We were away for 3 weeks and on our return the AC smells terrible

Editorial guide · Updated 18 September 2026

In short Short answer: you switched it off wet, in high summer, in a closed flat at 30 °C – you built a miniature greenhouse. Over 3 weeks the biofilm on the evaporator and pan had perfect conditions to grow, and at first start the unit blew all the spores into the room. That it 'calmed down' is your nose adapting, not the unit cleaning itself.

This page explains a situation in an editorial format. It is not an open discussion board or a verified customer review. Contact us on WhatsApp about your own appliance.

Situation discussed

Drumul Taberei, Valea Ialomiței street, 2-room flat, 12,000 BTU Whirlpool, 3 years old. I switched it off straight from the remote before the holiday, in the middle of a heatwave. On our return, first start: a mould smell so strong we opened all the windows. I let it run for a day and it calmed down, but it is still there. What happened in those 3 weeks?

Editorial explanation · Revizie AC

What is really happening

An evaporator switched off after cooling stays wet; surface humidity on the fins is above 80%, the threshold at which mould can grow, and it stays there for a long time in a closed flat. The pan keeps a layer of standing water at 28–30 °C – an incubator for bacteria. Studies on contaminated units show that at start-up the spore concentration in the room can jump from a few hundred to tens of thousands of units per cubic metre, exactly the 'wave' you felt. Running it for a day diluted the peak, but the source is unchanged.

Odours can have several sources — Deposits or moisture inside the unit.
The condensate drainage route.
Sources within the room, assessed separately.

What the studies say

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]
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]
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]
Manufacturers recommend running the unit in fan-only mode for several hours to dry the interior before long periods of non-use, precisely to prevent odour and mould. [Daikin Europe, 2020]

What we do on site

Collection bag under the unit, inspection and photos.
Cleaning solution on evaporator and roller, pressure wash in place; on units left wet the bag water comes out green-black – do not be alarmed.
Pan and drain hose cleaned and rinsed; we check no standing water remains.
Sanitising solution on the cleaned surfaces.
Outdoor unit on the balcony, washed in place.
Drying on 'Fan', smell test. See current rates on the Prices page.

What you can do yourself

Before any absence longer than 2–3 days: 2–3 hours on 'Fan' (or 'Dry', then 'Fan'), only then switch off. Manufacturers recommend exactly this in the manual.
If you can, leave a window on tilt or ask someone to air the flat once a week.
On return, first start with the window open for 15 minutes.

What NOT to do

Do not leave the unit running non-stop while you are away 'so it does not pick up smell' – it costs money and does not fix biofilm.
Do not run it on max 'Cool' on return to 'blow the smell out' – you send the spores into the room.
Do not put chlorine tablets in the pan.

Prevention

Golden rule: dry evaporator before shut-off, every time, and always before a holiday. Plus annual sanitising, in spring.

When to call us

If the smell persists after a day of running, it will not leave on its own: 0764 012 943, Monday–Saturday 08:00–20:00.

Ask your question Report an error

← Can the musty smell from my AC come from the upstairs neighbour?I bought an ozone generator for the AC smell – fine for two days, then back to square one →

Related threads

Editorial guide · Bad smell

After another company's 'sanitising', the unit has smelled of chemicals for a week

Explanations and useful checks for this situation: After another company's 'sanitising', the unit has smelled of chemicals for a week

Answered by technicianRead →

Editorial guide · Bad smell

Can the musty smell from my AC come from the upstairs neighbour?

Explanations and useful checks for this situation: Can the musty smell from my AC come from the upstairs neighbour?

Answered by technicianRead →

Editorial guide · Bad smell

I bought an ozone generator for the AC smell – fine for two days, then back to square one

Explanations and useful checks for this situation: I bought an ozone generator for the AC smell – fine for two days, then back to square one

Answered by technicianRead →

Editorial guide · Bad smell

Unit only 2 years old and already smelly – was I sold a dud?

Explanations and useful checks for this situation: Unit only 2 years old and already smelly – was I sold a dud?

Answered by technicianRead →

Editorial guide · Water and condensate

The hygrometer shows 65% with the unit on – should it not drop?

Explanations and useful checks for this situation: The hygrometer shows 65% with the unit on – should it not drop?

Answered by technicianRead →

Editorial guide · Cleaning

'Full dismantling' clean or in-place? One company says only dismantled is a proper clean

Explanations and useful checks for this situation: 'Full dismantling' clean or in-place? One company says only dismantled is a proper clean

Answered by technicianRead →

Read also

Sources (7)

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.