From A to C: how to get the most out of your window air conditioner

When one’s abode starts to feel more like the inside of an armpit, air conditioning can provide sweet relief. But if you live in an older home or apartment , you’re less likely to have central air. That means you have to drag out whatever machine you use to keep cool.

Most often, this is a window air conditioning unit, says Darcy Carbone, vice-president of sales at APA HVAC Technologies and a member of the residential buildings committee at the American Society of Heating, Refrigerating and Air-Conditioning Engineers. These units cool, circulate, filter and dehumidify air in a home.

But using a window unit means more than just sticking it on the sill and letting it rip. Here’s what you need to know to use your AC efficiently and safely.

How to choose between a window unit or portable AC

Robert Polchinski , a professor at the New York City College of Technology who helps students enter the HVAC industry, agrees that most people opt for a window unit because it is affordable. Double-hung windows – the windows that open and close vertically – are also most suited for a window unit.

Alternatively, a portable AC might be more compatible with your needs: for example, if your windows crank outward. However, portable units are typically more expensive , use more energy and are less effective at cooling spaces.

Related: ‘It’s unbearable’: in ever-hotter US cities, air conditioning is no longer enough

How to maintain your AC window unit

When you bring out your AC unit from wherever you stashed it through the winter, it’s wise to go through a quick list of check-ups before putting it in the window and flipping the switch. You should repeat these actions at the end of the season, too. Taking the appropriate steps can help your AC unit last longer and protect you from unintended side effects.

Clean the filters: Start by cleaning the unit’s filter. If you remove the front panel, you’ll see the filter, which you can slide out. These are washable, Carbone explains. Shake off any debris, wash the filter by hand with a mild dish detergent, then rinse. If you want to clean your filters again after the unit is in the window, be sure to turn off and unplug the AC first.

“Filters should be washed at the beginning of the season,” Polchinski says. “Generally, once a year should be enough. But depending on how dirty the air is in the environment, they may need to be cleaned more than once during the season.”

Check the coils: Next, Carbone recommends assessing the AC’s coils. A unit has two sets of coils: evaporator coils and the condenser coils. Evaporator coils help remove hot air from your home and are located behind the filter. Condenser coils expel heat and are located at the back of the unit, near the vents.

You’ll want to take care of these before the unit is in the window. Remove the unit’s outer casing, then use a vacuum with a hose attachment to clean off any dust and debris from the coils, says Polchinski.

Allow condensation to drain: Air conditioning units create condensation, and that water has to go somewhere. Some have built-in pans, and some need to be precisely level, while others need to sit at a slope. This last group should be installed so the unit’s rear is slightly tilted downward, which helps drain away any condensed water.

“You don’t want to accumulate sitting water, which could lead to all kinds of bad scenarios, like a mold build-up or a potential spot for mosquito breeding,” Polchinski says. He also cautions that if the water is warm enough, this could lead to legionella , a type of bacteria that grows in water and can cause sickness.

Store the unit correctly: Storage also matters, says Carbone. When summer is over, store the window unit as it would be in the window – don’t be tempted to flip it on its side to squeeze it into its winter hiding spot, which can damage the compressor.

How an AC window unit can affect your health

For a unit to make cold air, indoor air passes through a cooling coil, which results in condensation. This condensation needs to drain; otherwise, it can cause mold growth, explains Amy Li , an assistant professor at the University of Waterloo who studies how to improve indoor air quality.

While window unit filters do catch particles like dust and dander, they don’t perform the same function as an air purifier , for example. “The filters used don’t remove particles very effectively, so the air isn’t necessarily coming out much cleaner than it went in,” says Brent Stephens , a professor at Illinois Institute of Technology who researches indoor air issues.

Window units may look like they draw fresh air from outside, but they do not. Because a lack of fresh air increases how much indoor pollutants are inside, Li recommends using a carbon dioxide monitor to assess indoor air or alternating between using the AC and opening windows to encourage natural ventilation.

