Window AC Leaking Water — All Causes and How to Fix Each One

"Help — my window air conditioner is leaking water all over my floor! There's a puddle forming under the unit and I don't know what's causing it. Is something broken? Do I need a new AC?"

Don't panic. A window AC dripping water inside your home is one of the most common air conditioner problems, and in most cases, you can fix it yourself in under 30 minutes.

Water leaking from a window AC doesn't mean the unit is broken. It means the condensation your AC naturally produces isn't draining the way it should.

Below, we'll walk you through every cause of window AC water leaks, organized by where the water is coming from. For each cause, you'll get the exact steps to fix it.

Quick Diagnostic: Where Is the Water Coming From?

Before you do anything else, figure out where the water is leaking from. The leak location tells you the most likely cause and points you directly to the right fix.

Pro tip: If the water is dripping from the front vent area, start with Cause #1 below. Incorrect tilt angle is the #1 reason window ACs leak water inside.

How Window AC Drainage Works (30-Second Primer)

Understanding the drainage system helps you diagnose leaks faster. Every window air conditioner removes moisture from indoor air as part of the cooling process — this is how AC dehumidifies your home (learn more about AC dehumidification here).

That moisture condenses on the cold evaporator coil, drips into a drain pan (base pan) at the bottom of the unit, then flows through a channel toward the rear. At the back, a slinger ring attached to the condenser fan scoops the water up and flings it onto the hot condenser coil.

This is brilliant engineering. The slinger ring evaporates the condensate and boosts condenser efficiency by approximately 17%, according to research published in the MDPI Energies journal. Any excess water exits through the rear drain hole.

On a hot, humid summer day, a window AC can produce up to 2 gallons of water per day. That's a lot of water that needs somewhere to go. When any part of this drainage path gets blocked, cracked, or misaligned — you get a leak.

Cause #1: Wrong Installation Angle (No Rear Tilt)

What's Happening

Water is dripping from the front of the unit into your room instead of flowing to the back and draining outside.

Why It Happens

Window ACs are designed to drain condensate out the rear. If the unit is level or tilted slightly forward, water pools in the front of the drain pan and overflows into your room.

Every major manufacturer specifies a slight rear tilt during installation. GE recommends approximately 1/4 inch. LG specifies 1/4" to 1/2" toward the back. The Repair Clinic (Home Depot) standard is a 1-2 degree angle.

How to Fix the Window AC Tilt Angle for Proper Drainage

1. Step 1: Turn off and unplug the AC unit.

2. Step 2: Place a torpedo level on top of the unit, running front to back.

3. Step 3: If the bubble shows level or tilted toward the room, you need to adjust.

4. Step 4: Loosen the mounting brackets or window sash. Slide 1-2 wooden shims under the front (indoor) edge of the unit to raise it slightly.

5. Step 5: Re-check with the level. The back should sit 1/4" to 1/2" lower than the front.

6. Step 6: Tighten all brackets and verify the unit is stable. Close the window sash firmly against the AC frame.

This should stop the leak within 1-2 hours as existing water drains out the rear. If you're installing a new unit, get the tilt right from the start (see our complete window AC installation guide).

Cause #2: Clogged Drain Hole or Blocked Drain Channel

What's Happening

The tilt is correct, but water is still leaking inside. Water is accumulating in the drain pan because it can't flow to the rear.

Why It Happens

The drain trough that runs from front to back, or the drain hole at the rear, is clogged with dirt, dust, mold, algae, leaves, or pine needles. LG's official support page specifically warns about debris including leaves and residue causing restricted drainage.

How to Unclog a Window AC Drain Hole

1. Step 1: Turn off and unplug the unit.

2. Step 2: Go outside and locate the drain hole at the bottom rear of the unit.

3. Step 3: Use a pipe cleaner, thin wire, or small bottle brush to gently clear debris from the hole.

4. Step 4: Remove the front panel and check the drain trough (the channel that runs front to back). Wipe out any accumulated dirt or mold.

5. Step 5: Pour a small cup of warm water through the drain channel to verify it flows freely out the back.

6. Step 6: For persistent mold or algae, flush with a mix of warm water and white vinegar.

This should stop the leak immediately. Add this to your monthly AC maintenance routine during cooling season.

Cause #3: Dirty Air Filter → Frozen Coil → Ice Melt Leak

What's Happening

Your AC was running fine, then you suddenly discovered a large puddle of water. You may have also noticed the AC wasn't cooling well before the leak started.

Why a Dirty Filter Causes Your Window AC to Leak Water

Here's the chain reaction. A dirty air filter restricts airflow over the evaporator coil. Without enough warm air passing over it, the coil temperature drops below 32°F (normal operating temperature is around 40°F, per Grove Heating & Cooling). Moisture in the air freezes onto the coil instead of dripping into the pan.

