An a/c register.

Of course it’s on the hottest days that you notice your air conditioner is struggling. And it’s not always the equipment's fault. A poorly maintained system can perform poorly, failing to keep you cool while costing you more to run.

Here are eight reasons your air conditioner might not be working as well as it used to, from the experts at Consumer Reports.

Common Air-Conditioner Problems

Your filter is dirty. A clogged filter restricts airflow through the unit, decreasing its efficiency and reducing the ability to effectively cool the air. If you haven’t cleaned the filter in your room air conditioner recently, do it now. Filters on central AC units should be changed at least once a month, especially if your system is running constantly or if you have pets.

Warm air is leaking in. Check the seal around your window unit to make sure hot air isn’t getting in (or cold air isn't seeping out). If so, reseal around your unit with weatherstripping.

“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 weatherstripping around the perimeter of the unit. And always use the manufacturer’s installation and safety hardware.”

More on Air Conditioners & Keeping Cool

The TV is too close to the AC. Avoid placing lamps or TV sets near the thermostat that controls your central air or near a window unit. The thermostat senses heat from these appliances, which can cause the air conditioner to run longer than necessary.

The registers are 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.

The thermostat is on the wrong setting. If you have a programmable thermostat, make sure you programmed it so your home is at a comfortable temperature when you’re there and a bit higher when you’re not. Keep in mind that for every degree you raise the temperature, you’ll save 3 percent on your air-conditioning costs.

Window units, of course, don’t offer the whole-house control of a central system. If you’re depending on one window unit to cool a large space, you’ll have to experiment with the thermostat setting because the temperature will vary depending on where you are in the room.

It’s too sunny inside. If your room AC unit is in a sunny window, it’ll have to work harder to cool your space. And if the thermostat for your central system is in a sunny spot, it will register the wrong temperature. Your best bet? Keep your shades and curtains drawn all day during the heat of the summer.

Plants are 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 the unit and any plants or structures. There should also be 5 feet of clearance between the top of the unit and any trees above.

You forgot an annual checkup. An inspection by an HVAC technician can catch any serious problems before they get worse and leave you hot and bothered at the peak of summer. 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 as little as $140 for a small unit and as much as $370 for a large one. Below, models in each size that performed well in CR's tests. You can find more in our full air-conditioner ratings and recommendations.

Small (5,000 to 6,500 Btu/hr.)
• Kenmore 77060, $220
• SPT WA-6022S, $200

Medium (7,000 to 8,500 Btu/hr.)
• Friedrich Kuhl SQO8N10D, $710
• Kenmore 77080, $270

Large (9,800 to 12,500 Btu/hr.)
• Haier HWE12XCR, $320
• LG LW1216ER, $350

The stores are still stocked with room units, but getting a central air system at this time of year may be trickier because installers tend to be busy. Check out the most reliable central air systems, according to our members.