What to do if your AC window unit smells weird

Cleaning the unit’s filters can help it stay energy efficient because clogged filters block airflow and cause problems. Dirty filters also increase the chance of mold and mildew growing inside the unit. This can lead to health issues and cause the air that exits the unit to smell musty, explains Li.

Some AC units have a drain pan built into the unit to collect condensation. If this pan fills, mold can grow, which could also lead to a bad smell.

If drainage or filters don’t seem to be an issue, but you still notice a bad smell, Carbone says to open up the unit and clean the coil with bleach. If you try all the above and it still smells funny, “get rid of it”, she says.

When to replace your AC

Part of AC ownership also means knowing when to say goodbye. While window units can last for eight to 10 years , certain signs can indicate that something is amiss.

Polchinski explains that an AC may be at the end of its run if “your electricity bills are running much higher, but the price of electricity hasn’t changed”. If your usage hasn’t increased much from year to year, that suggests your unit is running less efficiently, he explains. That might mean that the motors are getting weaker, or there could be a “low charge in the refrigerant, which could be an indication that there’s a small leak”, Polchinski says. Frozen coils can also be a sign of a slow refrigerant leak, says Carbone. If you have frozen coils, there’s likely ice buildup around the coils and the unit isn’t efficiently cooling your home.

Window units use refrigerant, a chemical cooling agent, to remove heat and humidity from a room. During the process, the refrigerant changes state from liquid to gas. A leak can cause the refrigerant to escape as a gas. If an AC is leaking liquid, that’s more likely to be condensation buildup.

Older units also simply don’t work as well. “You’re just going to be paying for non-efficient equipment,” Carbone says.

You can bring an older AC to a specialist to see if they can fix whatever’s wrong. But Polchinski says that, because it’s expensive to repair and AC units are getting cheaper, “it probably doesn’t pay to repair it”.

Related: Air conditioning: the benefits, problems and alternatives

What to look for when buying a used AC

Carbone doesn’t personally recommend getting your AC unit secondhand and says she wouldn’t opt for one that’s more than five years old. She also advises that you make sure the seller has cleaned the filters and the coils.

Polchinski would also look for any signs of damage to the coil, adding, “I would want to plug it in somewhere and test it – even if it’s not in a window – just to make sure that it sounds right.” Listen for loud buzzing, hissing, banging and “pinging or scraping noises that could be an indication of rotating parts touching something that they shouldn’t be touching”.

When buying secondhand or new, you’ll also want to consider how the AC will work for the size of your room. A bigger unit isn’t necessarily better: if it’s too large, it will cool the room before removing humidity, making it feel clammy. You can do some simple math to figure out the size you need. Meanwhile, Polchinski says the most energy-efficient units are those that are Energy Star-rated .

Carbone says you’ll also want to consider what refrigerant the AC uses. Because refrigerants absorb heat, they are helpful in cooling systems, but if they leak, they can contribute to climate change . Newer machines use more environmentally friendly refrigerants.

Common AC misconceptions and mistakes

One major mistake Polchinski observes is when “people turn the AC unit to maximum cold thinking that is going to get the room cooler faster.” He equates it to people “constantly pressing an elevator button, thinking that’s going to make the elevator get somewhere faster”.

Ultimately, an AC can only reach a programmed temperature at a certain speed. Temperature control isn’t like volume control, he explains. It simply means the compressor keeps running until you get to the selected frigid setting.

Related: Air purifiers: how do they work and who should get one?

Another issue is that people tend to cool buildings when it’s “perfectly comfortable outside” or a ceiling fan would be sufficient, says Liz McCormick , an architect, and professor at the University of North Carolina at Charlotte who’s written a book about the impacts of air conditioning. That’s not great for the planet , and it changes our tolerance and perception of different conditions.

“It’s completely changed the way we live and work,” McCormick says of air conditioning. “But I do think we can continue to evolve the technology and find ways to balance this modern marvel with the healthful benefits of the outdoors.”