Dark stains on the wall around the split unit, or when turned on, the room fills with a musty smell that lingers in your throat. These are signs of mold, and it almost always appears on the air conditioner for the same reason: moisture that stagnates where it shouldn’t. The good news is that it can be prevented easily, and even when it has already appeared, it can be removed without major interventions. Let’s see where it comes from and what to do.
Why mold grows right there
When the split cools, it removes moisture from the air and turns it into condensation inside the indoor unit. If this water is not properly drained, or if the device remains damp inside, it creates the perfect environment for mold: dark, humid, and warm. The spores settle on the coil, in the tray, and in the filters, then with each activation they are blown into the room. Hence the smell and, over time, the stains on the wall near the vent.
The most common causes
- Dirty and never washed filters. They trap dust and moisture and become the first mold nest.
- Always abrupt shutdown. Turning it off suddenly leaves the inside wet with condensation, which never fully dries.
- Slow condensation drainage. If the tube is partially clogged, water stagnates in the tray and rots.
- Already humid room. Bathrooms, kitchens, and poorly ventilated homes start at a disadvantage.
- Cold air on the wall. Direct airflow on a cold wall creates external condensation that feeds mold on the plaster.
How to prevent it, the habits that matter
The first rule is to let the inside of the device dry. Many splits have a self-drying function, sometimes called dry or self-clean, which runs the fan for a few minutes after turning off to remove moisture from the coil. If yours has it, always use it. Alternatively, before turning it off, leave it on ventilation only for fifteen minutes. Then wash the filters every two or three weeks in summer and keep the drainage tube clear. Finally, don’t aim the airflow directly at a nearby wall; direct the louvers upward and diagonally.
How to remove it if it has already appeared
Disconnect the power, remove the filters, and wash them with warm water and a little white vinegar, which disinfects without being as harsh as bleach. Clean the louvers and visible grille with a cloth and the same solution, then let everything dry well in the shade. If the mold is deep inside the coil, you can’t reach it yourself: professional sanitization with specific products is needed, which costs a few dozen euros and eliminates odor and spores.
When the problem is the room, not the split
Sometimes the split is clean but the mold still returns because the room itself is too humid. In these cases, airing out daily helps, avoid drying laundry indoors, and keep humidity under control with a dehumidifier on the worst days. If mold has become a problem throughout the house and not just the air conditioner, check out our appliances section, where you’ll find dehumidifiers that keep humidity in the ideal range between 40 and 60 percent.
Frequently Asked Questions
Is mold from the air conditioner dangerous to health?
The spores blown into the air can cause respiratory irritation, coughing, and discomfort, especially for those with allergies or asthma. It’s not an emergency, but it’s a good reason to act early and not postpone cleaning.
Can I use bleach to clean mold from the split?
It’s better to avoid it. Bleach is harsh on plastic parts and filters and leaves a strong odor. White vinegar diluted in warm water disinfects sufficiently and is gentler on the device’s materials.
Why does the split smell like mold as soon as I turn it on?
Because the inside remained damp after the last shutdown and mold formed on the coil and filters. When turned on, the fan immediately blows that air into the room. Using the self-drying function before turning off prevents the problem.