The evaporator has a whitish-grey layer, not black – mould or limescale?
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Drumul Taberei, Prelungirea Ghencea, 12,000 BTU Daikin, 6 years old. I removed the filters and the evaporator fins have a uniform whitish-grey layer, like a thin felt. Not black. Slight smell, not sharp. One friend says it is limescale from condensate, another says white mould. Which is it?
What is really happening
Water condensing on the evaporator is practically distilled, so it leaves no scale. What you see is fine dust that got past the filters, stuck to the wet surface and was 'cemented' by bacterial biofilm – Methylobacterium and relatives form a colourless slimy layer. Cladosporium (black) often comes later or stays on the roller where you are not looking. Effects: a felt-covered fin exchanges heat worse, the fan pulls harder, the unit cools less for the same consumption – evaporator studies show 5–7% airflow loss and higher fan power as fouling builds. And the 'slight' smell is the sign the biofilm is active.
- 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.
- Dense bacterial biofilm (~11,000–47,000 CFU/cm²) dominated by Methylobacterium forms on aluminium heat exchangers. [Current Microbiology / PubMed Central, 2012]
- A typical evaporator fouls enough to double its pressure drop in about 7.5 years, reducing airflow by 5–7% and raising fan power by 1–10%. [Lawrence Berkeley National Laboratory / , 2002]
- Filter efficiency and indoor particle concentration are the most important factors determining how fast the evaporator fouls. [Lawrence Berkeley National Laboratory / , 2002]
- Mould odour is treated by WHO and epidemiological studies as an indicator of active microbial growth, even when nothing is visible. [PLOS ONE, 2012]
What we do on site
- Collection cover; alkaline cleaning solution over the full depth of the fins, contact time.
- Pressure wash in place – the felt peels off in strips, visible in the bag.
- Fan roller: checked and washed, that is usually where the black part is.
- Pan and hose cleaned.
- Sanitising solution on evaporator, roller, pan.
- Outdoor unit, washed in place.
- See current rates on the Prices page.
What you can do yourself
- Filters: every two weeks; if you have the Daikin fine (green/black) filter, follow the manual's cycle.
- Vacuum the room more often – less dust in the air means a cleaner evaporator.
- 'Fan' before shut-off.
What NOT to do
- Do not scrape the fins with anything – they bend and you block airflow.
- Do not apply 'descaler' to the evaporator – it is not scale and the acid corrodes aluminium.
- Do not jet the fins directly without a cover – water reaches the circuit board.
Prevention
The layer rebuilds depending on room dust; at 6 years, annual sanitising and clean filters keep the felt away.
When to call us
0764 012 943 / WhatsApp, 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.