How To Remove a Musty Smell From an Air Conditioner: 10 Tips

Old air conditioners can produce all sorts of odors, ranging from mildew to mustiness. If your AC unit makes your house smell gross, it’s time to take care of it. Ignoring it can be detrimental to your health.

Here are 10 tips on how to remove a musty smell from an air conditioner:

1. Replace the air conditioner’s filter.
2. Clean the cooling coils.
3. Deep clean the air vents.
4. Seal the air ducts.
5. Clean the drain line.
6. Ensure your AC is the right size for your home.
7. Blow the AC with pressurized air.
8. Wipe down the inside of the air conditioner.
9. Let the unit dry before sealing it.
10. Use a hygrometer to control the humidity.

The rest of this article will break down the steps to clean your air conditioner to eliminate the musty smell. We’ll also discuss a couple of preventative tips to keep the moldy smell away.

1. Replace the Air Conditioner’s Filter

Some air conditioner filters are single-use, while others can be washable and reusable. You should check which kind of AC filter you got before buying a replacement. 

Ensure you follow the manufacturer’s suggestions to get the best filter for your air conditioner. Using a multi-use filter in a single-use air conditioner slot will make your air filter ineffective.

With a dirty air filter, your air conditioner sends particles through the vents. You’ll smell musty odors wherever the vents are open.

Without any further ado, here’s how to replace a dirty air filter in your AC:

1. Turn off the air conditioner. It’ll suction the filter and make it very difficult to remove if you forget to turn it off. Moreover, you can get electrocuted when working on an air conditioner that’s turned on.
2. Remove the old filter, identify the type and size, and get a replacement or clean it if it’s reusable. Reusable air conditioner filters can be sprayed with a garden hose. Let them dry in front of a fan or in the sunlight before putting them back into the air conditioner. You can get a replacement air filter from the manufacturer or online.
3. Identify the marked arrows on the filter to know which way it goes. If you orient it the wrong way, it’ll cause airflow problems. The filter should fit snugly in the air conditioner’s filter slot. It shouldn’t fold or fall out on its own.
4. Seal the AC filter behind the vent. Most of them use retaining screws or clips to prevent them from falling out.
5. Turn on the air conditioner. Allow the AC to blow out the musty air through the new filter, then check if there are any signs of the smell. If it’s still there, proceed to the next step.

2. Clean the Cooling Coils

Cleaning air conditioner coils depends on whether you have an indoor or outdoor air conditioner. If you have an indoor unit, you’ll have to turn off the power at the circuit breaker, then unscrew the coil from inside of the AC. Those with outdoor units can spray the outside of the AC without having to worry about removing any components.

Here’s the process:

1. Turn off the power going to your air conditioner to prevent electrical hazards.
2. Remove the top of the air conditioner to reveal the inner components.
3. Pull out as much of the debris as you can (sticks, leaves, dust, etc.).
4. Spray the outside of the coils with the Arm & Hammer AC Coil Cleaner (Amazon.com) for a fume-free cleaning session to increase the air conditioner’s ventilation and prevent debris buildup.
5. Spray the foam away with a hose from the other direction to blow the debris away from the unit.

If you have an indoor air conditioner, you can take the coil outside and follow the same instructions.

Those who prefer a video tutorial can watch this helpful guide by Five Star Lawn Care:

3. Deep Clean the Air Vents

You’ve likely noticed that the odors come from the vents. That’s because your air conditioner pulls particles through the filter, especially if it’s old and dirty. No filter can remove 100% of the debris that flows into it. Each vent will have dust and other contaminants inside that need to be removed.

I recommend keeping the following tips in mind when cleaning your air conditioner’s vents:

• Turn off the air conditioner before cleaning the vents. The last thing you want is the AC to turn on and blow all of the debris into you. You can manually turn off the air conditioner at the meter or flip the circuit breaker, just to be safe.
• Remove the vent’s faceplate and scrub it thoroughly with a sponge. You can use a little bit of soap and warm water, but make sure you let the faceplate dry before replacing it. Wet vents invite rust, mold, and mildew, leading to gross odors throughout the house.
• If you think there’s debris trapped in the ducts, hire a professional. Deep-set debris can prevent airflow and send odors into the vents. Bad DIY repairs can severely damage the air conditioner and worsen the smell.
• Replace any screws that look rusted or corroded. Loose screws let moisture into the vents and wear down the vent’s structural integrity. You should also replace the faceplates if they’re bent or rusted.

4. Seal the Air Ducts

You can seal your air ducts to drastically reduce the unwanted odors in your home. Holes and leaks in the air ducts let moisture through the vents, especially during the humid months. Follow the air ducts in your attic, basement, and anywhere else you have access to them.

If you don’t want to replace the air ducts yourself, hire an expert. Ruining the ducts can void the warranty and make for an expensive repair.

If you decide to go down the DIY route, use the TapePlus Aluminum Foil Tape (Amazon.com) to seal minor holes and cracks. Each roll contains 210 feet of two-inch-wide tape that bonds with the duct to give you an airtight seal. Simply press and hold the tape over the hole for a minute. You can use the tape both inside and outside.

