How to Remove Mildew Smell from an Air Conditioner

Maybe you’re the exception to the rule and love the scent of cool, musty air.
Or maybe you’re like the rest of us who recoil at mildew odor.
Not only is mildew unpleasant — it’s potentially harmful to your health. And it may be coming from your air conditioner. Unlike mildew in the shower, you can’t just spray it away.
Mildew is a fungus that thrives in damp, dark places like your AC unit. In addition to its foul smell, it can cause respiratory problems, aggravate allergies, and even damage your air conditioning system.
What Causes Mildew Smell in Air Conditioners?
The mildew smell in your AC comes primarily from moisture buildup. The air conditioner cools warm air, which leads to condensation. If that condensation isn’t drained, the lingering moisture becomes a breeding ground for mold and mildew.
Here are some common causes:
- Blocked drain lines can prevent water from draining correctly.
- Air filters can collect dirt, debris, and moisture, creating the perfect environment for mildew growth.
- Dust and dirt in the evaporator coils can retain moisture, allowing mildew to thrive.
- Humid weather and high indoor humidity levels can exacerbate the problem.
- Limited airflow can lead to increased humidity inside the AC unit.
Step-by-Step Process to Remove Mildew Smell
1. Turn Off & Unplug Your AC Unit
- Ensure the unit is completely off to avoid accidents.
- If it’s a window AC or portable AC, unplug it for added safety.
- For central HVAC systems, turn off the power supply at the circuit breaker.
2. Inspect & Clean the Air Filters
- Remove the filters from your AC unit.
- Inspect the filters for signs of mold or clogging.
- Wash the filters with warm water and soap or a mild detergent.
- Allow them to dry completely before reinstalling. Replace the filters if they’re too dirty or damaged to clean.
Pro Tip: Some AC filters are disposable and must be replaced regularly. Check your manufacturer’s instructions for the recommended replacement schedule.
3. Clean the AC Drain Line
- Locate the drain line, typically a small PVC pipe near the outdoor unit or the condensate drip pan.
- If it’s clogged, use a wet vacuum to remove debris.
- For stubborn blockages, use equal parts vinegar and water to break down the buildup.
4. Clean the Evaporator Coils
- Purchase an AC coil cleaner spray from your local hardware store.
- Gently spray the cleaner onto the evaporator coils and wait for the prescribed time (usually listed on the product packaging).
- Wipe the coils with a clean microfiber cloth to remove dirt and mold residue.
5. Wipe Down Interior Components
Use a mixture of water and white vinegar (50/50 ratio) to wipe down the inside of the unit:
- Clean the fan blades, interior walls, and accessible components.
- Avoid soaking electrical parts. Use just enough moisture to clean effectively.
6. Deodorize Your AC Unit
After cleaning, deodorize your AC to remove any lingering smells:
- Place an AC deodorizer strip (available online or in stores) near the filter.
- Alternatively, place a small baking soda dish near the air intake vent to absorb odors naturally.
How to Prevent Mildew Smell from Returning
Once your AC is clean and odor-free, follow these proactive measures to maintain it.
Regular Maintenance
- Clean or replace your air filters every 1–3 months.
- Schedule professional HVAC maintenance annually to ensure the system is in optimal condition.
Keep the Drain Line Clear
To prevent clogs and buildup, pour a mixture of vinegar and water down the condensate drain line every few months.
Use a Dehumidifier
Running a dehumidifier in high-humidity areas of your home can reduce damp air, minimizing the chance of mildew forming in your AC.
Promote Airflow
Ensure vents aren’t blocked by furniture or curtains. Proper airflow reduces moisture accumulation in your unit.
Use Mold-Resistant Filters
Upgrading to mold-resistant, high-efficiency filters can lessen your AC’s mildew risk.
FAQs: Removing Mildew Smell from AC
Q: Why does my AC smell musty when I turn it on?
The musty smell usually stems from mildew or mold growth caused by trapped moisture in your AC system.
Q: Can a mildew smell from AC units be harmful?
Yes, mildew can affect indoor air quality and exacerbate respiratory issues like asthma and allergies. It’s best to address the problem quickly.
Q: How often should I clean my air conditioner?
Clean your AC unit at least once every six months or more frequently if you live in a humid climate.
Q: Can I use bleach to clean my AC?
Bleach may damage some components of your AC. Also, do you want to breathe it? White vinegar is a safer alternative.
Q: How do I know if the mildew is gone?
Run your AC for a few minutes after cleaning. If the smell persists, repeat the cleaning process or call a professional.
Q: What if I can’t find the source of the smell?
If you’ve cleaned all visible components and the smell continues, a professional HVAC technician can inspect the internal system.
Q: Are there professional services for cleaning AC units?
HVAC professionals offer deep-cleaning services that can address difficult-to-reach areas of your AC.
Q: Can running the fan-only mode help?
Running the fan-only mode for 20–30 minutes after using the AC can help dry out internal moisture, preventing mold growth.
Related Air Conditioning Pages
- 10 Troubleshooting Tips for a Leaking Air Conditioner
- 14 Easy Ways to Improve Indoor Air Quality in Your Home
- The Benefits of Air Duct Sealing for Energy Efficiency & Health
- What Is AC SEER Rating & Why Should You Care?
- What to Do If Your Air Conditioner’s Not Cooling
- Air Conditioning Compressors & What They Do
- 6 Tips to Improve Window Air Conditioner Performance
- 11 Air Conditioning Repairs You Can Handle DIY
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