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Air Conditioner Leaking Water Inside? Clear the Drain Line First

Most indoor AC leaks come from a clogged drain line, a full pan or a tilted unit. Clear the line with vinegar, check the pan, then keep sludge from returning.

By Jamilur Rahman Updated November 5, 2026 · 7 min read

Time
30 min
Cost
Under $20
Difficulty
Medium
A puddle and a wet towel on the floor under an indoor air conditioner with a drip falling from its bottom edge
In this guide
  1. Quick answer
  2. Free vs. permanent fix
  3. What you'll need
  4. Why it happens
  5. Try this first (free)
  6. When free fixes aren't enough
  7. Buy the fix
  8. Window, central or portable? Where the water comes from
  9. What to skip
  10. FAQ
  11. Related problems

Quick answer

An air conditioner leaking water inside is most often a clogged condensate drain line, which backs up and spills over the drain pan. Turn the system off, clear the line with vinegar and a wet/dry vacuum, and check the filter and the unit's tilt. A drain line cleaning kit helps keep the algae sludge from building up again.

Free fix vs. permanent fix

Free fix · 30 minutes

Vinegar flush and a vacuum

  • Switch the AC off at the thermostat
  • Pour distilled white vinegar into the drain access port
  • Suck the line clear from the outdoor end with a wet/dry vacuum
  • Empty and wipe the drain pan

How long it lasts: Often a few months. Algae and sludge grow back in a warm, damp line, so a cleared line can clog again within a season.

Most readers choose Permanent fix
AC condensate drain line cleaning kit (illustration)
Illustration

An AC condensate drain line cleaning kit

Sludge is what clogs the line. A kit with a flexible brush or vacuum adapter and drain pan tablets removes the sludge and slows its regrowth. It turns the yearly flood into a five-minute monthly job.

What to get: AC condensate drain line cleaning kit

How long it lasts: A whole cooling season when you flush the line every month or two.

Check price on Amazon

As an Amazon Associate I earn from qualifying purchases.

What you'll need

  • Distilled white vinegar

    To flush algae from the line

    You have it

  • Wet/dry vacuum

    To pull the clog out from the outdoor end

    You have it

  • Towels and a bucket

    For overflow

    You have it

  • Level or phone level app

    Checks that a window unit tilts the right way

    Free
  • Our pick
    AC condensate drain line cleaning kit (illustration)
    Illustration

    AC condensate drain line cleaning kit

    A brush or adapter plus tablets for the monthly flush

    Check price

Why it happens

1

A clogged condensate drain line

Moisture from the coil drips into a pan and leaves through a drain pipe, usually 3/4-inch PVC. Algae, dust and sludge build up in that pipe. When it blocks, the pan fills and overflows into the house.

2

A cracked or rusted drain pan

In older units, the pan can crack or rust through. Water then drips from the bottom of the unit even though the drain line is clear.

3

A coil that froze and is now melting

A frozen coil melts faster than the pan can drain, and the water spills over. A dirty filter is often behind the freeze.

4

A unit that is not level

A window unit should tilt slightly toward the outside so water runs out the back. If it tilts inward, the water drains into the room. A central air handler that is out of level pushes water to one side of the pan.

5

A kinked or sagging drain hose

Drain lines need a steady downhill slope. A sag or a kink holds water in the line, and it overflows at the first low spot.

Try this first (free)

Do these in order, starting with the simplest.

  1. Turn the AC off

    Tool used: Towels and a bucket

    Set the thermostat to off. If water is near wiring or the pan is overflowing, switch the system off at the breaker as well. Lay towels and a bucket under the leak.

  2. Find the drain line and its access port

    On central systems, look for a 3/4-inch white PVC pipe leaving the indoor unit, with a T-shaped vent and a cap. Outside, find where the line ends, usually near the outdoor unit or the foundation.

  3. Clear it from the outdoor end

    Tool used: Wet/dry vacuum

    Hold the hose of a wet/dry vacuum against the outdoor end of the drain pipe and seal it with a rag or tape. Run the vacuum for a minute or two. Sludge and slime often come out.

  4. Flush the line with vinegar

    Tool used: Distilled white vinegar

    Pour about a cup of distilled white vinegar into the access port, wait 30 minutes, then flush with a little water. Skip bleach, since many HVAC guides advise against it because it can corrode metal parts.

  5. Check the pan, filter and tilt

    Tool used: Level or phone level app

    Wipe out the drain pan and look for cracks or rust holes. Replace a dirty filter. For a window unit, set a level on top and check that it tilts slightly toward the outside.

  6. Add a monthly flush

    Tool used: AC condensate drain line cleaning kit

    During the cooling season, flush the line with vinegar every month, or drop a drain pan tablet in the pan. Write the date on the thermostat or in your phone so it does not slide.

