How to Fix a Window AC Unit Leaking Water Inside

Frequently Asked Questions

Why is my window AC dripping water inside?

The most common cause is improper tilt. Window AC units should slope backward (outside) about 1/2 inch per foot so condensation drains outside. If the unit is level or tilts inward, water backs up and drips inside. Other causes: a clogged drain hole (blocked by debris), a dirty filter causing coils to freeze and thaw repeatedly, or a cracked condensate drain pan.

How do I fix the tilt on my window AC?

Turn off and unplug the unit. Gently lift the outside edge and place a thin wedge or piece of weatherstripping under the front edge of the unit (inside sill side) to create the correct outward slope. Most manufacturers specify 1/2 inch per foot of unit depth. Reinstall and test: during operation, water should drip from the outside back of the unit, not from the inside.

Is it normal for a window AC to drip water?

Dripping from the outside and back of the unit is completely normal — that's condensation draining as designed. Dripping water inside the room is not normal and signals a tilt issue, clogged drain, or dirty coil. A properly installed and maintained window AC should shed all condensation outside.

How much does it cost to fix a leaking window AC?

DIY repair is usually free (adjust the tilt, clean the drain) or under $10 (shims, drain cleaner). If the drain pan is cracked, a replacement pan for common models costs $15-$40. If the issue is a failing compressor or severe icing (coils frozen solid), repair may not be cost-effective — a new window AC unit costs $150-$600 depending on BTU.

Window AC leaking inside: (1) Check the tilt — the unit should slope slightly downward toward the outside (about 1/2 inch), so condensate drains out not in. Adjust with shims.

A window air conditioner that leaks water inside the house is one of those problems that starts as a minor annoyance — a wet windowsill, a small puddle on the floor — and escalates quickly into water damage, mold, and ruined trim if ignored. The good news is that the vast majority of window AC water leaks have simple mechanical causes that you can diagnose and fix yourself without any specialized HVAC knowledge.

This guide covers the four most common causes: a clogged drain hole, incorrect tilt angle, a clogged air filter causing freeze-then-thaw flooding, and a cracked or overflowing drain pan. Work through each cause in order and you will almost certainly find and solve your problem.

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What You Need

Stock these supplies before you start. Having everything ready means you will not have to stop mid-repair.

• Wet/dry shop vacuum — for vacuuming out drain pan water, dirt, and debris
• Pipe cleaners or a thin bottle brush — for clearing the drain hole and drain channel
• White distilled vinegar — for dissolving algae and mineral scale in the drain pan
• Replacement AC filter for your unit model — a clogged filter is a top cause of freeze-and-flood leaks
• Waterproof epoxy or HVAC drain pan sealer — for repairing small cracks in the plastic drain pan
• Level tool — for checking and correcting the unit’s tilt angle

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How a Window AC Unit Handles Water

Understanding the water management system in a window AC helps you diagnose which part has failed. When your AC cools the air, it does so by passing warm indoor air over a cold evaporator coil. Water vapor in the air condenses on this coil just like a cold glass sweats on a humid day. This condensate drips down into a drain pan inside the unit.

From the drain pan, the water is supposed to travel to the back of the unit and drain outside. Most modern window ACs use a “slinger ring” design — the fan blade at the back of the unit dips into the collected condensate and slings it against the condenser coil, where it evaporates and assists with cooling. Some older units rely instead on a simple gravity drain hole at the back of the unit.

In either case, water flows inside → collects in drain pan → exits to the back → evaporates or drains outside. When any part of that chain breaks down, water backs up and overflows into your home.

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Cause 1 — Clogged Drain Hole or Drain Channel

The most common cause of indoor leaking is a clogged drain. The drain hole at the back of the unit — typically a small notch or hole in the bottom of the outer case — can become plugged with dirt, algae, dust bunnies, or mineral scale from hard water. When the drain hole blocks, condensate fills the drain pan and eventually overflows into the room.

How to inspect and clear the drain:

First, turn off the AC and unplug it from the wall. Pull the unit out from the window sill far enough to access the front and sides. Remove the front filter panel (most slide out without tools) and clean or replace the filter while you have it out.

Look at the bottom interior of the unit — the drain pan that catches condensate. Shine a flashlight into it. You are looking for standing water, slime buildup (usually green, black, or reddish), or mineral deposits that look like white or tan crusty scale.

