11 Air Conditioner Blunders to Avoid on Hot Summer Days


Hot, wet, and muggy—truly the worst kind of summer weather to endure. It’s even worse when you turn to your air conditioner for deliverance, only to find that it isn’t cooling. Or maybe it is, but you’re worried that your energy bill will take a huge hit.

Fear not! We’ll walk you through several common air conditioning mistakes that you might be making at home. Fixing these issues will result in faster cooling, greater comfort, less wasted energy, and smaller cooling bills.

Mistake 1: Using a Wrongly Sized AC Unit

The worst mistake you can make happens before you even turn your AC on for the first time. All AC window units are designed with a particular amount of “air space” in mind, and your unit won’t be effective if your room is too big or too small for the unit.

All AC window units have a British Thermal Unit (BTU) rating, typically in the range of 5,000 to 15,000. The rule of thumb is that every square foot of room space needs 20 BTU, plus 600 BTU for every person that regularly occupies the room. You may need to increase it by up to an additional 10 percent if the room has high ceilings, receives a lot of sunlight, etc.

And don’t buy an AC unit that’s too big either. An oversized AC unit will pump out a lot of cool air but won’t stay on long enough to dehumidify the room. The lingering humidity will make the room feel muggy and warmer than it actually is.

Mistake 2: Opening Your Windows and Doors

Here’s a quick rundown of how air conditioners work: The AC unit pulls in warm air from the room; the warm air passes over a refrigerant that absorbs the heat from that air; the now-cooled air is blown back into the room while the absorbed heat is expelled out the back of the unit.

This is why window AC units need to be mounted in a closed window (or piped outside) and why central AC units are located outside of the house. The “inside air” and the “outside air” need to be isolated from each other in order for the air conditioning to work. If your windows or doors are open, the heat that gets expelled is just going to come right back in.

best value window air conditioner units
Image Credit: Pbroks13/Wikimedia

Mistake 3: Keeping the AC on All the Time

Let’s say you leave for work at 8am and come home at 5pm. Your ideal home temperature is 72F. Is it more energy-efficient (i.e. bill-friendly) to leave your AC unit on at 72F all day long? Or should you turn it off as you leave and turn it back on when you return?

Most people think the first method is better, but actually the second one uses less energy—and the savings can be quite significant.

The hotter it is outside, the more you’ll save by using the second method. To learn more about why this is, check out our post on how to optimally set your thermostat

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Mistake 4: Setting the Temperature Too Low

After coming home from a long day at work, you step in only to be slammed by a thick wall of stale 90F air. So what do you do? You might crank your AC way down to 65F so the room will cool faster.

But that’s not how air conditioning works.

Whether your room is 70F or 90F, the AC unit pumps out the same “strength” of cool air. The temperature setting only tells the unit when to stop pumping out cool air—changing the setting from 70F to 65F won’t speed anything up.

It’s actually worse because the AC unit will keep working even after it reaches your ideal temperature! For example, if your ideal temperature is 72F and you set the unit for 65F, it will keep going until the room hits 65F, at which point you’re now too cold and have unnecessarily wasted a lot of energy.

So, set your AC unit to your ideal temperature and let it do the work. If you’re tired of coming home to a hot and stuffy house, consider getting a smart or programmable thermostat

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Mistake 5: Leaving Rooms and AC Vents Open

If You Have a Window AC Unit

Remember how window units are designed with a certain amount of air space in mind? That air space assumes that doors are closed.

For example, your bedroom might be 150 square feet, but if you leave the door open while the unit runs, the cool air will leak out and warm air will leak in. In effect, the unit is trying to cool the air in your room and outside the room!

To maximize efficiency, keep the doors closed in any room that has a window unit running. This will limit the amount of air that needs to be cooled to that room only, and it will also speed up how quickly the air is cooled.

If You Have a Central AC Unit

You probably have AC vents located in every room of your house. If all of your vents are open, then the central unit is trying to cool every single one of those rooms.

The more air space that needs to be cooled, the longer it takes to cool down all of that air space. This also means your unit will need to stay on longer, which means wasted energy and higher bills.

By closing vents, you cut off rooms from the central unit and reduce the total amount of air that needs to be cooled. This speeds up the cooling of the rooms where vents are open. (You should also keep the doors to such rooms closed.)

best value window air conditioner units
Image Credit: Africa Studio/Shutterstock

Mistake 6: Not Circulating Air With a Fan

Most people use a fan when it’s warm and switch to the AC when it’s hot. But fans and ACs should not be seen as either-or; in fact, ACs work better when combined with fans. (Especially automated ceiling fans!)

Cool air accumulates where it gets blown out. For window AC units, this means the area right in front of the unit is the coolest. For central units, the coolest areas are right around the vents.

