Mould on the wall under the indoor unit – from the unit or from the building?
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Berceni, Giurgiului road, 1976 block, top floor. Under the air conditioner (Midea 12,000, 5 years old) a mould patch has appeared on the wall, about 20 cm, right under the left edge of the unit. It is an exterior wall. No visible dripping. The building manager says it comes from the roof terrace. How do I find out?
What is really happening
The drain pan collects litres of water a day in summer. When biological sludge builds up in it (the pan is the most bacterially loaded spot in the unit), the water level rises a few millimetres and, with even a slight tilt, water spills over the pan edge and seeps down the wall behind the housing, where you cannot see it. It does not drip – it soaks. The exterior wall is colder, surface humidity exceeds 80%, and the spores coming out of the unit anyway find perfect ground. So you have two problems from the same source: the dirty pan and the spores in the unit.
- 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 condensate pan of household units had the highest bacterial load (~12,300 CFU/cm²), over 20 times the 500 CFU/cm² limit for public spaces. [Frontiers in Public Health / PubMed Cent, 2024]
- EPA requires condensate pans to be clean and drain lines unobstructed; standing water must be fixed immediately. [US EPA, 2024]
- 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]
- Mould spores are everywhere in air and dust and cannot be fully eliminated; the goal is moisture control and cleaning surfaces. [US EPA, 2024]
What we do on site
- We check the unit with a spirit level and the back of the housing with a torch – usually the water track is visible on the wall behind the unit.
- Collection cover; we clean the pan and hose until fully clear, with plenty of water.
- Cleaning solution on evaporator and roller, pressure wash in place.
- Sanitising solution on pan, evaporator, roller.
- Test with 1–2 litres of water poured over the evaporator – it must all exit via the hose, nothing on the wall.
- Outdoor unit on the facade, washed in place.
- If the unit is mounted out of level, we tell you – realigning is an installer's job, not ours. See current rates on the Prices page.
What you can do yourself
- The wall patch: after sanitising the unit, clean it with detergent and water, let it dry completely, then paint with anti-mould additive paint.
- Keep the room below 60% humidity.
- Check monthly, with your palm, whether the wall under the unit is cold and damp.
What NOT to do
- Do not paint over the patch before fixing the source – it returns within a month.
- Do not cut the housing or fit an improvised 'gutter' under the unit.
- Do not pour drain unblocker into the hose.
Prevention
A clean pan does not overflow. Annual sanitising and a simple water test once a season keep the wall dry.
When to call us
Call 0764 012 943, Monday–Saturday 08:00–20:00; send a WhatsApp photo of the patch and the unit so we come prepared.
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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.