The end-of-season routine for a window air conditioner may need one more step before you pack the unit away.
An expert warned that buildup in the drain area of some window AC units can sit in standing water for months during the summer, with air then moving across it and into the room. If not taken care of properly, you could be looking at a mold-covered unit next spring.
Here's what to know
In a TikTok post, Keystone Mitigation Services (@KeystoneMitigationServices) pointed to the back of a window AC unit and explained how moisture can linger inside after months of use.
"That thing's been pulling water out of your air all summer," the host said. "Some of it drains out the back. The rest sits in a tray inside the unit."
Dark, damp spaces can become a breeding ground for grime, mildew, mold, or other biofilm-like buildup.
"Every time the fan kicked on, it pulled air right across that into the room," the expert said.
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The video also highlighted unit angle as something homeowners may overlook. If the AC is tilted the wrong way, water can "drip inside instead of out," which can leave unwanted moisture behind.
Commenters also noted that not every model is designed the same way.
One wrote, "Yeah I've lived in Maine all my life and all the window units sold here do not have any kind of tray that pulls out."
More background
Window air conditioners naturally pull humidity from indoor air as they cool, creating condensation. In a properly installed unit, much of that water drains away. However, trapped moisture can still collect in pans, channels, or internal compartments, especially if drainage is blocked or the unit is not pitched correctly.
Repeated exposure to moisture can even affect nearby walls or window frames.
"Feel your wall underneath," the expert told viewers. "It could be soft."
Storing the unit before it has fully dried can also allow any existing buildup to keep spreading in the dark over the colder months.
What can be done?
The expert recommended emptying the tray, cleaning the area, and leaving the AC sitting in the sun until it is thoroughly dry before putting it away for storage. That extra drying time may help prevent growth from continuing.
People should still check their owner's manual before taking anything apart, as some units do not have removable trays and others may require partial disassembly for safe cleaning. If there is visible buildup near electrical components, use caution — or call a professional.
One commenter shared a DIY drainage fix, writing, "I always drill a hole on the bottom of the tray. Let it drain."
Still, handle modifications like that carefully, since altering a unit could affect its performance or warranty.
"Because if you store it wet, it doesn't stop," the expert said. "It grows in the box all winter. And next June, the first thing that blows into your bedroom is whatever grew in there."
Where can I learn more?
With a window AC, the bigger issue can be the damp buildup that sticks around after the water collects. If you neglect proper cleaning, the situation can quickly get worse.
• A homeowner said using an AC's dry mode to curb humidity can help stop mold issues.
• Cleaning pros at NovaCleaners warned some mold removal habits can backfire instead of fixing the problem.
• Indoor air quality monitors can flag pollution spikes at home before smells or irritation become obvious.
Drying, cleaning, and checking airflow around a window unit are important steps before stashing it away.
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