Hot, wet, and muggy—truly the worst summer weather to endure. It's even worse when you turn to your air conditioner for help, only to find it isn't doing anything to alleviate the heat. Or, maybe it is, but you're worried you'll feel the pain when your energy bill arrives at the end of the month.

Fear not: we'll walk you through several common air conditioning mistakes you might be making at home. Fixing these issues will result in faster cooling, greater comfort, less wasted energy, and a much more modest utility bill.

1. Using an AC Unit That Isn't the Right Size

AC in a room

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 small to handle.

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

We also don't recommend buying 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 muggier and warmer than it is.

2. Opening Your Windows and Doors

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

This is why window AC units need to be mounted in a closed window, and split-type AC units have an indoor unit connected via hoses to an outdoor unit; this is also why central AC units are located outside the house. The "inside air" and the "outside air" must be isolated for the air conditioner to work. If your windows or doors are open, the expelled heat will just get sucked back in.

A diagram of airflow throughout the AC unit.
Image Credit: Pbroks13/Wikimedia Commons

In fact, even if you don't have an AC unit, you shouldn't open the windows when the heat is at its peak. You learn more about this in our guide on how to keep your room cool without AC.

3. Keeping the AC on All the Time

AC in living room

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

Most people think the first method is better, but the second one uses less energy—the savings are significant.

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

4. Setting the Air Conditioner's Temperature Too Low

modern air conditioner

After coming home from a long day at work, you step inside, only to be slammed by a thick wall of stale, 90F air. What do you do? You might think that the best course of action is to crank your AC all the way down to 65F so the room will cool faster—but that's not how air conditioning works.

Your AC unit continuously pumps out the same "strength" of cool air, whether your room is 70F or 90F. The temperature setting only tells the unit when to stop pumping out cool air; changing the setting from 70F to 65F won't speed up the process. It's the worst thing that you can do in this situation. This is because the AC unit will keep working even after it reaches your ideal temperature.

If, for example, your ideal temperature is 72F, and you set the unit for 65F, the unit will keep going until the room hits 65F. Now, you're simply too cold and have unnecessarily wasted a lot of energy.

The lesson: set your AC unit to your ideal temperature and let it work. If you're tired of coming home to a hot and stuffy house, consider getting one of the best smart thermostats for your home.

5. Leaving Rooms and AC Vents Open

An open door with sofa in the background
Image Credit: Africa Studio/Shutterstock

There are a couple of different scenarios to run through here.

If You Have an AC Unit in Each Room

Remember how window units are designed with a certain amount of air space in mind? That air space assumes all the doors connected to the room are closed.

Your bedroom might be 150 square feet, but if you leave the door open while the unit runs, you have a whole lot of hallway that you need to factor into the equation. 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, a volume it might not be rated to accommodate.

Keep the doors closed in any room with a window unit running to maximize efficiency. This will limit the hot air it will deal with to what is contained in that room, helping speed up how quickly the air is cooled.

If You Have a Central Home AC Unit

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

The more air space that needs to be cooled, the longer it takes to cool it all down. This also means that your unit must stay on longer—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. The air will be cooled much faster in rooms with open vents. You should also keep the doors to these rooms closed for the same reason we described above.

6. Not Circulating Air With a Fan

bedroom with a ceiling fan

Most people use a fan when it's warm and switch to the AC when it's hot. Fans and ACs should not be seen as either-or, though. In fact, ACs work way better when working together with fans.

Cool air accumulates wherever it gets dumped out. This means the area right in front of the unit is cooler for window AC units. For central units, the coolest spots are right around the vents.

To cool down the rest of your home, you'll have to wait for the thermal transfer to get the job done naturally. Unfortunately, this can often be a painfully slow process; you can force the cool air to circulate throughout the space by using a fan to move things along.

Imagine squeezing a drop of blue food coloring into a glass of cold water. If you just watch it sink without stirring, it could be some time before the particles dissipate, achieving equilibrium and distributing themselves throughout the water homogeneously. The same principle applies here.

Circulating air also helps your sweat evaporate faster, which has a cooling effect on your skin. This means you can set your AC to a higher temperature while feeling just as comfortable, shaving a lot off your energy bill on hot days. And if you're away, you can use these simple ways to automate your fans, helping your AC stay efficient all the time.

To help you with this, check out the best ceiling fans to keep you cool.

7. Neglecting to Change the AC Filter

Image Credit: Mindy w.m. Chung/Shutterstock

No matter how clean your room is, particles are always floating around in the air that gets sucked into the AC unit during operation. All AC units are outfitted with particle filters to prevent these particles from causing problems inside the machine.

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 and disrupt the unit's function, leaving you with maintenance issues to deal with long-term.

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

8. Forgetting to Clean Between Seasons

air conditioner

Have you ever seen a window unit dripping all the time? If an AC unit is used frequently, condensation on the inside can wreak havoc. However, well-designed AC units have ways of ridding themselves of condensation before it causes a problem.

Normally, condensation runs off and drips out on its own, so you don't have to worry about it. If something goes wrong and the condensation pools, mold and bacteria might grow inside.

