Can the musty smell from my AC come from the upstairs neighbour?
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Titan, Liviu Rebreanu street, 11-storey block. 9,000 BTU Electrolux, 4 years old, in the bedroom. The upstairs neighbour's outdoor unit sits directly above mine and drips its condensate onto it. I blamed the musty smell at start-up on that. Does it make sense? Or is it my own unit?
What is really happening
Outdoor and indoor units exchange only refrigerant through closed copper pipes, never air. The bedroom air is recirculated exclusively by the indoor unit, and the smell is produced by biofilm on its cold fins and fan roller – the roller being the most fungus-laden component of the entire machine. The neighbour's condensate can at most leave limescale and algae stains on your outdoor casing, which does not smell indoors. Four bedroom years without sanitising are plenty for a healthy layer of biofilm.
- Deposits or moisture inside the unit.
- The condensate drainage route.
- Sources within the room, assessed separately.
- 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]
- When a contaminated air conditioner starts, airborne fungal concentration in the room can jump from 40–920 to 5,500–100,000 CFU/m³. [Indoor Air, 2023]
- Mould odour is treated by WHO and epidemiological studies as an indicator of active microbial growth, even when nothing is visible. [PLOS ONE, 2012]
- 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 indoor unit and show you the source – on the roller, with a torch.
- Collection cover, cleaning solution, pressure wash in place: evaporator, roller, pan, drain.
- Sanitising solution on the clean surfaces.
- Outdoors we wash both fins and casing – we also remove the neighbour's condensate stains as far as possible, without climbing the facade.
- See current rates on the Prices page.
What you can do yourself
- Talk to the neighbour about extending his condensate hose to the downpipe – not for the smell, but for the facade and for peace of mind.
- Your filters every two weeks, 'Fan' for 20 minutes before shut-off.
What NOT to do
- Do not pour bleach on the outdoor unit 'to kill the algae' – it runs down the facade and onto the balcony below.
- Do not block the outdoor grille with an improvised sheet-metal awning – you overheat the compressor.
Prevention
Annual sanitising of the indoor unit, with the outdoor washed in the same visit. Whatever the neighbour does above does not affect your indoor air.
When to call us
Bookings: 0764 012 943, Monday–Saturday 08:00–20:00. Titan is 20 minutes from us.
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Read also
- Bad smell from the air conditioner
- The air we breathe at home
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Sources (3)
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.