The outdoor unit is green with algae/mould on the north side – does it matter?
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House in Voluntari, the outdoor unit of a 12,000 BTU Mitsubishi sits on the north wall in permanent shade, next to a hedge. It is green on the fins and the casing, a visible layer. Indoors the unit does not smell. Does that green get into the house or is it just ugly?
What is really happening
The two units are linked only by copper pipes carrying refrigerant; the outdoor air stays outdoors. Algae and mould on the outdoor fins are the result of permanent shade plus moisture from the hedge – they have no path to your lungs. What they do, along with dust and poplar fluff, is clog the gap between fins: the condenser cannot shed heat, pressure rises, the compressor works harder and hotter, and cooling capacity drops. Efficiency guidance recommends checking both coils annually for exactly this reason. And if the indoor unit 'does not smell' on a 12,000 BTU unit used in a house, it is still worth a look: smell shows up late.
- 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.
- Fouling of the outdoor condenser raises pressure drop and condensing pressure, reducing the split unit's capacity and efficiency. [Science and Technology for the Built Env, 2019]
- The evaporator and condenser coils accumulate dirt over time and should be checked every year and cleaned as necessary (US DOE). [US Department of Energy – Energy Saver, 2024]
- 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]
- 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
- Outdoors: we protect the circuit board, apply cleaning solution to the condenser fins, let it act, wash with controlled pressure from inside out, in place – green comes out first, then clear.
- Casing and outdoor fan, wiped and washed.
- Indoors: collection cover, cleaning solution, pressure wash of evaporator, roller and pan, then sanitising solution.
- We show you the roller inside – you decide whether 'does not smell' also means 'is clean'.
- See current rates on the Prices page.
What you can do yourself
- Trim the hedge to at least 50 cm from the unit on all sides so air circulates and the fins dry.
- In spring, brush poplar fluff off the grille with a soft brush, dry.
- Do not wrap the unit in airtight film in winter – it traps moisture; a ventilated awning is fine.
What NOT to do
- Do not wash the fins with a car pressure washer from 10 cm – you flatten them and block airflow for good.
- Do not pour bleach or algaecide on the unit – it ends up in the soil and the hedge, and the aluminium corrodes.
- Do not bend fins back with a screwdriver.
Prevention
The outdoor unit is washed once a year, together with the indoor one, in spring, after the poplar fluff has passed. With the hedge trimmed, the green comes back much more slowly.
When to call us
For houses with several units we schedule all in one visit: 0764 012 943, Monday–Saturday 08:00–20:00.
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Read also
Sources (6)
- www.tandfonline.com/doi/full/10.1080/23744731.2019.1605197
- www.researchgate.net/publication/233277421_Evaporator_Air-Side_Fouling_Effect_on_Performan
- www.energy.gov/energysaver/maintaining-your-air-conditioner
- www.osti.gov/servlets/purl/805122
- journals.plos.org/plosone/article?id=10.1371/journal.pone.0047526
- www.ncbi.nlm.nih.gov/books/NBK143940/
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.