Since air conditioners blow cool air into the room, those little baddies can catch a ride and spread, colonizing elsewhere. This could lead to health issues like asthma, pneumonia, black mold invasion, and even Legionnaire's disease.

It's essential to clean and service your AC units at the start of every cooling season. Never let anything disgusting live rent-free in your home.

9. Putting Off AC Maintenance

Image Credit: Andrey_Popov/Shutterstock

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 throughout its lifetime, all while minimizing the energy it uses.

If you hear or see something weird, such as grinding or internal dripping, get it checked immediately. If something smells off, same deal.

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. If you can't afford an annual AC checkup, avoid procrastinating when something does go wrong.

10. Not Insulating the Attic

house attic

During the summer, your house's roof absorbs the heat from the sun's rays. This means that if you've not insulated your attic, the heat could trickle down from the top into your house. Of course, if that happens, the air conditioner will work harder to keep your rooms cooler during the summer.

To solve the problem, insulate your attic with fiberglass or injection foam to put less strain on the air conditioner. However, make sure that you seal any air gaps in your attic before you insulate it.

We recommend injection foam insulation if you want to seal any air gaps. If you use injection foam insulation, you probably don't need to worry about it for the next 80 years—but fiberglass insulation should be replaced after 10 or 20 years.

11. Ignoring Heat Sources

house interior

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

  • Replace single-pane windows with double-paned windows.
  • Seal any cracks that might be letting heat in 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 the time spent cooking on the stove or in the oven. To reheat, use the microwave.

Heat sources, no matter small, add load to your AC. So, if you leave them unnecessarily running, you're adding load to your AC unit, making it work harder.

12. Expecting Too Much From Your AC

air conditioning condenser

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 whatever you've got going on outside. With the proper setup, you might be able to bring the temperature down by 20F or 25F, but if it's 110F outside, you might still be left sweating.

Common sense goes a long way here. We live in the real world, not some fantasy version of reality.

Additional Recommendations to Cool Your Home

Aside from the things you should avoid, we have a few more recommendations to help you stay cool.

The Best Temperature Setting for Your AC

There are two schools of thought when setting your AC's temperature:

  1. The Temperature Best for Comfort and Productivity. Hot weather can affect both your ability to work and your capacity to rest.
    • According to one study by Cornell University, the ideal temperature for office work productivity is around 77F.
    • According to WebMD, the ideal temperature for falling and staying asleep is between 60F and 65F, adjusted to the most comfortable temperature.
  2. The Temperature 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 the room becomes tolerable. What's most important here 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, cranking it down to your preferred temperature when you return. Most central AC units should not be set below 68F, or they will freeze up, which can damage or destroy the compressor.

Remove Hot Air to Cool Down a Room Fast

Let's say you've been away from home, and your AC has been off for the entire time. It's only about 80F outside, but your house is sweltering at 90F. Before turning on the AC, you should do this 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 the second to blow inwards. This circulation sucks the hot air from your house and sends it outside.

If you live on 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.

Make Your AC Feel Cooler

Remember, heat is amplified by humidity. When there's extra moisture in the air, your body's sweat will just sit on the surface of your skin instead of evaporating and cooling you down. This is why "dry heat" is more tolerable than "wet heat," even when the dry heat is many degrees hotter.

If you live in a wet climate and your house isn't adequately insulated, weather-sealed, and ventilated, you might feel more humidity than you should. This humidity could be causing your home to feel much hotter than it is.

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 neglecting this excess moisture, letting it just float around. In that case, a dehumidifier might work wonders for you.

Dehumidifiers are similar to air conditioners, but instead of being optimized to absorb heat, they're designed to absorb moisture. If you have a humidity problem, using a dehumidifier with your AC could make your house feel 10F cooler.

Need to Upgrade? Best Value Room AC Units

If you need to upgrade your AC, check out these options.

For Small Rooms (~150 sq. ft.)

Keystone 5,000 BTU AC image

The Keystone LCD Window-Mounted Air Conditioner offers your home a lot of 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 adequately cool the air and will run constantly.

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

The Keystone 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 ideal.

For Medium Rooms (~300 sq. ft.)

LG 12,000 BTU AC image

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 18,000 BTU AC image

This powerful Frigidaire Window-Mounted AC unit produces 18,000 BTUs of cold air and can cool all but the largest rooms within minutes. While the product description claims to work for rooms up to 1,000 square feet, you probably won't want to push it 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 airflow in two directions.

This model requires a 230V outlet and is incompatible with regular 115V outlets. This is typical for window AC units in the 18,000 BTU range; buyer beware, but a great choice if your home can accommodate it.

Stay Cool During the Summer

Like everything you own, your AC unit is a companion with needs. Start by choosing the best one for your home and giving it everything it needs to provide you with the cold air you desire. Optimal placement, energy-efficient habits, regular maintenance, and a watchful eye will all help ensure it can perform for years.

If you've followed everything in our guide, but you're still finding it challenging to keep your home cool during the summer, you should consider doing an energy audit. This can be done using a thermal camera to determine the parts of your home where heat is coming through.

Once you've done an energy audit, you will know exactly where to insulate, so you can make your air conditioner more effective during the summer.