC off at the thermostat so the coil stops making water while you work.
Find the condensate drain line, a white PVC pipe near the indoor unit, and trace it to its outdoor termination.
Hold a wet/dry vac tightly over the outdoor end for a minute or two to pull the clog free.
Check the drain pan under the air handler for rust holes or water that will not drain away.
If the system has a condensate pump, confirm it has power and that its reservoir is not full and stalled.

Prevent it next time

A clogged drain is the easy one to prevent. Flush the condensate line with a cup of distilled white vinegar every month or two through cooling season, a task worth pinning to your home maintenance checklist, and have the tech clear it at the annual before-summer AC test. Add a $15–40 float switch so a future clog shuts the system off instead of soaking the ceiling below. A frozen-coil leak, by contrast, is an airflow fix, not a drain one.

When to call a pro

Bring in a pro when a wet/dry vac will not clear the line, when the drain pan is rusted through, or when the leak keeps returning. Clearing a stubborn clog or replacing a pan runs $75–600, and a new condensate pump is $150–350 installed. If water has already reached drywall or flooring, get the source stopped fast: a soaked ceiling costs far more than the AC repair, and a chronically leaking old unit may be due for replacement.

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Frequently asked questions

Is a leaking AC an emergency?

It is urgent, not a call-911 emergency. Water from the pan will warp drywall, stain ceilings, and start mold within a day or two, so shut the AC off and mop up right away, then clear the drain. What makes it worse is ignoring it: a slow drip behind a wall or above a ceiling can rot framing before you ever see a stain. Stop the source the same day and dry everything thoroughly.

How do I unclog an AC condensate drain myself?

Find the white PVC drain line and its outdoor end, then hold a wet/dry vac tightly over the opening for a minute or two to suck the clog out. Back inside, pour a cup of distilled white vinegar into the access port to kill the algae, and repeat monthly through cooling season. If the vac will not pull the blockage free, it may sit deeper in the line and be worth a $75–250 service call.

What is the float switch everyone recommends?

It is a small $15–40 sensor that sits in the drain pan or on the line and shuts the AC off when water rises too high. Since the system stops making condensate the instant it powers down, the pan stops filling and the ceiling stays dry. It is the cheapest insurance in HVAC: a few dollars in parts against a repair well north of $1,000. Any technician can add one during a routine service visit.