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Few things are more frustrating than uncomfortable heat, especially when that heat is wet and muggy. It’s even worse when you turn to your air conditioner for deliverance, only to find that it isn’t helping all that much. Or maybe it is helping, but you cringe in anticipation of the energy bill.

In this article, we’ll walk you through 11 common mistakes that you might be making in your AC usage. Rectifying these issues will result in faster cooling, greater comfort, less wasted energy, and smaller cooling bills.

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Mistake 1: Using a Wrongly-Sized AC Unit

The first, and perhaps worst, mistake you could 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 may not be effective if your room is too big or too small.

All window units have a British Thermal Unit (BTU) rating, typically in the range of 5,000 to 15,000. The most common rule of thumb is 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.

That said, make sure you don’t buy a unit that’s too big either! An oversized unit will constantly turn on and off because it’s too effective at pumping out cool air, but won’t be able to dehumidify the room. Humidity can make a room feel much warmer than it is.

Mistake 2: Opening Your Windows and Doors

Most people don’t seem to understand how air conditioners work. Long story short? The AC unit pulls in warm air from the room, which passes over a refrigerant that absorbs the heat from the 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 units need to be mounted in a closed window and why central 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.

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Mistake 3: Keeping the AC on All the Time

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 (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 the truth is that the second one actually 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 The Most Energy-Efficient Way to Set Your Thermostat The Most Energy-Efficient Way to Set Your Thermostat In the dead of winter and peak of summer, one question that pops up at least once throughout each season is: how should you set your thermostat so that both comfort and savings are maximized? Read More .

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 up anything.

It’s actually worse because the AC unit will keep working even once it’s reached your ideal temperature. 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 wasted a lot of unnecessary energy.

So set your 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 Ecobee3 vs. Nest Thermostat: A Head-to-Head Comparison Ecobee3 vs. Nest Thermostat: A Head-to-Head Comparison Did you know that a smart thermostat can boost your home value? It's one of many reasons why a smart thermostat is well worth the initial investment. But which one should you get? Read More .

Note that most central AC units should not be set below 68F or else they will freeze up, which can damage or destroy the compressor. Yet another reason to only set your unit to an ideal room temperature.

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Mistake 5: Leaving Rooms and AC Vents Open

If You Have a Window 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 Unit

You probably have AC vents located in every room of your house. If all of your vents are open, then the central unit tries to cool down 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 a vent, you cut off that room from the central unit and reduce the total amount of air that needs to be cooled, thus speeding up the cooling of other rooms where vents are open. You should also keep the doors to such rooms closed.

Mistake 6: Not Circulating Air With a Fan

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

Cool air tends to remain in the area where it’s blown out. For window units, this means the area in front of the unit is coolest. For central units, the coolest areas are right around the vents. In order to cool down the rest of the air space, you have to wait for thermal transfer, and that can be slow. But with a fan, you can circulate the cooled air around the room.

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.

Furthermore, the circulation of air helps your sweat to evaporate faster, which has a cooling effect on your skin. This allows you to set your AC to a higher temperature while still feeling just as comfortable, which can shave off huge chunks of your energy bill on hot days.

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Mistake 7: Neglecting to Change the Filter

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

A clean filter is crucial for optimal AC efficiency. Particle buildup can reduce airflow, forcing 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 result in greater frequency of maintenance issues.

If your AC is on all day every day, change the filter once every month. If you don’t use it so 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.

Mistake 8: Forgetting to Clean Between Seasons

Due to all the hot and cold air that AC units experience, condensation is a big concern. It’s normal to see water dripping during operation, but if the unit isn’t properly maintained, condensation can build up and lead to the growth of mold or bacteria.

And since air conditioners work by blowing cool air back into the room, the growing mold and bacteria can easily go airborne. This may lead to health issues like asthma, pneumonia, black mold invasion, and even Legionnaire’s disease. As such, your AC unit should be cleaned and serviced at the start of every cooling season.

Humidity in general can be a huge problem in the summer months, so consider these health benefits of monitoring home humidity 4 Health Benefits of Monitoring Temp & Humidity at Home 4 Health Benefits of Monitoring Temp & Humidity at Home Did you know that there are health benefits of using smart home products? That's right. One or two smart purchases could drastically improve your quality of life at home. Read More . Even if you don’t have mold or bacteria, there are other benefits to consider.

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Mistake 9: Procrastinating 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. And, 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 unit.

Mistake 10. Ignoring Other Heat Factors

For optimal cooling, you should help your AC unit out as much as possible. Anything you can do to reduce heat in your room, apartment, or house will go a long way towards making your unit more effective.

  • 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.
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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 an optimized setup, it may bring the temperature down by about 20F or 25F. But if it’s 100F outside, there’s almost no chance of getting your home down to 65F!

Other Ways to Improve Your Summer

Computers and laptops can generate a lot more heat than you might expect. If the heat can’t dissipate fast enough, it can cause irreparable damage to the device. But in the summer, that heat can also contribute to ambient temperatures. See our tips for reducing how much heat your PC emits How to Prevent Computer Overheating & Keep Your PC Cool How to Prevent Computer Overheating & Keep Your PC Cool Overheating computers may lead to unexpected shutdowns, lost data, and hardware damage. We'll show you how to read computer temperatures and simple ways to keep your PC cool. Read More .

If you live in a house, we highly recommend getting a smart thermostat 13 Things You Didn't Know You Could Do With a Nest Thermostat 13 Things You Didn't Know You Could Do With a Nest Thermostat You may be surprised just how many little tricks you can accomplish with your new Nest thermostat! Read More . 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.

Going camping? Check out these nifty solar-powered camping gadgets The Best Solar Tech for Camping This Summer The Best Solar Tech for Camping This Summer Solar gadgets can make your time in the woods more convenient and more satisfying. What benefits do they offer? Read More that will make your time out in nature more enjoyable. Rather stay inside? Sit back and relax with these summer vacation movies on Netflix 10 Summer Vacation Movies You Can Watch on Netflix 10 Summer Vacation Movies You Can Watch on Netflix If you spend your summer vacations relaxing, you may want to watch some movies to pass the time. These films, all available to watch on Netflix right now, were made to enjoy on summer vacation. Read More .

How troublesome is your air conditioner? Got any other tips that we missed? Let us know in the comments below!

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  1. Mac
    June 14, 2017 at 6:34 am

    I have been told that the most effective and efficient way to use a/c is to set the temp. to 4 degrees below ambient. In other words if it is 28 degrees in the house set the a/c to 24.
    Does this make sense?

  2. J Masters
    June 13, 2017 at 6:40 pm

    Also a mistake to close doors with central air conditioning and having carpet so high it restricts air return under the door. It's similar to closing off vents.

  3. Fik of Borg
    June 6, 2017 at 7:02 pm

    My tongue is numb ... not from the cold but from explaining to my coworkers that lowering the setting won’t speed up the cooling.
    Most of them keep on falling to the placebo effect of lowering the setting from a confortable and reachable 24°C to an unreal 18°C, going back to their desks and triumphantly proclaim "NOW it's cooling"

  4. Mike
    June 6, 2017 at 2:42 pm

    If you have a central a/c system with an indoor blower be careful shutting off too many vents. The unit is "sized" for a certain total airflow and if you reduce the airflow too much it has the potential to form ice on the cooling coil. If the unit freezes up you have to turn it off and wait for many hours to completely defrost before using the unit again. A frozen unit can also leak water and damage floors or ceilings. Avoid closing more than 25% of your available vents.