You expect a window AC unit to drip somewhere. That’s normal. What’s not normal is coming home to a puddle on your windowsill, water running down the wall, or your carpet squishing under the unit like a bad water balloon. If your AC is leaking inward instead of dumping condensate outside where it belongs, something’s off, and it’s usually one of a handful of fixable problems.
It’s Probably the Tilt, Honestly
Window units are designed to sit tilted slightly toward the outside, usually about a quarter to half an inch lower on the outdoor side. That tiny slope is doing all the work of directing condensate away from your living room and out onto the ground or your window ledge. If the unit is level, or worse, tilted the wrong way, all that water has nowhere to go but back inside.
This happens more than you’d think, especially with units installed in a hurry during the first heat wave of the season. A cheap torpedo level takes the guesswork out of it. Check the tilt before you assume something’s broken, because a five-minute adjustment with some shims fixes a huge chunk of these complaints.
A Frozen Coil Is Also a Common Culprit
If your filter is caked with dust, or the unit’s been running nonstop trying to fight off a heat wave, the evaporator coil can ice over. That ice eventually melts, and it melts faster than the drain pan and tray were ever designed to handle, so it overflows inside the housing and drips into your room instead of routing outside.
The fix here isn’t glamorous: pull the filter, wash it or replace it, and let the unit run on fan-only mode for a couple hours to fully thaw before switching back to cooling. Do this monthly during peak summer use and you’ll avoid the whole mess. A clogged filter also just makes your unit work harder and burn more electricity, so keeping it clean pays off twice.
Clogged Drain Holes and a Dirty Base Pan
Most window units have small weep holes or a drain channel on the outdoor-facing side that lets excess condensate escape. Dust, pollen, and general grime build up in there over a season or two and can block the path entirely. Water backs up, finds the path of least resistance, and that path is usually your window frame.
Pull the unit’s front panel (unplug it first, obviously) and take a look at the base pan. A quick rinse and a pipe cleaner or old toothbrush run through the drain openings usually clears it right out.
Sometimes the Unit Is Just Too Small for the Room
An undersized AC unit runs constantly trying to cool a space it’s not rated for, which means it never gets a break to properly manage humidity and condensate the way it’s designed to. If yours is leaking on the worst, muggiest days specifically, oversizing the room might be the real issue rather than anything mechanical. The ENERGY STAR room AC sizing guide is a solid reference if you’re not sure whether your unit is actually matched to your square footage.
Quick Checklist Before You Call Anyone
- Check the tilt with a level, adjust with shims so the outdoor side sits lower
- Clean or replace the filter and let the unit thaw if there’s visible ice
- Clear the drain holes and wipe out the base pan
- Reseal the window with foam insulation strips so nothing’s dripping in through gaps around the unit itself
- Confirm the unit’s BTU rating actually matches your room size
Most of this takes less time than it took to read this post. If you’re already pulling the panel off, it’s worth grabbing a foam insulation kit while you’re at it, since gaps around the unit let in humid outside air that just makes the condensate problem worse. And if you’re shimming for tilt, a basic torpedo level is cheap enough that there’s no excuse to eyeball it.
None of this is exciting work, but a leaking window unit in late July isn’t just annoying, it can warp your windowsill and encourage mold if it goes on long enough. Fifteen minutes now saves you a much bigger repair later.
