Window AC Leaking Water Inside? Tilt, Drain Holes and Pan Fixes

Why a window AC makes water

Every air conditioner removes moisture as it cools. Water condenses on the cold indoor coil, drips into a base pan, and flows back toward the outdoor side. Many units then pick up some of that water with a slinger ring on the condenser fan and fling it onto the hot outdoor coil, where it evaporates and helps cooling. Whatever is left drains out of the back of the unit. On a humid day a typical window unit can produce several gallons of water, so a small fault in the drainage path quickly shows up as a puddle on the sill or floor.

A steady drip outside is normal. Water inside the room, running down the wall or onto the sill, is not.

Causes, ranked

1. Wrong tilt. If the unit is level or tilts into the room, condensate runs forward over the front lip of the pan. Most manufacturers call for a slight backward slope of roughly 1/4 to 1/2 in (6–13 mm) over the depth of the unit; check your manual.
2. Clogged drain channel or base pan. Dust, algae slime, leaves and insect debris block the passage between the indoor and outdoor sections.
3. Blocked drain hole or plug left in. Some units ship with a rubber drain plug, or have a drain port you can open; others depend on the slinger. A blocked opening lets the pan overflow.
4. Frozen coil thawing. Ice built up on a dirty-filter coil melts all at once and overwhelms the pan. See window AC freezing up.
5. Poor seals. Gaps around the unit let warm, humid outdoor air hit the cold front panel, so condensation forms on the outside of the case. Rain can also blow in through unsealed gaps.
6. Cracked base pan on an older unit.

Step-by-step diagnosis and fixes

1. Check the tilt. Put a short spirit level front-to-back on top of the unit. The bubble should show a slight drop toward the outside. Adjust with the support bracket, a shim under the outdoor side of the bracket, or the mounting rail, following the window AC installation guide. Never tilt it so much that the unit is unstable.
2. Clean the filter. A clean filter keeps the coil from icing. Rinse and dry it every two weeks in summer.
3. Look for ice. If the front coil is frosted, switch to fan-only and thaw before going further.
4. Clear the drain path. Unplug the unit. Remove the front grille and look along the bottom of the indoor section for a channel or opening leading to the back. Clear it with a pipe cleaner or a length of stiff wire, then flush with a cup of water mixed with a little dish soap. A wet/dry vacuum on the outdoor drain hole can pull out stubborn clogs.
5. Check drain holes and plugs. Read the manual to see whether your model expects the drain plug in or out. Some units should keep it in during cooling so the slinger can use the water; others should have it removed in humid weather or when using a drain hose.
6. Seal the gaps. Refit side panels and add foam weatherstrip between the window sash and the top of the unit and around the side curtains.
7. Clean the base pan thoroughly. For heavy slime, the unit has to come out of the window and the case has to be removed. This is a two-person lift, and the job involves the unit's wiring and sharp fins. Many owners prefer a service shop for it.
8. Replace if the pan is cracked or the unit is old enough that a deep clean costs more than it is worth.

Water near outlets and cords is a shock risk. Unplug the unit before touching wet areas, and dry the outlet area before plugging back in. If water has reached the receptacle, have it checked.

Leaking outside onto people or the building below

In apartments, drips onto balconies or the sidewalk below are a common complaint even when the unit is working correctly. Options include a drain hose fitted to the unit's drain port and run down to a safe spot, a small drip tray, or a model with a full slinger design that evaporates most of the water. Check your building's rules.

Prevention

• Set the correct backward tilt at installation and recheck it each spring.
• Clean the filter every two weeks and the coils every season.
• Flush the drain path at the start and end of each season.
• Seal gaps around the unit to keep rain and humid air out.
• If the room is very humid, consider whether the unit is oversized, since short cycles remove less moisture; see AC not removing humidity.

Frequently asked questions