How to fix a mini split AC that smells bad

There’s nothing worse than a musty or downright nasty smell filling your space when your mini split AC kicks on. These odors usually mean mold, mildew, or some other buildup is lurking inside the system. In this piece, we’ll dive into common causes of these smells and give you practical steps to wipe them out. Let’s figure out what’s making your AC stink and how you can fix it yourself.

The info in this article might not match your specific appliance model. Even though we’re trying to offer handy advice for all sorts of DIY appliance repairs, differences in models and setups can change the troubleshooting steps, error codes, part swaps, and so on. For spot-on guidance, we highly recommend checking the manufacturer’s documentation for your exact model or contact us if you’ve got questions.

Common causes of mini split AC odors

Your mini split AC is meant to circulate clean, cool air throughout your home. But sometimes, common issues can cause unwanted smells. Understanding these factors helps you pinpoint where the odor’s coming from so you can jump into your DIY fix. Let’s look at the usual suspects.

• Mold or mildew growth: Mold and mildew love damp, dark places. Your mini split AC is a perfect spot for them since moisture condenses on coils and drips into drains. If spores settle in these areas, they can grow like crazy, leading to that unmistakable musty odor.
• Air filters clogged or dirty: Air filters catch dust, pet hair, and other particles. Over time, if you don’t change or clean them, they get clogged. When that happens, airflow is restricted, moisture doesn’t evaporate properly, and stale odors can develop.
• Bacteria growth in the drain pan: The drain pan (or drip tray) collects water from the evaporator coil. If the water doesn’t drain right, it can become stagnant. Bacteria might start growing in the standing water, creating unpleasant smells that blow into your living space when the AC runs.
• Drain line clogged: The drain line carries excess water outside. A clog in the drain line can trap water, inviting mold, mildew, or bacteria to flourish. You might notice a musty or sour smell if the line is blocked.

Check and clean the air filters

Air filters are usually the first thing to check when you’re troubleshooting odors from your mini split AC. A clean filter promotes better airflow and helps prevent moisture-related problems. 

1. Turn off your AC. Make sure the unit is switched off before you get started. Even though you’re just handling the filters, it’s a good idea to minimize any risks.
2. Access the filters. Depending on your unit, you might need to remove the cover or front panel. If you’re unsure, see how to find your appliance’s model number or check the manufacturer’s instructions for details on removing the panel.
3. Remove the filters. Carefully slide or pull them out. Take note of how they fit so you can put them back correctly.
4. Look for buildup. Check for dust, dirt, or any visible mold or mildew. If the filters look clogged or discolored, it’s time to clean them.
5. Clean the filters. Use a soft brush or vacuum to remove loose debris. Then, rinse the filters with lukewarm water. If you spot mildew, you can use a mild detergent or a commercial coil cleaner approved by your AC’s manufacturer. Avoid harsh chemicals that might damage the filter materials.
6. Dry thoroughly. Wipe off excess water and let the filters air-dry completely before putting them back. Moisture in your filter can encourage more mold growth.
7. Reinstall the filters. Slide or push them back into place, making sure they’re aligned just as they were before. Secure the covers on your mini split.

Remove mold and mildew from your evaporator coil

If the filter’s clean but the odor sticks around, mold or mildew might be growing on the evaporator coil. Since the coil’s job is to cool and dehumidify air, it stays damp, which can lead to unwanted growth.

1. Switch off the unit. Safety first. Turn off the power at the main switch or circuit breaker if you can, just to be safe.
2. Open up the indoor unit. Gently remove any panels blocking the evaporator coil. Check the manufacturer documentation or review your model number details for guidance.
3. Inspect the coil. A flashlight might help you see if there’s mold, mildew, or heavy dust. If you see dark or fuzzy patches, that’s likely mold.
4. Use a soft brush or vacuum attachment. Carefully remove any loose debris. Be careful not to bend the coil’s fins.
5. Apply a coil cleaner. Look for an AC-safe coil cleaner designed to remove mold and mildew. Follow the directions on the cleaner’s label. Spray it evenly and let it sit for the recommended time. Many coil cleaners foam up to get into the fins.
6. Rinse if needed. Some cleaners need to be rinsed with water. Others are self-rinsing. Check the instructions. Avoid hitting the coil with high-pressure water—you could damage the fins.
7. Dry and close the panel. Make sure any excess moisture drains away. Wipe up any leftover water, then close up the unit. Restore power when the coil is fully dry.

Inspect and clear the drain line

A clogged drain line can trap water, which stagnates and fosters foul-smelling bacteria or mold. You might notice water leaks around the indoor unit, or you might see a drip pan that’s always full of water.

1. Turn off the power. Shut off the AC unit to avoid any electrical hazards and to stop the flow of water.
2. Find the drain line. On most mini split units, you can find the drain line by following the drip pan outlet. If you’re unsure, check the manufacturer’s schematic or repair help resources.
3. Check the drain pan. If you see standing water or slime, that suggests a blockage. You might also notice algae or mold growth.
4. Clear any clogs. Some drain lines can be unclogged by pushing a thin, flexible brush through the tube. Others might need a vacuum or flushing with water or vinegar to dislodge gunk.
5. Flush the line. If your unit’s design allows, flush a diluted vinegar solution or warm water through the line to remove any stubborn buildup. If you want detailed instructions for your model, figure out your specific model number first.
6. Make sure it’s draining properly. Once the clog is cleared, check that water flows freely out of the line.
7. Put everything back and test. Make sure everything is reattached securely. Turn on the AC and watch for normal drainage and no odors. If the smell’s still there, you might have more buildup in the piping or other parts of the system.

Other potential odor sources

While mold, mildew, and clogs are the usual suspects, other sources might be behind that bad smell in your mini split AC. Here’s how to handle a few less common issues.

The condensate pan

Your condensate (or drip) pan sits beneath the evaporator coil to catch excess water. This area can collect dust and debris, leading to bacterial growth.

1. Turn off the unit. Always keep the power off while you’re handling internal components.
2. Find the drain pan. It’s usually accessible once you remove the front panel or the bottom panel of the indoor unit.
3. Empty any standing water. Use a sponge or small vacuum to remove stagnant water. Dispose of it safely.
4. Clean with approved solutions. Use mild soap, a commercial coil cleaner, or diluted vinegar to wipe away residue. A soft cloth or sponge usually works best. If there’s crusted debris, you might need a small brush.
5. Dry it thoroughly. Make sure the pan is as dry as possible before putting everything back together. Moist, dark areas attract more mold and bacteria.
6. Reinstall and turn on the power. Check that the pan is correctly placed. Restore power to your AC once everything is properly in place.

Wrap it up

Whenever you need new filters or cleaning supplies, check out AppliancePartsPros.com for a variety of parts and resources. Our repair help center can guide you through loads of common DIY fixes for your mini split AC and other household appliances.

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Got questions about your mini split AC or need a hand? Reach out to our customer support team at AppliancePartsPros.com. We’re happy to help you troubleshoot or find the exact part you need to keep your appliance running smoothly.