In order to cool down the rest of your air space, you either have to wait for thermal transfer—which is painfully slow—or you can force the cooled air to circulate using a fan.

Imagine a drop of blue food coloring in a glass of water. If you wait, it could take hours before the color seeps throughout the water. But if you stir it, the color goes everywhere within seconds.

In addition, the circulation of air helps your sweat to evaporate faster, and this has a cooling effect on your skin. Which means you can actually set your AC to a higher temperature while feeling just as comfortable. This trick can shave significant amounts off your energy bill on hot days.

Mistake 7: Neglecting to Change the AC Filter

No matter how clean your room is, there are always particles floating around in the air that get sucked into the AC unit during operation. To prevent these particles from causing internal problems, all AC units have particle filters.

A clean filter is crucial for optimal AC efficiency. Particle buildup reduces airflow, which forces the unit to work harder to pull in the same amount of air. At worst, a dirty filter can increase your energy bill by up to 15 percent—not to mention the increased likelihood of maintenance issues.

If your AC is on all day every day, change the filter at least once per month. If you use it less often, change the filter once every three months. Some window units have removable filters that you can clean and rinse, otherwise you’ll need to buy replacement filters as necessary.

best value window air conditioner units
Image Credit: Mindy w.m. Chung/Shutterstock

Mistake 8: Forgetting to Clean Between Seasons

Regular operation of an AC unit results in condensation forming inside. This is normal behavior, and well-designed AC units have ways to get rid of that condensation before it causes issues. That’s why you see window AC units dripping all the time.

Normally condensation runs off and drips out on its own and you don’t have to worry about it. But if something goes wrong and the condensation pools, mold and/or bacteria could grow inside.

And since air conditioners work by blowing cool air into the room, that mold and/or bacteria can easily go airborne. This could lead to health issues like asthma, pneumonia, black mold invasion, and even Legionnaire’s disease.

This is why it’s important to clean and service your AC units at the start of every cooling season.

Mistake 9: Putting Off AC Maintenance

Regular maintenance doesn’t just extend your unit’s lifespan. Maintenance keeps your machine operating at tip-top shape, which means faster and more efficient cooling while minimizing energy usage.

But if you can’t afford annual AC checkups, at the very least avoid procrastinating when something does go wrong.

If you hear something weird, like grinding or internal dripping, then get it checked. If something smells off, get it checked. If you see unusual black growths or leaking liquid, get it checked!

It’s better to be safe than sorry, and fixing the problem early is often cheaper than replacing the entire AC unit when it breaks down.

best value window air conditioner units
Image Credit: Andrey_Popov/Shutterstock

Mistake 10: Ignoring Heat Sources

Anything you can do to reduce heat in your room, apartment, or house will go a long way towards making your AC unit more effective and efficient.

  • Replace single-paned windows with double-paned windows.
  • Seal any cracks that might leak in heat around windows.
  • Cover sunlit windows with thick, light-colored curtains. Keep them closed during the hottest hours of the day.
  • If possible, add insulation to sun-facing walls.
  • Avoid CPU-intense activities on your computer or laptop.
  • Avoid using heat-generating devices, like washers and dryers.
  • Limit time spent cooking on the stove or in the oven. To reheat, use the microwave.
  • Run your wrists under cold water whenever you feel hot.

Mistake 11: Expecting Too Much From Your AC

At the end of the day, AC units are not magic bullets. Even if you follow everything above perfectly, there may still be days when you feel hot with the AC running.

On average, your AC unit can bring your indoor temperature down to about 15F lower than the outside temperature. With a powerful and optimized setup, you might be able to bring the temperature down by 20F or 25F. But if it’s 110F outside, there’s virtually no chance of getting your home down to 65F!

Bonus Room AC Tips and Tricks

The Best AC Temperature to Set in the Summer

When talking about “best” temperatures, there are two ways to think of it:

  • The temperature that’s best for comfort and productivity.
    According to a study by Cornell University, the ideal temperature for office work productivity sits somewhere around 77F. According to chief of sleep medicine Ralph Downey III, the ideal temperature for falling and staying asleep is between 65F and 72F, tweaked to the temperature that feels most comfortable to you.
  • The temperature that’s best for balancing comfort and cost.
    Energy Star recommends going no lower than 78F. If that’s too warm for you, drop it one degree at a time until it becomes tolerable. Note that what’s most important is the difference between your AC setting and the outside temperature—the greater the difference, the more it has to work and the more it’ll cost.

When you’re away from home, it’s best to set your AC to around 85F then crank it down to your preferred temperature when you return. Don’t want to come home to a hot house? Get a smart thermostat and turn the AC on remotely once you start heading home.

