Window AC Unit Leaking Water Inside: Causes & Fixes
A window AC unit leaking water inside usually means the condensate drain is clogged, the unit is tilted wrong, or the drain pan is cracked. Most fixes cost under $20 in parts and can be completed in under an hour. Addressing this quickly prevents water damage to your window sill and interior walls.
Step-by-Step Fix
- 1
Unplug the unit
Disconnect the AC from the outlet before any work. This keeps you safe from electrical shock and prevents the compressor from running while you troubleshoot.
- 2
Check the tilt and seal
Remove the unit from the window and inspect the front edge—it should slope slightly downward toward the outside. Reinstall it with a 1/4-inch downward tilt using shims or brackets. Verify the window seals are tight; use weatherstripping foam tape ($3–5) to close gaps where water can seep inside.
- 3
Clean the condensate drain
Locate the small drain hole on the back or bottom of the unit. Use a thin wire, pipe cleaner, or turkey baster to clear algae and dust buildup blocking the line. Flush the drain with warm water to confirm water flows freely out the back of the unit.
- 4
Inspect and clean the drain pan
Pull out the removable drain pan at the bottom of the chassis and rinse it thoroughly with warm water. Look for cracks, rust, or mold; if the pan is damaged, purchase a replacement pan kit ($15–30) matching your unit model.
- 5
Check the evaporator coils
Look at the front intake fins and coils through the filter area. If they are visibly clogged with dust, carefully vacuum them or spray with a commercial AC coil cleaner ($8–12) to restore drainage. Dirty coils reduce cooling and trap water.
- 6
Plug in and test
Reinstall the unit, plug it in, and run it on cool mode for 10–15 minutes. Place a cup under the outdoor drain outlet to verify water exits outside, not inside. If water still pools indoors, the compressor may be faulty and require professional replacement.
⚠️ Safety Notes
- Always unplug the AC unit before opening the casing or touching internal parts to avoid electrical injury.
- Do not force the drain or insert sharp objects that could puncture the aluminum fins or plastic tubing.
- If water is pooling inside your wall or drywall is soft, stop and call a professional—mold and structural damage may already be present.
When to Call a Pro
If cleaning the drain and resealing the unit does not stop the leak, or if water is seeping into your walls or ceiling, call an HVAC technician to check the compressor and refrigerant levels; internal leaks require professional diagnosis and repair.
Frequently Asked Questions
- Why is my window AC leaking water inside instead of outside?
- The unit is tilted backward instead of forward, the drain line is clogged with dust or algae, or the seal between the unit and window frame is loose. All three issues prevent condensate from draining outdoors.
- How do I fix a clogged AC drain line?
- Use a thin wire, pipe cleaner, or compressed air to clear the drain hole on the back of the unit, then flush it with warm water until water flows freely. This usually costs nothing and takes 5–10 minutes.
- Should I tilt my window AC unit toward the inside or outside?
- Tilt it slightly downward toward the outside (rear) of the window at about 1/4 inch per foot of depth. This gravity-fed slope ensures condensate drains away from your home, not toward it.
- Can a cracked drain pan cause indoor water leaks?
- Yes, a cracked or rusted drain pan will leak collected condensate onto your window sill and interior wall. Replacement pans cost $15–30 and are specific to your AC model.
- How often should I clean my window AC unit to prevent leaks?
- Clean the filter, drain pan, and drain line every 2–4 weeks during cooling season, especially if you live in a humid area or near dust sources. Regular maintenance prevents clogs and water damage.
Not sure which step applies to your situation?
Describe your specific problem to ProAssist and get a tailored diagnosis — free, no sign-up required.
Diagnose My HVAC Issue →