Short answer
Set it to "Cool", not "Fan" or "Dry", a few degrees below the room temperature. Unplug it, slide out the front filter, rinse it and let it dry. Seal the gaps around the side panels and close the blinds on that window. Plug it straight into the wall and press "Reset" on the plug if it has one. Ice on the coil? Run "Fan" for an hour. Still warm? Call a technician.
Safety first
Unplug the unit before you clean it. Plug it straight into a wall outlet, never an extension cord or power strip. Do not take a window unit out of an upper-floor window by yourself: it is heavy and can fall.
How to check a window AC that is not cooling
1Running, but the room stays warm
It hums and blows, but the air is not cold. These checks take about 20 minutes and fix many window units.
Read all the steps
- Running, but the room stays warm. It hums and blows, but the air is not cold. These checks take about 20 minutes and fix many window units.
- Cool mode, a few degrees lower. Make sure it is on "Cool", not "Fan", "Dry" or "Eco". Set it a few degrees below the room temperature.
- Unplug it and wash the filter. Slide the filter out of the front. Rinse it with warm water and a little dish soap, and let it dry completely.
- Seal the gaps around the sides. Hot air sneaks in around the side panels and the window sash. Close the gaps with foam strips and close the blinds on that window.
- Plug it in and press Reset. Plug it straight into the wall. If the plug has "Test" and "Reset" buttons, press "Reset". Give it 15 minutes on "Cool".
- Still not fixed? Call an HVAC contractor Especially if: it still blows warm air with a clean filter and the right mode.
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Ice on the front coil
Frost on the coil behind the filter means poor airflow or a very cold setting on a cool night. Switch to "Fan" for an hour or two so it melts, wash the filter and raise the setting a few degrees. If it frosts again with a clean filter, the unit may be low on refrigerant. Window units are sealed, so ask a technician whether it is worth repairing.
Is it big enough for the room?
A unit that is too small runs nonstop and never catches up. Compare the BTU number on its label with the room size chart in the manual or on the box. Sunny rooms, kitchens and rooms with several people need more. A big or open room may need a larger unit or a second one.
What not to do
- Do not use an extension cord or a power strip.
- Do not spray water into the controls or the back of the unit.
- Do not block the outside part with plants or covers: it needs to push the heat out.
- Do not tilt it toward the room. It should lean slightly to the outside so the water drains.
When to call an HVAC technician
- It still blows warm air with a clean filter and the right mode.
- Ice comes back on the coil with a clean filter.
- It buzzes or clicks, but the compressor never starts.
- The plug trips again right away, feels hot or the cord looks damaged.
Frequently asked questions
Why is my window AC running but not cooling?
Most often a dirty filter, the wrong mode, a setting too close to the room temperature or gaps that let hot air in. A frosted coil or low refrigerant can also cause it.
Where is the reset button on a window AC?
On most units it is on the plug, next to a "Test" button. Press "Reset" firmly. If it trips again right away, unplug the unit and have it checked.
How do I clean a window AC filter?
Unplug the unit and slide the filter out of the front. Rinse it with warm water and a little dish soap, and let it dry completely before you put it back. Do it every couple of weeks in summer.
Why is my window AC freezing up?
A dirty filter, a very low setting on a cool night or low refrigerant. Run it on "Fan" until the ice melts, clean the filter and set it a few degrees higher.
Can I recharge a window AC myself?
No. Window units are sealed, and handling refrigerant requires certification. Low refrigerant means a leak, and a technician can tell you if it is worth fixing.
Can I plug a window AC into an extension cord?
No. Plug it straight into a wall outlet that can handle it. Extension cords and power strips can overheat and start a fire.