The ice builds progressively, further restricting airflow, creating a cascading freeze cycle. When the AC cycles off or you turn it off, all that ice melts at once — producing far more water than the drain pan can handle. The result: a sudden flood.

According to Trane, dirty air filters are one of the top causes of frozen evaporator coils. Carrier estimates it takes 1-4 hours for a frozen coil to fully thaw, and up to 24 hours in severe cases.

How to Fix It

1. Step 1: Turn the AC off immediately. Do not chip away at ice — this can damage the coil and cause a refrigerant leak.

2. Step 2: If your AC has a "fan only" mode, switch to it. The warm air helps thaw the ice faster.

3. Step 3: Place towels or a shallow pan under the unit to catch melt water.

4. Step 4: Remove and inspect the air filter. If it's clogged with dust, clean it with warm soapy water and let it dry completely (or replace it).

5. Step 5: Wait 2-4 hours for the coil to fully thaw before restarting the unit.

6. Step 6: Restart the AC with the clean filter. Monitor for the next few hours to confirm no re-freezing.

This should resolve the leak within 4-6 hours. Replace or clean your air filter every 30-90 days during cooling season to prevent this from happening again. If the coil keeps freezing with a clean filter, the issue is likely low refrigerant (see Cause #4) or your AC may be freezing up for other reasons (see our guide to window AC freezing up).

Cause #4: Low Refrigerant → Frozen Evaporator → Leak

Window AC Freezing Up and Leaking Water

This cause looks identical to Cause #3 (frozen coil, sudden melt, big leak), but the root problem is different.

Why It Happens

Low refrigerant drops pressure inside the evaporator coil, which makes it too cold. The coil temperature falls below freezing, and moisture in the air ices over. Per Complete Air Mechanical, this is ironic but real: less refrigerant means a colder coil, not a warmer one.

Since window ACs use a sealed refrigerant system, low refrigerant almost always means there's a leak somewhere in the system. A field-experience guide from acacos.com notes that partial ice formation near the start of the evaporator coil specifically points to a low refrigerant charge.

How to Fix It

This is not a DIY fix. Refrigerant handling requires EPA certification and specialized equipment.

1. Step 1: Turn off the AC and let the ice thaw completely (2-4 hours).

2. Step 2: Check and clean the air filter to rule out Cause #3.

3. Step 3: If the coil freezes again with a clean filter, the refrigerant charge is likely low.

4. Step 4: Call an HVAC technician or consider whether the unit is worth the repair cost. Recharging a window AC often costs $100--$200+, which may approach the price of a new unit.

For older window ACs, this may be the point where replacement makes more financial sense (see how long AC units typically last). You can also read our guide on how to recharge a window AC unit for more details on the process.

Cause #5: Cracked or Overflowing Drain Pan

Window AC Unit Drip Pan — How to Check and Replace

The drain pan (also called the base pan) sits at the bottom of the unit and collects all condensation. Most window ACs use a polystyrene foam tray for this, per Repair Clinic.

Why It Happens

Over time, the pan can crack, warp, or rust through. Older units are especially susceptible. Even a small crack will allow water to bypass the drainage system entirely and drip straight down onto your floor or windowsill.

How to Fix It

1. Step 1: Turn off, unplug, and remove the front panel.

2. Step 2: Inspect the drain pan for visible cracks, rust holes, or warping. A flashlight helps here.

3. Step 3: Minor cracks can be temporarily sealed with waterproof epoxy or sealant.

4. Step 4: Significant damage means replacement. Search for your model number + "drain pan" to find the correct part. Frigidaire's keyhole drain part, for example, is EA150D / 5304481680.

The leak will stop as soon as the pan is sealed or replaced. If the pan is severely corroded, it's usually a sign the unit is aging and may need replacement soon.

Cause #6: Window Seal Gap Letting Humid Air In

How to Seal a Window AC to Stop Condensation Leaks

If water is dripping down the sides of the unit or you see condensation forming on the AC body itself, the problem isn't drainage — it's air infiltration.

Why It Happens

Gaps between the AC unit and the window frame allow hot, humid outdoor air to contact the cold AC surfaces. When 90°F humid air hits a cold AC body, moisture condenses instantly — just like a cold glass of water on a summer day. Networx specifically identifies air leaks around the unit as a common cause of side and front dripping.

This also happens when the accordion side panels aren't fully extended or secured, or when the foam seal between the window sashes has deteriorated.

How to Fix It

1. Step 1: Extend the accordion side panels fully to the edges of the window frame and screw them into place.

2. Step 2: Replace the foam seal strip between the upper and lower window sashes (most units include one — replacements are available at any hardware store).

3. Step 3: Apply weatherstripping or caulk around any visible gaps between the unit and the window frame.

4. Step 4: Check the seal after heavy rain to ensure no water enters around the perimeter.

Proper sealing not only stops leaks but improves cooling efficiency by preventing warm air from fighting your AC's output (related: why your AC isn't blowing cold air).