    AC condensate drain line cleaning kit (illustration)
    Illustration

    Tool for this step

    AC condensate drain line cleaning kit · A brush or adapter plus tablets for the monthly flush

    Check price on Amazon

When free fixes aren't enough

If water returns within days of a clean flush, or the pan is cracked, the line may be partly blocked deeper in or sagging between supports. A cleaning kit helps with sludge. It cannot fix a cracked pan, a bad slope or a leaking refrigerant line. Call a pro if you see ceiling stains, a bulging ceiling or water near the breaker panel, and turn the AC off. Ice on the coil points to a freezing problem, so treat that first. A window unit that tilts the right way and still drips indoors may have a failing pan or poor seals.

Buy the fix

Still stuck after the free fixes? These are the products we would look at.

Our picks compared
PickProductFixesWho it's forBuy
Best pick
AC condensate drain line cleaning kit (illustration)
Illustration
AC condensate drain line cleaning kit
sludge that clogs the condensate line and floods the pan Owners of central air, mini splits or ducted systems with an accessible drain line. Check price
Budget pick
AC drain pan tablets (illustration)
Illustration
AC drain pan tablets
algae regrowth that clogs the drain over a season Someone who has already cleared the line and wants it to stay clear. Check price
Best pick
AC condensate drain line cleaning kit (illustration)
Illustration

AC condensate drain line cleaning kit

Fixes: sludge that clogs the condensate line and floods the pan

Look for: A flexible brush or vacuum adapter that fits a 3/4-inch PVC drain; Drain pan tablets or a cleaner made for condensate lines; Clear steps for flushing the access port; Fittings that attach without tools.

Who it's for
Owners of central air, mini splits or ducted systems with an accessible drain line.
Honest downside
You have a window unit with no drain line. Use the tilt check, a drain hose and a seal check instead.
Check price on Amazon
Budget pick
AC drain pan tablets (illustration)
Illustration

AC drain pan tablets

Fixes: algae regrowth that clogs the drain over a season

Look for: Made for air conditioner drain pans; Safe for plastic and metal pans; Slow-dissolving so one tablet lasts weeks.

Who it's for
Someone who has already cleared the line and wants it to stay clear.
Honest downside
The line is blocked now. Clear it first, since tablets do not unclog.
Check price on Amazon

As an Amazon Associate I earn from qualifying purchases. We don't show prices because they change, so use the button to see today's price.

How we picked: How we picked: this page is based on HVAC maintenance guides, air conditioner owner manuals and the problems and fixes people report in owner reviews. We do not test products by hand.

Window, central or portable? Where the water comes from

  • Window unit: condensation drips out the back by design. Water inside means a wrong tilt, a bad seal or a cracked pan.
  • Central air: water from the indoor unit or ceiling means a clogged line or a full pan. Water outside near the outdoor unit can be normal.
  • Portable unit: the tank or hose is full. Empty it, or attach a drain hose that slopes down.

What to skip

What to skip

Skip bleach and caustic drain openers in the condensate line. Strong chemicals can damage the pan and nearby metal, and they are dangerous to splash. Do not ignore a slow leak either, because weeks of damp can stain ceilings and grow mold.

Frequently asked questions

Why is my air conditioner leaking water from the bottom?
Water at the bottom of an indoor unit means the drain pan has overflowed or cracked. A clogged line is the usual cause. Turn the unit off, clear the line and check the pan for holes.
Why does my air conditioner leak water, and how do I stop it leaking inside the house or apartment?
Clear the condensate drain line, replace the filter, and check that the unit is level. For a window unit, make sure it tilts slightly toward the outside. In an apartment, report the leak in writing so maintenance can fix the pan or the pipe.
Air conditioner spitting water or too much condensation dripping: why, and how do I fix it?
A window unit sprays water onto its hot coil to cool it, so some spitting is normal in humid weather. Too much condensation indoors is usually a drain, tilt or window-seal problem. Check the seals around the unit.
Is it normal for an air conditioner to leak water outside?
Yes. Dripping from the outdoor drain line or from the back of a window unit is normal in humid weather. Constant pooling near the foundation, or water from a refrigerant line, is not. Extend the drain pipe away from the wall.
How often should I clean the AC drain line?
Once or twice a cooling season in most homes, and monthly in a humid climate. A cup of vinegar poured into the access port, then a rinse, is enough for routine upkeep.

As an Amazon Associate I earn from qualifying purchases. Affiliate disclosure

AC condensate drain line cleaning kit (illustration)
Illustration

Our pick: AC condensate drain line cleaning kit

Check price