Mold in the Air Conditioner? Here’s How to Tackle It—and Keep It Away For Good

The first time you turn on your AC unit in the spring, you expect the instant relief of cool air. What you don’t expect is a weird, musty smell that makes you cough and sneeze. If you experience this, you may have mold growing in your AC unit.

There are several surprising places mold can grow, including the AC unit. Mold has a tendency to grow inside air conditioners that sit unused for a while. Since you don’t use the AC unit during the colder months, you probably won’t the signs of mold until it’s had ample time to spread. And left unchecked, mold spores can produce allergens that can lead to sore throats, headaches, and various respiratory symptoms. While odds are the mold in your AC unit is not the scary black mold (Stachybotrys chartarum) that you’ve heard about, inhaling mold spores of any type can result in respiratory issues.

Successfully banishing mold from an AC depends on where it’s located. If the mold is accessible, you may well be able to remove it yourswlf. But if it’s growing in an area that’s difficult to access, you’ll either need to hire a pro or replace the unit entirely. Read on for the sleuthing tips and cleaning steps you need to bring clean, safe, cool air into your home again.

Turn Off the AC

If you suspect there’s mold inside your AC unit, the U.S. Environmental Protection Agency recommends turning it off so no more mold spores can circulate and contaminate your home. Turning off the unit also allows you to test for mold to see exactly what types of spores you’re dealing with.

Inspect the Unit

Unplug the unit, remove the front grille cover (most snap off but some are held in place by screws), and then pull out the filter beneath the grill. Grab a flashlight and inspect the inside of the unit for the signs of mold growth, which may appear as streaks or clusters of brown, black, or greenish stains, some of which may appear fuzzy. Mildew, a common type of mold, produces powdery gray or white stains. If you find just a few traces of mold on the hard surfaces inside the unit, proceed with cleaning. If it’s filled with heavy mold growth, indicated by mold and mildew deposits that cover one-third or more of the surface area of the case and the internal workings, it’s probably time to replace the unit.

If you suspect mold, consider contacting one of the best mold removal companies (such as ServiceMaster Restore or ServPro) to investigate. Mold inspection costs around $660, which may seem like one more thing to budget for, but a professional can easily determine what kind of mold you’re dealing with and can come up with a removal plan. Alternatively, you can DIY this part using one of the best mold test kits.

Clean the Mold

AC units vary in the way they are assembled, so check your owner’s manual to determine how to identify and disassemble the exterior cabinet so that you can access the interior. The manual may also have information on how to clean the air conditioner correctly. Before you start, assemble your tools and materials and then don a dust mask, goggles, and gloves. You’ll need to arm yourself with the following materials and tools:

• Screwdriver
• Wet/dry vacuum with a nozzle attachment
• Commercial AC coil cleaner
• Garden hose
• Dish soap
• Bleach
• Large plastic bucket
• Scrubbing sponge

Once you have your supplies, it’s time to start removing mold from the AC system by following a series of steps.

• Remove the unit from the window, using a screwdriver to remove the frame that secures it. AC units are heavy, so recruit someone to help you take it to your yard or driveway and place it on a concrete surface or on a large piece of plywood.
• Remove the grille and the filter from the front of the unit. Soak the filter soak in a sink with hot soapy water to which you’ve added approximately 1/2 cup of bleach.
• Remove the top and the back of the cabinet, following the directions in your owner’s manual.
• Vacuum out dust and debris from the inside of the air conditioner. Use a nozzle attachment to get out as much as possible.
• Spray the coils (U-shaped metal tubes near the front and the back of the unit) with commercial coil cleaner and let the product dwell as directed by the manufacturer to dissolve anything that’s collected on AC coils. Cleaning the gunk on the coils is crucial because it provides a breeding ground for mold.
• Spray the coils with a garden hose to remove the coil cleaner solution. It will have dissolved the gunk and it should spray right off. While window AC units are fairly waterproof, avoid spraying the controls and the spot where the electrical cord is attached.
• Fill a plastic bucket with a few gallons of hot water and about 1/2 cup of household bleach.
• Saturate a scrubbing sponge in the bleach solution and use it to wipe the inside surfaces of the AC unit, removing all traces of visible mold. The bleach will kill residual mold spores.
• Spray the filter that’s been soaking in the sink with the hose to remove all debris.
• Allow the unit to air-dry completely, which could take up to 24 hours, before reassembling the unit and reinstalling it in your window.

If mold removal is out of your comfort zone, it’s best to call a professional. Mold remediation costs an average of $2,250, but knowing that the mold has been addressed properly is priceless.

Take Precautions

Once you’ve had mold in the air conditioner unit, there’s an increased risk that it will develop again despite your cleaning efforts. This is because mold spores could remain within the inner workings of the unit that you were unable to reach. The presence of dust in the unit helps mold spores adhere and grow, so make it a practice to remove the grille and filter every few weeks and vacuum the interior of the unit to keep dust from settling and get rid of any dust that has accumulated.

In addition, don’t shut off your AC if you won’t be using it for a few days or longer during hot weather. This is a common practice when people go on vacation, but when the weather is warm and humid, mold is more likely to get a foothold in the AC unit. The air movement that occurs when the unit is running helps prevent mold growth. If you want to save on cooling costs when you’re gone, set the thermostat on your air conditioner to a higher-than-normal temperature. For example, if you usually set the temperature to 75 degrees Fahrenheit when you’re home, set it 10 degrees warmer to 85 degrees Fahrenheit before you leave. That way, the AC will not run as often, but it will still cycle on occasionally when the temp in the house rises. This will allow air to circulate through the unit and reduce the risk of mold growing while you’re gone.

Once the weather cools down and you no longer need to run your AC unit, invest in one of the best air conditioner covers to protect it from dust and debris while it’s not in use. And consider purchasing one of the best air purifiers for mold to improve the air quality of your home year-round.

Replace the Unit

If upon your initial inspection, you discover rampant mold and mildew in the air conditioner, do not attempt to clean it. Visible heavy mold indicates that more mold is flourishing in places you cannot see or easily reach, such as within the fan motor casing. Alternately, you could call a mold remediation specialist to inspect the unit to see if it can be professionally cleaned, but the consultation could cost as much as purchasing a new air conditioner.

Bottom line: It’s unhealthy to breathe air contaminated with mold spores, so if you can’t effectively clean the unit, it should be replaced. A professional HVAC technician can advise you on the correct air conditioner size for your home and can recommend the best brands and models in your budget.