Use a shop vacuum to suck out all standing water and loose debris. Then pour about half a cup of white distilled vinegar into the drain pan and let it sit for 15 to 20 minutes. Vinegar dissolves both algae biofilm and light mineral scale. Flush the pan with clean water from a spray bottle or small cup, then vacuum again.

Locate the drain hole at the lowest point of the back or bottom of the unit. Probe it with a pipe cleaner or a thin stiff wire to confirm it is clear. You should be able to pass the wire completely through without obstruction. If the hole was blocked, clearing it often solves the problem immediately.

Preventing future drain clogs: Place a drain pan tablet (available at HVAC supply stores) in the drain pan each season to prevent algae growth. Run the unit on fan-only mode for 30 minutes at the end of each season to dry the pan before winter storage.

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Cause 2 — Incorrect Tilt Angle

Window AC units are designed to be installed with the back of the unit sitting slightly lower than the front — typically about a quarter inch lower per foot of unit depth, creating a gentle rearward tilt. This tilt uses gravity to move condensate from the front evaporator area toward the rear drain or slinger ring.

If the unit is level, tilted forward, or tilted sideways, water that should drain to the back instead flows toward the front and inside the room. This is especially common when a unit is installed on a sloped windowsill or when the window is not perfectly level.

How to check and correct the tilt:

Use a torpedo level placed along the side of the unit from front to back. The bubble should indicate that the back of the unit sits a quarter inch to a half inch lower than the front over the full depth of the unit. If the unit is level or tilted the wrong way, you need to shim the front mounting bracket upward.

Check the window mounting bracket or the window sill itself. Most installation brackets have slotted mounting holes or adjustment tabs that let you set the tilt. Loosen the bracket hardware, tilt the unit to the correct angle, and re-tighten. Alternatively, cut small wooden shims and place them under the front edge of the window mounting bracket to achieve the correct angle.

Re-level after adjustment and test by running the unit and observing where condensate appears. Within 15 to 30 minutes of operation, you should see water appearing on the back exterior of the unit or hear the slinger ring flinging water — not seeing any dripping inside.

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Cause 3 — Frozen Evaporator Coil from a Dirty Filter

If you notice that water leaks occur in larger amounts, somewhat periodically, and the front of the unit feels unusually cold — or you can see ice on the visible part of the evaporator coil — you are dealing with a freeze-and-thaw flood cycle rather than a simple drain issue.

How a frozen coil causes flooding: When airflow over the evaporator coil is restricted — most often by a clogged air filter but also possible due to blocked return air, a fan malfunction, or a low refrigerant charge — the coil temperature drops below freezing. Condensate that would normally drip into the drain pan instead freezes on the coil. This buildup can continue for hours. Then when the thermostat cycles the compressor off, or when the unit runs in fan-only mode, the accumulated ice melts all at once — flooding the drain pan and overflowing into the room.

The fix:

Turn off the unit and let it thaw completely — this can take two to four hours. Do not try to chip off the ice; you can puncture the refrigerant lines. Place towels in front of the unit to catch the melt water.

Once thawed, remove and clean or replace the filter. The filter should be washed with mild soap and water or vacuumed clean, then fully dried before reinstalling. If the filter is degraded or torn, replace it.

Also check for anything blocking the return air path — furniture pushed against the front of the unit, curtains draped over the intake, or objects placed inside the window on either side that restrict airflow.

After cleaning the filter, run the unit and watch the evaporator coil for 30 minutes. If it starts to ice over again with a clean filter and clear airflow, you likely have low refrigerant — a condition that requires a certified HVAC technician to diagnose and refill. Most residential window units are sealed systems and refrigerant leaks are rare, but they do occur after many years of service.

Filter cleaning schedule: Clean the filter every two weeks during heavy use. A two-minute filter rinse twice a month eliminates the most common cause of freeze-and-flood incidents entirely.

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Cause 4 — Cracked or Overflowing Drain Pan

If the drain hole is clear, the tilt is correct, and the filter is clean, but water still appears inside, inspect the drain pan itself. In units more than 5 to 10 years old, the plastic drain pan can develop hairline cracks, especially along the corners or at the junction where the front section meets the back section. These cracks allow water to drip through the bottom of the unit before it ever reaches the drain hole.