Note: Most central AC units should not be set below 68F or else they will freeze up, which can damage or destroy the compressor.

How to Cool Down a Room Fast

Let’s say you’ve been away from home and your AC has been off the whole time. It’s only about 80F outside, but your house is sweltering and reaching 90F. Before turning on the AC, there’s something else you should do first.

Open two windows on opposite ends of your house. Mount window fans into each one. Set the first window fan to blow outwards and set the second window fan to blow inwards. This circulation sucks the hot air right out of your house and sends it outside.

Note: If you have multiple floors, the outward-blowing window fan should be on the top floor while the inward-blowing window fan should be on the bottom floor.

Obviously this only works when the inside temperature is hotter than the outside temperature. Learn more about this technique in our article on how to keep your house cool even without an AC

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How to Make Your AC Feel Cooler

Remember, heat is amplified by humidity. When there’s excess moisture in the air, your body’s sweat can’t evaporate as well and you start to feel hotter. That’s why “dry heat” is more tolerable than “wet heat,” even when the dry heat is many degrees hotter.

Does your home have excess humidity? If you live in a wet climate and your house isn’t properly insulated, weather-sealed, and ventilated, then it probably does. That humidity could be causing your house to feel much hotter than it is.

11 Air Conditioner Blunders to Avoid on Hot Summer Days weather heat index chart
Image Credit: National Weather Service

Air conditioners naturally pull moisture out of the air, but if your AC is set to 78F or higher and isn’t running all the time, it could be leaving excess moisture in the air. In that case, a dehumidifier might work wonders for you.

Dehumidifiers are very similar to air conditioners

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, but instead of being optimized for absorbing heat, they’re optimized for absorbing moisture. Running one in tandem with your AC could make your house feel 10F cooler—assuming your house has a humidity problem. Humidity can be a huge problem in the summer months for other reasons too, so consider these health benefits of monitoring home humidity

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Need to Upgrade? Best Value Room AC Units

For Small Rooms (~150 sq. ft.)
Keystone KSTAW05B

best value window air conditioner units

Keystone 5,000 BTU Window AC

Keystone 5,000 BTU Window AC

Buy Now On Amazon

The Keystone KSTAW05B offers a strong value for the price. For a room that’s 150 square feet, you don’t really want to go any lower than 5,000 BTU. If you do, the AC may not be able to adequately cool the air and will end up running all the time.

This particular model has an easy-to-use front panel, and comes with a remote control with remote temperature sensing. This means rather than gauging the temperature around the unit itself, it will keep running if a particular area of the room is still warm.

And not only does it perform well, but the Keystone KSTAW05B is quieter than other units of its size. Of course it still makes noise, but it’s a solid option for bedrooms and offices where less noise is preferred.

For Medium Rooms (~300 sq. ft.)

best value window air conditioner units

LG 12,000 BTU Window AC

LG 12,000 BTU Window AC

Buy Now On Amazon

With its 12,000 BTU rating, the LG LW1216ER can effectively cool rooms up to 400 square feet in size. You’ll be hard-pressed to find a similarly priced unit that performs as well as this one.

It comes with all the convenience features you’d expect: auto-cooling, multiple fan speeds, remote control (but no remote temperature sensing), and a washable filter that’s easy to pull out from the front.

For Large Rooms (~500 sq. ft.)
Frigidaire FFRE1833S2

best value window air conditioner units

Frigidaire 18,000 BTU Window AC

Frigidaire 18,000 BTU Window AC

Buy Now On Amazon

The powerful Frigidaire FFRE1833S2 produces 18,000 BTUs of cold air, able to cool all but the largest of rooms within minutes. While the product description claims to work for rooms up to 1,000 square feet, you probably don’t want to go much higher than 800 square feet.

Convenience features include a remote control with remote temperature sensing, multiple fan speeds, an ionizer that cleans and removes particles from the air (including pollen), and two separate vents that allow you to direct the cool air in two directions.

Note: The Frigidaire FFRE1833S2 requires a 230V outlet and is not compatible with regular 115V outlets. This is typical for window AC units in the 18,000 BTU range.

Other Ways to Stay Cool During Summer

Computers and laptops generate more heat than you might think. During summer months, that heat can contribute to a rise in ambient temperature. See our tips for reducing how much heat your PC emits. (If the heat can’t dissipate fast enough, it can also cause irreparable damage to the device!)

If you live in a house, we highly recommend getting a smart thermostat. Smart thermostats are designed to strike a balance between maximizing your comfort and minimizing energy usage. They often pay for themselves within the first year.

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that will make your time out in nature more enjoyable. Rather stay inside? Sit back and relax with these summer vacation movies on Netflix

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Image Credit: Butsaya/Shutterstock

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