Water dripping from a window AC unit onto an inside floor or down interior drywall is one of the most common summer service calls, but nearly every cause is fixable in under 30 minutes with basic tools. A window air conditioner leaking water indoors signals an installation problem, a drainage issue, or a maintenance failure, all of which a homeowner can diagnose and fix without calling an HVAC tech. The same leak outside the window is usually normal condensate discharge and not a problem at all. This guide walks through the seven most common causes, how to identify each one, and the specific repair steps that resolve them.
Part of our Windows & Doors guide — explore the full guide →
- Indoor Versus Outdoor Leaks: Identifying the Direction
- Cause 1: Improper Tilt Angle
- Cause 2: Clogged Drain Hole or Pan
- Cause 3: Frozen Evaporator Coils
- Cause 4: Damaged or Missing Seals
- Cause 5: Excessive Humidity and Undersized Unit
- Cause 6: Clogged Filter and Dirty Coils
- Cause 7: Cracked Drain Pan or Internal Damage
Indoor Versus Outdoor Leaks: Identifying the Direction
The first question is always where the water is coming out. Window ACs produce condensate as they cool indoor air, and that water has to go somewhere. The system is designed to drain it outside through a hole in the rear pan.
- Water dripping from the back of the unit outdoors: normal in humid weather, no action needed
- Water on the indoor windowsill or floor: indicates a real problem requiring repair
- Water down the interior wall below the unit: usually a tilt or seal issue
- Ice forming on the indoor coils: frozen evaporator, separate problem
- Pooling on the AC top from outside rain: missing or damaged rain cover
If the leak is outdoors and the volume seems excessive, check the local humidity. A 10,000 BTU unit running in 80 percent humidity can produce 1-3 gallons of condensate per day, all of which exits the back of the unit. That is normal operation, not a malfunction.
Cause 1: Improper Tilt Angle
The most common indoor leak comes from a unit installed level or tilted forward instead of slightly back. Window ACs need a 1/4 to 1/2 inch tilt toward the outside so condensate flows to the rear pan and drains out. A unit installed perfectly level lets water pool in the front pan and overflow into the room.
Check the tilt with a small bubble level placed front-to-back on top of the unit. If the bubble centers or shifts toward the indoor side, the unit needs adjustment. Two options:
- Loosen the mounting bracket screws and shift the bracket down 1/4-1/2 inch on the outside
- Add thin spacers (cedar shims work well) under the front edge of the unit inside the window frame
Re-check the level after adjustment. A typical 5,000-15,000 BTU window unit should sit with its outside edge 1/4 to 1/2 inch lower than its inside edge. Larger 18,000-25,000 BTU units may need closer to 1/2 inch of tilt because they produce more condensate.
Cause 2: Clogged Drain Hole or Pan
The drain hole in the back pan can clog with leaves, insects, mineral buildup, or biofilm. When the drain backs up, water collects in the front pan and overflows. Unplug the unit, pull it from the window, and inspect the rear pan.
Clear the drain hole with a soft pipe cleaner, a small zip tie, or compressed air. Avoid sharp metal tools that can punch through the pan. After clearing the hole, flush the entire pan with warm water and a tablespoon of bleach in a spray bottle to kill algae and slime. Some units have a removable drain plug for winterizing; make sure it is installed for summer operation.
Cause 3: Frozen Evaporator Coils
If the indoor coil ices over, melting ice exceeds the drain capacity and overflows into the room when the unit cycles off. Frozen coils have several possible causes:
- Dirty air filter restricting airflow across the coil
- Low refrigerant from a slow leak (requires HVAC tech)
- Outdoor temperatures below 60°F overnight while running
- Fan motor running too slowly
- Blocked supply vents or grilles
To diagnose, turn the unit off and let it thaw for 4-6 hours. Remove and clean the filter (most window AC filters are washable; rinse and air dry). Run the unit on fan-only mode for an hour, then switch to cool. If the coil freezes again within a few hours, you likely have low refrigerant and need professional service. Refrigerant leaks in window units are rarely worth repairing because parts and labor often exceed the cost of a replacement unit.
Cause 4: Damaged or Missing Seals
Window units rely on foam seals at the top, bottom, and sides to prevent outdoor rain from blowing into the gap between the unit and the window frame. Old or compressed seals let rainwater enter and run down the interior wall.
Replacement foam weatherstrip costs about $8 at any hardware store. Apply it around all four sides of the unit where it meets the window frame, paying particular attention to the underside where water tends to wick. The side accordion panels also need to seal tightly against the window frame; replace any cracked plastic side panels with new accordion sections from the manufacturer or use a generic AC side panel kit for $15-25.
Cause 5: Excessive Humidity and Undersized Unit
An undersized AC running constantly in high humidity produces more condensate than the drain pan can handle. Symptoms include a unit that never cycles off, water overflow during the most humid hours, and a room that stays warm despite the AC running flat-out.
Calculate the proper BTU rating for the room. The rule of thumb is 20 BTU per square foot, with adjustments:
- Add 10 percent for sunny rooms
- Add 600 BTU for each person over two in the room
- Add 4,000 BTU if the room is a kitchen
- Subtract 10 percent for heavily shaded rooms
A 250 square foot bedroom needs 5,000-6,000 BTU. A 400 square foot living room with two sunny windows needs 9,000-10,000 BTU. Running an oversized unit causes a different problem (short-cycling without dehumidifying), so size as accurately as you can.
Cause 6: Clogged Filter and Dirty Coils
Dirty coils trap moisture instead of letting it drip cleanly off into the pan. Pull the front grille and inspect the evaporator coil. A coil coated in dust, pet hair, or kitchen grease cannot shed water properly.
Clean the coil with a no-rinse coil cleaner (Nu-Calgon, Frost King, around $10), applied with the unit unplugged and partially pulled from the window. Spray the foam cleaner directly onto the coil surface, let it sit for 5-10 minutes, and rinse with a gentle stream of water from a spray bottle. The condensate drain carries the dirty water out the back. Reinstall the filter only after it has fully dried. A clean coil also improves cooling efficiency by 15-25 percent compared to a heavily fouled coil.
Cause 7: Cracked Drain Pan or Internal Damage
The least common but most serious cause is a cracked drain pan or damaged internal tubing. Drop damage during installation, freeze damage from running the unit below 60°F outdoor temperature, or simple age can crack the plastic pan and allow water to escape inside the cabinet.
Look for visible cracks by removing the front grille and shining a flashlight on the pan from above. A crack longer than 1 inch usually cannot be reliably repaired. JB Weld or marine epoxy can patch small cracks under 1/2 inch on a thoroughly dried pan, but the fix often fails within 1-2 cooling seasons. For a unit over 5-7 years old with a cracked pan, replacement is almost always more economical than repair. A new 10,000-12,000 BTU window unit runs $250-450 and lasts 8-12 years with basic maintenance. Diagnosing a window air conditioner leaking water issue early prevents the water damage and mold problems that can cost thousands of dollars in repairs to drywall, flooring, and trim.
