In winter on heating mode the unit gives off a burnt/dusty smell – not in summer
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Popești-Leordeni, new block, 12,000 BTU Gree also used for heating in winter, since it is cheaper than our boiler. At the first warm starts in October it smells of hot dust, slightly burnt, for about an hour. In summer no smell at all. I read on a forum it is 'dust burning' – but where from, if the filters are clean?
What is really happening
In summer the evaporator is cold and wet, and any mould smell is diluted by condensate. In winter the same coil becomes a hot 'radiator': the dried biofilm, dust and fine grease trapped in it heat up and release volatile compounds, just like the dust on a radiator at first switch-on, except there is far more of it and it is mixed with biological material. Nothing is burning; it is thermal desorption. Clean filters stop new dust, but they do not clean what has already settled on the fins over the 3–4 summers you have had the unit.
- Deposits or moisture inside the unit.
- The condensate drainage route.
- Sources within the room, assessed separately.
- The 'dirty sock' or musty smell from an air conditioner comes from volatile organic compounds produced by microbial biofilm on the evaporator and blower. [Current Microbiology, 2000]
- 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]
- 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]
What we do on site
- Your ideal moment is autumn, before you switch to heating – not spring – so you do not bake the summer's layer.
- Collection cover, cleaning solution on evaporator and roller, pressure wash in place; both dust and biofilm come out.
- Sanitising solution on the clean surfaces.
- Pan and drain – cleaned, even though they produce no condensate in winter.
- Outdoor unit – on heating it works as the evaporator and collects ice and dust; we wash it in place so it does not strain.
- See current rates on the Prices page.
What you can do yourself
- At the season's first heating start, open the window for 15 minutes and run 'Fan' before 'Heat'.
- Filters washed and fully dry before the cold season.
- Wipe the dust off the louvres and the top intake grille.
What NOT to do
- Do not spray 'anti-dust' spray or silicone on the fins – it bakes and smells worse.
- Do not vacuum the fins directly – you bend them and never reach the roller.
- Do not ignore the smell if it turns clearly to 'burnt plastic' or the unit produces smoke – that is electrical, not cleaning; switch off at the breaker.
Prevention
Units used all year (cooling + heating) need sanitising once a year, ideally in autumn. With clean filters and a short airing at first start, the hot-dust smell does not come back.
When to call us
Call 0764 012 943 (WhatsApp) when you are about to switch to heating; we come beforehand so you start clean.
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Read also
- Bad smell from the air conditioner
- The air we breathe at home
- Symptoms of an uncleaned air conditioner
Sources (6)
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.