8 Air Conditioner Problems and How to Fix Them

Top reasons your unit isn't keeping you cool

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close up of AC vent on light blue wall Photo: iStock

Like any appliance, air conditioners need a little TLC to deliver optimal performance. And it’s the ACs that haven’t been kept in top-notch condition that tend to struggle most.

You don’t need to lose your cool because your AC goes on the blink. We asked air conditioner pros and Consumer Reports test engineers to identify the most common problems that crop up with window and central air conditioners. They highlighted eight to troubleshoot. Plus, you’ll find our list of top air conditioners for small, medium, and large rooms.

Here are eight questions to ask that will help you keep your AC at peak performance. As it turns out, the No. 1 problem affects both window units and central AC systems, and—we’re happy to report—it’s the easiest to fix.

More on Air Conditioning

1. Is the filter dirty? No matter which type of air conditioner you have, a clogged filter restricts airflow, decreasing efficiency and reducing the unit’s ability to cool the air effectively. If you haven’t cleaned the filter in your window or portable air conditioner recently, make sure you do and then check it periodically. For central AC systems, check the filter manufacturer’s recommendation for when to swap in a new one. In CR’s tests, HVAC (heating, ventilation, and air conditioning) filters last three to 12 months.

Just keep in mind that if your central air system is running constantly or if you have pets, you’ll need to change your filters more often. “When we get calls about central air units that aren’t working well, nine times out of 10 it’s because of a dirty filter,” says Stewart Unsdorfer, the owner of Central Heating & Air Conditioning in Cleveland.

Window AC Checklist

2. Is it too sunny inside? If your AC is in a window that gets direct sunlight in the heat of the day, it will have to work harder to cool your space. If you have a choice, move it to a shadier spot. If not, keep your shades and curtains drawn during the day to block the sun and prevent it from heating up your house.

3. Is warm air leaking in? Check the seals around your window AC to make sure hot air isn’t getting in and cold air isn’t seeping out. Reseal around your unit with weather stripping if necessary.

“Most new window units come with insulation panels to place over the plastic adjustable side panels,” says Chris Regan, CR’s senior air conditioner tester. “But to maximize efficiency, you’ll still need to use weather stripping around the perimeter of the unit. And always use the manufacturer’s installation and safety hardware.”

4. Is your window AC vibrating? Air conditioners make plenty of noise, including the whir of the fan and the sound of the compressor cycling on and off. But if you hear a vibrating noise, it may mean that your unit was installed incorrectly. Make sure it’s sitting securely in the window, and review the installation instructions in your owner’s manual to confirm that no steps were missed.

Central AC Checklist

5. Is the thermostat on the right setting? With a smart or programmable thermostat, you can control the temperature in your home more easily so that it’s comfortable when you’re there and a bit warmer when you’re not. If the temperature seems off, make sure the thermostat isn’t exposed to direct sunlight, which may cause it to register the wrong temperature.

6. Are the registers dirty or blocked? If you have a forced-air heating and cooling system, regularly vacuum the registers to remove any dust buildup. Make sure that furniture and other objects aren’t blocking the airflow through your registers.

7. Are plants crowding the compressor? The outdoor compressor for central air needs adequate airflow to work correctly, so make sure there’s at least 2 to 3 feet of space between it and any plants or structures. There should also be 5 feet of clearance between the top of the unit and any trees above.

8. Did you remember the annual checkup? An inspection by an HVAC technician can catch any serious problems before they get worse and leave you hot and bothered. The technician will check all the moving parts as well as the refrigerant, and recharge the system if necessary.

Need a New AC?

If your room air conditioner is beyond repair, you can find one that did well in our tests for about $200 for a small unit and upward of $800 for a large one. Make sure you match the size of the air conditioner to the room it’s meant to cool. Below, we list models in each size that performed well in CR’s tests. You can find even more in our full air conditioner ratings.

ACs for Small Rooms (100 to 300 square feet)

For a small bedroom, home office, or guest room, look for an air conditioner with a capacity of 5,000 to 6,500 Btu. You’ll want one with good scores for comfort and noise. Here are three top picks to consider.

ACs for Midsized Rooms (250 to 400 square feet)

To cool a bigger or busier room, you’ll want to step up to a midsized air conditioner rated 7,000 to 8,500 Btu. If it’s for a bedroom, heed our noise scores. Here are three top picks.

ACs for Large Rooms (350 to 650 square feet)

For a living room or family room, you’ll want a large air conditioner, 9,800 to 12,500 Btu, especially if you have an open floor plan. Here are three good choices.

Maintaining Your Air Conditioner

We rely on window air conditioners to keep us cool when temperatures climb. On the “Consumer 101” TV show, Consumer Reports expert John Galeotafiore explains to host Jack Rico how to maintain a window AC unit to ensure cool, clean air throughout the summer.

Mary H.J. Farrell

Knowing that I wanted to be a journalist from a young age, I decided to spiff up my byline by adding the middle initials "H.J." A veteran of online and print journalism, I've worked at People, MSNBC, Ladies’ Home Journal, Good Housekeeping, and an online Consumer Reports wannabe. But the real thing is so much better. Follow me on Twitter.