Cause #7: Oversized AC Unit Short-Cycling

What's Happening

The room feels cool but clammy, the AC turns on and off frequently (every few minutes), and you're finding excess water or condensation around the unit.

Why It Happens

An oversized window AC (too many BTUs for the room size) cools the air so quickly that it reaches the thermostat set point and shuts off before it can adequately dehumidify. Fire & Ice HVAC calls this creating "a cold jungle — cool but muggy." The Mayo Clinic recommends indoor humidity between 30-50% for comfort.

Short cycling means the evaporator coil doesn't run long enough to properly manage condensation. The constant on-off also accelerates component wear. Energy Star warns that oversized systems "will not remove humidity effectively."

How to Fix It

There's no quick fix for an oversized unit. Your options are to run the AC on its lowest fan setting to extend run cycles, use a separate dehumidifier, or replace the unit with a properly sized one. Use our BTU calculator to determine the correct size for your room.

Cause #8: Blocked or Broken Condensation Slinger Ring

What's Happening

You hear water sloshing inside the unit, or there's an excessive amount of water pooling in the base pan. The unit may also be louder than usual from the fan hitting standing water.

Why It Happens

The slinger ring — a ring attached to the condenser fan blade — is supposed to scoop water from the base pan and spray it onto the condenser coil. If the ring is damaged, bent, or if debris is preventing the fan from reaching the water level, condensate accumulates instead of evaporating.

GE states that the slinger ring design "improves the operational efficiency of the air conditioner, so you do not want to drain the water or add a drain hole." Some newer units use self-evaporating technology in place of a traditional slinger ring.

How to Fix It

1. Step 1: Turn off and unplug the unit.

2. Step 2: Remove the outer casing to access the condenser fan area (consult your model's manual).

3. Step 3: Inspect the slinger ring for damage, bending, or debris wrapped around it.

4. Step 4: Clean the fan area and remove any obstructions.

5. Step 5: If the slinger ring is damaged, order a replacement part using your model number.

As a temporary measure in extremely humid conditions, LG notes you can remove the drain plug and attach a 1/2" drain hose to route water away from the unit. But this should be a last resort since the slinger ring is critical to efficiency.

Parts and Replacement Costs at a Glance

How to Stop Your Window AC From Leaking Water (Summary)

Here's the quick-action checklist:

1. Check the tilt. The back of the unit should be 1/4" to 1/2" lower than the front. This alone fixes most leaks.

2. Clear the drain hole and drain channel of any debris.

3. Clean or replace the air filter. Dirty filters cause frozen coils, which cause sudden floods.

4. Inspect the drain pan for cracks.

5. Seal all gaps around the window frame with foam weatherstripping.

6. Verify your AC is the right size for the room — too many BTUs causes short-cycling and humidity problems.

When to Call a Pro vs. DIY Fix

GE's support team recommends calling a professional for "persistent water leaks after cleaning the drain." If you've worked through all the causes above and the leak continues, the issue may be internal — a cracked evaporator housing, warped internal channel, or manufacturing defect.

Frequently Asked Questions

Why is my window AC leaking water inside?

The most common cause is incorrect installation angle. If the unit doesn't tilt slightly toward the outside (1/4" to 1/2"), condensate pools in the front pan and overflows into your room. Other causes include clogged drain holes, dirty air filters causing frozen coils, and cracked drain pans.

Window AC not draining water — what's wrong?

If your window AC isn't dripping water outside at all, the drain hole is likely clogged with dirt or debris. Clear it with a pipe cleaner. If the unit has a removable drain plug (many LG models do), verify the plug is properly seated. In dry climates, no dripping may actually be normal — the slinger ring evaporates all condensate internally.

Why is my window AC spitting water inside and making noise?

Water spitting from the front vent happens when the fan picks up water from an overfilled base pan and flings it forward. LG confirms this occurs when humidity is extremely high or when the unit isn't tilted correctly toward the back. Fix the tilt, clear the drain hole, and the spitting should stop.

How much water does a window AC produce per day?

On a hot, humid day, a window AC can produce up to 2 gallons of water per day. This is completely normal. The water comes from moisture the AC pulls out of your indoor air. In drier climates, you may see little to no dripping. LG notes that drain frequency ranges from every 6-8 hours to several weeks, depending on your local humidity.

Should my window AC drip water outside?

Yes — some dripping from the back of the unit is perfectly normal and means the drainage system is working correctly. GE explicitly states: "It is normal to see water dripping on the outdoor side." If there is no dripping at all during humid weather, check for a clogged drain hole or a malfunctioning slinger ring.

When should I replace a leaking window AC instead of fixing it?

Consider replacement if the unit needs a refrigerant recharge (cost often approaches a new unit price), the drain pan is severely corroded, the compressor is struggling, or the AC is more than 10-12 years old. Check our guide on how long AC units typically last to help you decide.