Another option is to replace the ducts. If you live in an old home, there’s a good chance that the air ducts are worn out. Extreme temperatures are more likely to harm the air ducts over the years. Getting new ducts will stop moisture and debris from getting into the air conditioner.

Note: If you hire a pro to replace sections of air ducts, consider having them clean the ducts as well. Leaks often happen due to lime and dust buildup. Replacing portions of the ducts won’t remove the blockages, so it’s important to remove them beforehand.

5. Clean the Drain Line

Most air conditioners have a drain line, also known as a condensate line. As the name suggests, condensate lines get rid of condensation, which is a breeding ground for bacteria and mold. Not cleaning the drain line will undoubtedly cause musty smells in your home. You should flush the drain line regularly.

Most drain lines are made of PVC pipes. If yours has soldered metal pipes, you might have to call a professional to clean the line. Most metal lines can be unscrewed, cleaned, and screwed back into place. If your lines can’t be unscrewed, contact a professional to carry out the procedure.

Here’s how to clean a PVC air conditioner drain line:

1. Locate the condensate line and wrap a towel around the open end.
2. Attach a wet vacuum hose over the towel to achieve a tight seal.
3. Turn on the vacuum to drain as much of the water out of the condensate line as possible.

Watch this video by Silver Cymbal if you want to do it yourself:

Another quick and easy way to clear the line is to use the Homiom Drain Gun (Amazon.com). This tool uses CO2 to blast clogs through the line, removing the mildew, mold, and other debris.

6. Ensure Your AC Is the Right Size for Your Home

Unfortunately, having an air conditioner that’s too large or too small can cause all sorts of problems for your home. You might think bigger is better or getting a smaller unit will save money, but you’d be surprised by the negative effects.

Here’s what can happen if the AC doesn’t match your house:

Small Air Conditioners Don’t Filter Enough

If your air conditioner is too small for the house, it won’t have enough power to remove all particles. The slow-moving air won’t prevent moisture buildup, leading to mildew, mold, and rot. You must have the correct size AC to keep your home feeling, looking, and smelling fresh.

Large Air Conditioners Damage the Ducts

Getting a massive air conditioner can put too much pressure on the ducts and vents. It’ll break, bend, and warp the vents, covering the HVAC system in holes and leaks. As I’ve mentioned earlier, leaks invite moisture, mold, and gross odors.

So, how do you know what AC size you need? Try the calculator by AC Direct to know exactly what square footage, weight, and BTUs you should have.

7. Blow the AC With Pressurized Air

Pressurized air is perfect for cleaning air conditioners because it removes dust and debris without harming the internal components. You can use a standard automotive air compressor, but some are unnecessarily loud and take up a lot of space. Another option is to use a handheld air compressor.

The Compressed Air Duster (Amazon.com) packs up to 33,000 RPM, pushing more than enough air through the air conditioner. You can use it to clean the vents and ducts, too. This handheld tool uses a rechargeable battery that lasts up to 30 minutes of continuous use. Choose between automatic and manual cleaning for an odorless AC.

Keep these suggestions in mind:

• Turn off the air conditioner to prevent electrical hazards and dust from blowing through the ducts and vents.
• Keep the air compressor about 18 inches (45.72 cm) from the air conditioner, then gradually move it closer if necessary to avoid damaging the parts.
• Wear a mask and safety glasses to keep the inevitable clouds of dust and debris out of your mouth and eyes.
• Use a handheld vacuum or vacuum hose attachment to remove the dust piles when you’re done with the air compressor.

8. Wipe Down the Inside of the Air Conditioner

You can use wet wipes to clean the AC, but don’t let them drip into the electrical parts. These wipes are useful for vents and other exposed parts. They’re also perfect for the air conditioner’s housing since they can get covered in dust and grime. Never use wet wipes to clean the filter, wires, or buttons.

Outdoor air conditioners are somewhat weatherproof, but you should take caution with indoor units. Too much moisture can exacerbate the issue or lead to other problems. Air compressors and handheld vacuums can handle most odors associated with indoor units.

9. Let the Unit Dry Before Sealing It

It doesn’t matter how well you clean the inside of your air conditioner if you don’t let it dry. 

The moisture from wet wipes, sponges, and other cleaning products can grow mold if they’re left undried. You’ll have a clean air conditioner for a few days, then the musty odors will return and make you think there’s another issue.

You can leave the door open for indoor air conditioners. Outdoor ACs have removable top panels that can be set aside. Let the air conditioner dry for a couple of hours (or until it’s dry to the touch).

10. Use a Hygrometer to Control the Humidity

Some house air conditioner meters have built-in hygrometers. 

These meters measure the humidity in the air. Lower the humidity to about 45% to prevent moisture from growing mildew and mold in the air ducts. You can hire a professional to install a hygrometer in your house. Also, use a dehumidifier to adjust the ambient humidity.

Final Thoughts

You don’t have to throw the whole AC away if you notice funky smells coming from the vents. 

Cleaning the indoor and outdoor AC unit thoroughly and ensuring everything is sealed is an easy way to eliminate odors, lower energy bills, and improve air quality.

Steve Rajeckas

Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he’s not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.