How to inspect the drain pan:

With the unit unplugged and the filter removed, shine a bright light into the interior and look carefully at the drain pan surface. Pour a small amount of water into the pan and watch to see if it seeps through any point before reaching the drain hole. Cracks are sometimes visible as discoloration or darkened lines in the plastic.

Repairing minor cracks: Allow the drain pan to dry completely — use a fan or heat gun on low — then apply waterproof epoxy rated for plastics, or use HVAC drain pan sealer, which is sold in squeeze tubes specifically for this repair. Apply to the crack, smooth it level with the pan surface, and allow to cure per the product instructions before running water over it again.

Drain pan replacement: For units with severe cracking or a pan that has failed at a seam, replacement pans are available for many popular window AC brands. Search by brand and model number. However, drain pan replacement requires more significant disassembly of the unit — if the unit is older and in otherwise marginal condition, a new unit may be more economical.

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Preventing Window AC Leaks Long-Term

Annual start-up inspection: Before turning on the unit each summer, vacuum out the drain pan, clear the drain hole, and inspect the filter. Five minutes of preventive maintenance avoids the most common causes of in-season flooding.

Store properly in winter: Before storing the unit, run it on fan-only mode to dry the interior, then store it in a clean dry location with the drain hole facing down or to the side so any residual moisture drains out rather than sitting in the pan.

Check installation angle every year: Brackets and sill materials can shift over winter. Re-verify the tilt angle each spring before turning the unit on for the season.

Do not run the unit in very cold weather: Window AC units are not designed to operate in outdoor temperatures below about 60 degrees Fahrenheit. The coil will freeze, which stresses the drain system and can damage the compressor.

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FAQ

• question: “My window AC is dripping outside — is that normal?” answer: “Yes. Water dripping from the back exterior of the unit is completely normal and expected. This is condensate that has properly drained to the rear of the unit and is exiting outside. The problem only exists when water appears on the interior side — on the windowsill inside, on the floor, or dripping from the front of the unit.”

• question: “How often should I clean my window AC filter?” answer: “During heavy summer use, every two weeks is the standard recommendation. If you run the unit constantly, live in a dusty environment, or have pets that shed, clean it weekly. A filter that takes more than 10 minutes to dry after washing is probably due for replacement — they degrade over multiple seasons.”

• question: “My AC leaks only when it rains hard. Is that a different problem?” answer: “Yes. Leaking specifically during rain is usually a window sealing issue — water is infiltrating around the unit through gaps in the window frame sealing foam or around the side accordion panels, not from condensate inside the unit. Inspect and replace the foam sealing strips around the unit and check that the accordion side panels are fully extended and sealed against the window frame.”

• question: “Can I use bleach to clean the drain pan?” answer: “A diluted bleach solution (one tablespoon of bleach per gallon of water) is effective for killing algae and mold in the drain pan. However, avoid getting bleach on metal components as it can accelerate corrosion. Rinse thoroughly with clean water after use. Vinegar is gentler and works well for routine cleaning; reserve bleach for situations where there is visible mold growth.”

• question: “The drain pan repair held for a week and now it is leaking again. What should I do?” answer: “A repair that fails quickly usually means the pan surface was not fully dry when the epoxy was applied, or the crack has continued to propagate beyond the repair area. Strip the old epoxy, allow the pan to dry completely over 24 hours with airflow directed at it, and reapply. If cracking continues to spread, the plastic has likely become brittle from age and UV exposure — a replacement pan or new unit is the better solution.”

• question: “My window AC is only 2 years old but is already leaking. Is it defective?” answer: “A unit that young leaking water is almost always an installation angle issue rather than a defect. Re-verify the tilt angle from front to back and from side to side — the unit should tilt slightly rearward and sit perfectly level left to right. Also confirm the filter is clean, as freeze-and-thaw flooding can occur in new units with restricted airflow just as easily as old ones.”

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Related Reading

• How to Install a Window Air Conditioner Safely
• How to Upgrade to a Smart Thermostat
• How to Fix a Window Air Conditioner — full troubleshooting when the leak is just one of several problems
• How to Unclog a Drain Fast