Dark patches on the wall around your split unit, or a stale smell that fills the room as soon as you switch on. Both are signs of mould, and on air conditioners it almost always comes from the same source: moisture sitting where it shouldn't. The good news is it's easy to prevent and, once it appears, straightforward to remove without major works.
Why mould grows right there
When the unit cools, it extracts moisture from the air and converts it into condensate inside the indoor unit. If that water isn't drained properly, or if the interior never dries out fully after use, you get ideal conditions for mould: dark, damp, and warm. Spores take hold on the evaporator coil, in the condensate tray, and on the filters. Every time you switch on, they get blown into the room. That's the source of the smell, and over time you get the dark staining on the wall next to the outlet grille.
The most common causes
- Filters that have never been cleaned. Dust and moisture trapped in the filter pads create the first mould habitat.
- Always switching off abruptly. Cutting power immediately leaves the interior wet with condensate that never fully dries.
- Slow condensate drain. A partially blocked drain pipe lets water pool and stagnate in the tray, where it decomposes.
- Already-humid rooms. Bathrooms, kitchens, and poorly ventilated spaces start at a disadvantage.
- Cold airflow directed straight at a cool wall. Direct airflow onto a wall creates external condensation that feeds mould in the plaster.
Prevention: habits that make the difference
The first rule is letting the interior dry after each use. Many units have an automatic drying function, sometimes called Dry or Self-Clean, that keeps the fan running for a few minutes after shutdown to dry the coil. If yours has it, always use it. If not, switch to fan-only mode for ten to fifteen minutes before turning the unit off. Clean the filters every two to three weeks in summer, keep the drain pipe clear, and don't point the airflow straight at a nearby wall. Angle the vanes upward and diagonally.
How to remove mould that's already there
Switch off at the mains, remove the filters and wash them with lukewarm water and a little white vinegar. Vinegar disinfects without attacking the plastic the way bleach can. Wipe down visible coil fins and the housing with the same solution and a soft cloth, then let everything dry fully in the shade before reassembling. If the mould is deep inside the coil or the cross-flow fan, those areas aren't accessible without disassembly. A professional sanitisation service can treat those components with the right products and is usually quick and inexpensive.
When the room is the problem, not the unit
Sometimes the unit is clean and the mould keeps returning because the room itself is too humid. In that case, ventilate daily, avoid drying laundry indoors, and use a dehumidifier on the worst days. If mould is a whole-home problem rather than just around the AC, take a look at our appliances section, where you'll find dehumidifiers that keep indoor humidity in the healthy 40 to 60 percent range.