Window AC leaking water inside
A window air conditioner pulls water out of the air as it cools, and that water is supposed to run to the back of the unit, outside. When it drips into the room instead, the unit is usually tilted the wrong way, the water channel in its base is clogged, or the coil iced over and is melting faster than it can drain. Most of these fixes need only a level, a soft brush and an hour. Because these units are heavy and sit in an open window, anything that means pulling the unit out is a job for two people.
Where the water in a window AC goes
The coil behind the front filter gets cold, and moisture in the room air condenses on it, the same way a cold glass sweats. That water drips into the base pan, a shallow tray that runs under the whole unit. Channels in the pan carry it past the divider wall to the back half of the unit, outdoors. Many units have a slinger ring on the fan blade that picks up some of that water and throws it onto the hot outdoor coil, where it evaporates, which helps efficiency. Extra water drains out the back or over the sides of the pan outside.
All of this depends on gravity. Most manufacturers tell you to install the unit with a slight tilt toward the outside, often about 1/4 to 1/2 inch or a few degrees, but check your manual, since some newer units are made to sit level. If the unit sits level when it should tilt, or tips toward the room, water collects at the front and spills over. If the channels clog with dirt, algae or debris, water backs up. And if the coil freezes because of a dirty filter, low airflow or cool weather, it melts all at once and floods the front of the pan.
Why a window AC leaks inside
The unit sits level or tips toward the room, so water runs forward.
Dirt, algae or debris blocks the path to the back of the pan.
Low airflow lets the coil freeze, then the ice melts into the room.
Below about 60°F outside, many units ice up on cool settings.
Rain or outdoor condensation gets in around the unit.
A coil that keeps icing with a clean filter needs a technician.
| You notice | Likely cause | Start at |
|---|---|---|
| Water drips from the front grille | Tilt is wrong | Step 2 |
| Filter is gray and dusty | Low airflow | Step 3 |
| Frost or ice on the coil | Coil froze | Step 4 |
| Tilt is fine, filter clean, still drips | Clogged base pan channel | Step 5 |
| Leaks only when it rains | Gaps or worn foam seals | Step 6 |
| Not sure where it's coming from | Find the source | Step 1 |
Let's fix it
- Unplug it and find where the water comes outTurn the unit off and unplug it. Dry the area with a towel and put a dry towel on the sill under the front of the unit. Look closely. Water running out from under the front grille points to the tilt or a clogged pan. Water dripping from the top or sides of the window frame points to gaps or rain, and water only after long runs on a cool day points to ice. If the cord or plug ever got wet, let everything dry completely before you plug it back in, and stop using it if the plug or outlet looks scorched, or if the plug's test/reset button won't reset.
- Check the tiltSet a short level on top of the unit, running front to back, or across the side of the cabinet. Check the owner's manual for the recommended slope. Many call for the back (outdoor) side to sit slightly lower, about 1/4 to 1/2 inch, though some newer models are designed to sit level. If the unit is level when it should tilt, or tips toward the room, adjust it. On units with a support bracket outside, change the bracket's leveling bolt. Otherwise add a thin wood shim under the front of the unit on the inside sill. Make sure the unit is still firmly held by the sash and bracket afterward. Never tilt it so far that it could slide, and don't tilt it side to side.
- Clean the filterPull off the front grille or open the filter door (most clip or slide out without tools) and remove the filter. Hold it up to a light. If it's gray or you can't see through it, rinse it under warm water from the back side, the side that faced the coil, so the dust washes out the way it came in. A drop of dish soap helps with greasy kitchen dust. Shake it off and let it air dry completely before putting it back. Never run the unit without the filter. Most manufacturers suggest checking it every two weeks during the cooling season.
- Let an iced coil thawWith the filter out, look at the coil behind it with a flashlight. If you see frost or ice, leave the unit off, or run it on fan only, until it's completely melted, usually one to a few hours. Keep towels under the front, since a lot of water comes off at once. Then figure out why it froze. A dirty filter is the usual cause. Running it on the coldest setting when it's cool outside (many units start to ice below about 60°F outdoors), or with the fan on low in a humid room, can also do it. Use a moderate setting and a higher fan speed. If it ices again with a clean filter and normal weather, it may be low on refrigerant, which needs a technician.
- Clear the drain channel in the base panWith the unit unplugged and the grille off, shine a light into the bottom of the cabinet under the front coil. You'll see the base pan and a channel leading under the divider wall to the back. Clear debris and sludge with a pipe cleaner, bottle brush or wet/dry vacuum. Wear gloves and keep your hands off the sharp coil fins. Wear an N95 mask for heavy mold. Pour a cup of water slowly into the front of the pan and watch it flow toward the back. If it pools at the front, the path is still blocked. Some units have a drain plug on the back; check your manual before removing it, since many are designed to keep a little water in the pan. If the pan is caked with grime you can't reach, the chassis has to come out, a two-person job or one for a technician.
- Reseal the gaps and test itCheck the foam strips on top of the unit and the accordion side panels. If the foam is crushed, crumbling or missing, replace it with window AC foam seal or adhesive weatherstrip so rain and outside air can't get in around the unit. Close any gaps at the sides with the side panels or foam, and make sure the sash rests down on the unit's top rail or foam. Put the filter and grille back, plug it in, and run it on a moderate setting for an hour with a dry paper towel on the sill. The towel should stay dry. Outside, you may see water dripping from the back or hear the fan slinging water, which is normal.
Watch it done
- The sill and floor stay dry after an hour of cooling
- No ice on the coil behind the filter
- Water drains or drips only on the outside
- No water around the unit after rain
Still dripping, or the coil keeps icing? Call an HVAC or appliance tech.
If it comes back
If the leak returns after a few weeks, check the filter first. In dusty or pet-heavy homes it can clog in as little as two weeks during heavy use. Then check that the unit hasn't settled level, which happens when the sash or a wood sill shifts.
If the room also has water running down the inside of the glass, that's condensation on the window rather than the AC leaking. See the guide for condensation on windows. For drafts around the unit in fall, see how to stop drafts around windows.
If your home has central air and the leak is near the indoor unit or furnace, the condensate drain line is the usual cause. See the guide for a clogged AC condensate drain or, for ice on the lines, the guide for a frozen AC.
What a pro typically charges (a rough estimate; prices vary by area): A service call for a window unit typically runs about $100–$250. For an older or smaller unit, replacing it often costs less than a refrigerant repair.
Find an AC repair tech near you