Black spots on the unit's louvres – is it mould? How bad is it inside?
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Titan, Nicolae Grigorescu street, 1982 block, 12,000 BTU Gree, 4 years old, living room. I noticed black dots on the swinging louvre and along the bottom edge of the housing. I wiped them with a damp cloth and they came back within two weeks. If it looks like that outside, what does it look like inside?
What is really happening
Mould grows where a surface stays wet: the evaporator is permanently wet in operation, the fan roller receives spray and condensate, and the louvre is the last cold point where the humidity-saturated air exits. Spores spread from inside outwards, not the other way, so spots on the louvre mean a mature colony behind. Studies on split units show the fan roller is the most fungus-laden component, and the dominant genera – Cladosporium (the black one), Aspergillus, Alternaria – are precisely the allergenic ones. Wiping the louvre removes surface spores, but the 'factory' stays behind and repopulates within two weeks.
- A picture can document visible deposits.
- Colour and appearance do not reliably identify microorganisms.
- The appropriate assessment depends on the issue; a photo is not a laboratory test.
- The blower (cross-flow fan) is the most fungus-contaminated component of a split unit, more than the filter or the front of the evaporator. [Indoor Air, 2023]
- On filters of residential split units, Cladosporium (~32%), Aspergillus (~9%), Acremonium and Alternaria dominate – genera known as allergens. [Building and Environment, 2025]
- Mould does not grow below roughly 80% surface relative humidity, but the cold surfaces inside an air conditioner constantly exceed this threshold. [WHO / NCBI Bookshelf, 2009]
- Washing with alkaline detergent, brushing and high-pressure rinsing reduces fungi inside the unit by more than 99%. [Indoor Air, 2023]
What we do on site
- We open the housing and turn the roller by hand under a torch – we show you the layer on its blades so you can compare with the louvre.
- Sealed collection cover under the unit, film over sofa and TV.
- Cleaning solution on evaporator, roller, pan and inner housing, a few minutes' contact.
- Pressure wash, in place: evaporator fins front and back, roller blade by blade, pan, hose.
- Louvres and outlet grille washed separately at the sink.
- Sanitising solution on everything washed.
- Outdoor unit on the balcony, washed in place. See current rates on the Prices page.
What you can do yourself
- You can keep wiping the louvre with dish soap and water (unit off at the breaker), but now you know it is cosmetic.
- Filters every two weeks.
- 20–30 minutes on 'Fan' before shut-off in the evening.
What NOT to do
- Do not use bleach on the louvre – you discolour the plastic and inhale fumes; it does not fix the roller.
- Do not push a brush through the grille towards the roller – you break the blades, which are thin plastic.
- Do not ignore the spots 'because they are small'.
Prevention
Once cleaned, the unit stays spot-free for 1–2 seasons if you dry it before shut-off. Annual sanitising, ideally before the warm season.
When to call us
When you see mould on the louvre, that is the moment: 0764 012 943, Monday–Saturday 08:00–20:00.
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The information on this page is educational and based on cited sources; it does not replace medical advice. We do not do repairs, installation or refrigerant circuit work.