Window AC Unit Leaking Water Inside – 5 Causes

A window air conditioner unit that leaks water inside your home isn't just an inconvenience—it's a sign that something needs attention. Water damage can lead to mold growth, electrical hazards, and costly repairs if left unchecked. The good news is that most window AC leaks stem from preventable issues you can diagnose and fix yourself.

Understanding why your AC is leaking helps you decide whether you can handle the repair or need professional help. Let's walk through the five most common causes and what you can do about each one.

1. Clogged or Frozen Condensation Drain

Your window AC unit produces condensation as it cools air—sometimes up to 5-20 gallons per day depending on humidity levels and system size. This water needs somewhere to go, and that's where your condensation drain comes in.

How the Drain System Works

Modern window units have a small drain pan and drainage holes (typically 1/4 inch diameter) that direct moisture either outside through the rear of the unit or down through the window track. When these drains clog with dust, algae, or debris, water backs up and spills inside your home instead.

Identifying a Blocked Drain

Look for these warning signs:

• Water pooling in the interior window sill
• Dampness around the window frame
• Musty odors coming from the AC unit
• Visible water dripping from the unit's front panel rather than the back

How to Clean the Drain

Follow these steps to clear the blockage:

1. Unplug the unit completely from the wall outlet
2. Locate the drain opening — this is usually a small hole on the back or bottom of the unit (consult your manual if unsure)
3. Use a thin, flexible object like a pipe cleaner, straightened coat hanger, or plumbing snake to gently probe the drain hole
4. Flush the drain with warm water using a turkey baster or small funnel — do this carefully to avoid pushing the clog deeper
5. Check the drain pan inside the unit for accumulated sludge and clean it with a damp cloth
6. Repeat flushing until water flows freely

For stubborn clogs, a mixture of equal parts white vinegar and water can dissolve algae buildup. Let the solution sit for 15-20 minutes before flushing.

Preventing Future Clogs

Clean your AC's drain every 2-4 weeks during the cooling season. This simple maintenance prevents 90% of drain-related leaks.

2. Damaged or Misaligned Condensation Pan

The condensation pan is a small container, usually made of plastic or aluminum, that catches water before it reaches the drain. When this pan cracks, corrodes, or shifts out of position, water flows directly into your home instead of toward the drainage system.

Signs of Pan Damage

• Persistent leaks even after cleaning the drain
• Visible cracks or holes in the pan (inspect this when the unit is unplugged)
• Water leaking from the unit's interior rather than outside
• Unit recently moved or reseated

Checking Pan Position

Window AC units should be slightly tilted backward (typically 1-2 degrees) to encourage water to flow toward the drain. If your unit has shifted forward or sits level, gravity works against proper drainage.

How to adjust:

1. Unplug the AC unit
2. Carefully remove it from the window
3. Place a shim or thin wedge under the back edge of the mounting bracket
4. Reinstall the unit, ensuring it tilts slightly backward
5. Test for leaks by running the unit in a low-humidity area first

Pan Replacement Options

If the pan is cracked:

• Temporary fix: Use waterproof epoxy putty ($8-15) to seal small cracks. This buys you time but isn't permanent
• Permanent fix: Order a replacement pan assembly from the manufacturer (costs $25-60) and follow your unit's manual for installation
• Professional service: An HVAC technician can replace it in 30-45 minutes for $150-250

3. Clogged Fins or Evaporator Coils

The evaporator coil is the cold, metal component inside your AC that absorbs heat and produces condensation. When dust, pet hair, or debris accumulates on the fins (thin metal sheets surrounding the coil), water can't drain properly and backs up inside the unit.

How Dirty Coils Cause Leaks

Clogged fins restrict airflow, which causes several problems:

• Reduced cooling efficiency (you might notice weak airflow)
• Ice buildup on the coils, which melts and overwhelms the drain system
• Water pooling instead of flowing toward the drain opening

Cleaning the Evaporator Coil

Important: Never use high-pressure water on AC fins—they're delicate and bend easily, worsening the problem.

1. Unplug the unit
2. Remove the front cover or filter panel (this varies by model)
3. Gently vacuum the coils and fins with a soft brush attachment
4. Use compressed air (from a can or compressor) at low pressure (30-40 PSI) to blow dust away from the fins, working from the top down
5. For stubborn buildup, use a coil cleaner spray ($12-20) designed for AC units, applied according to product directions

This cleaning typically takes 20-30 minutes and can solve leaking issues while improving cooling performance by 15-20%.

4. Poor Installation or Window Sill Slope Issues

How you install your window AC unit significantly impacts whether water leaks inside. Many leaks happen because the unit wasn't installed correctly from the beginning.

Installation Problems That Cause Leaks

• Unit tilted forward instead of backward (prevents proper drainage)
• Gaps between the unit and window frame where water can enter
• Sagging window sill that doesn't slope downward to the outside
• Improper seal around mounting brackets

Checking Your Installation

Stand outside and look at your window from a low angle. The window sill should slope downward toward the exterior—typically dropping 1/4 inch per 12 inches of width. If it's level or slopes backward, this is a drainage problem waiting to happen.

Fixing Installation Issues

For tilting problems:

1. Fully remove the unit from the window
2. Check the mounting brackets for damage
3. Adjust the rear support brackets to create a slight backward tilt (about 1/4 inch rise from front to back)
4. Reinstall and test

For air gaps:

1. Remove the unit again
2. Use weatherstripping foam tape ($8-15 for a roll) around the edges where the unit meets the window frame
3. Apply caulk ($5-10) around the outside perimeter to seal completely
4. Let caulk cure for 24 hours before running the AC

5. Frozen Evaporator Coils

When outdoor temperatures drop below 62°F, or if your AC runs constantly in very dry conditions, the evaporator coil can freeze solid. As it thaws, the enormous volume of water overwhelms the drainage system, causing leaks inside.

Why Coils Freeze

• Running the AC below 62°F (uncommon but happens with overzealous cooling)
• Extremely low humidity levels (below 30%)
• Restricted airflow from dirty filters or blocked vents
• Thermostat set to maximum cooling for extended periods

Recognizing a Frozen Coil

• Sudden reduction in airflow or cooling
• Ice visible inside the unit or on exterior surfaces
• Water leaking shortly after the ice melts
• AC running but producing no cold air

Thawing Frozen Coils

1. Turn off the AC immediately — continuing to run it wastes energy and can damage the compressor
2. Turn on the fan only (if your unit has this feature) to help melt ice faster
3. Wait 3-4 hours for natural thawing, or accelerate by opening windows to allow warmer air circulation
4. Check and clean the filter — a dirty filter often causes freezing
5. Adjust thermostat settings — avoid running AC in cold weather or on maximum indefinitely

Common Mistakes to Avoid

Many homeowners inadvertently make leaks worse:

• Using high-pressure water or power washers on coils or fins (causes permanent damage)
• Ignoring leaks for weeks (allows mold growth and electrical damage)
• Covering drain holes with tape or sealant (traps water inside the unit)
• Removing the front panel without unplugging (electrical shock risk)
• Assuming a leak will fix itself (it won't—the underlying problem persists)
• Blocking the exterior drain opening with siding or landscaping (defeats the entire drainage system)

When to Call a Professional

You should contact an HVAC technician if:

• The evaporator coil is visibly cracked or damaged
• You're uncomfortable removing the unit from the window
• The drain system still backs up after cleaning
• Water is leaking inside the wall or wall cavity (suggests installation problems beyond DIY repair)
• The unit is still under warranty (professional service preserves it)
• You suspect electrical damage from water exposure

Quick Troubleshooting Checklist

Before calling for help, run through this checklist:

• Unplugged the unit and visually inspected for obvious cracks
• Cleaned the condensation drain with a pipe cleaner and flushed with water
• Vacuumed the evaporator coil fins gently
• Checked that the unit tilts slightly backward
• Verified the exterior drain opening isn't blocked
• Cleaned or replaced the air filter
• Ensured thermostat isn't set to cool in cold weather
• Checked for visible ice on coils
• Inspected the drain pan for cracks

Moving Forward

A leaking window AC unit demands attention, but most causes are fixable with basic tools and a little time. Start with the simplest solutions—cleaning the drain and inspecting the pan—before moving to more involved repairs.

Your next steps:

1. Unplug your AC unit immediately to prevent electrical hazards
2. Identify which cause matches your situation using the descriptions above
3. Address the problem using the specific steps provided
4. Run the unit briefly in low-humidity conditions to test for leaks
5. Establish a maintenance routine (drain cleaning every 2-4 weeks) to prevent recurrence

Most window AC leaks can be resolved in under an hour with supplies you might already have at home. Take action now to prevent water damage, mold growth, and the higher costs that come with ignoring the problem.