It smells on Dry (dehumidify) mode, not on Cool – what does that mean?

Editorial guide · Updated 18 September 2026

In short Short answer: on Dry the fan runs very slowly, so the air lingers longer over the wet evaporator and comes out more concentrated in the compounds the biofilm produces; on Cool the fast fan dilutes the same smell. You already have biofilm – Dry does not create it, it just makes it obvious.

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

Voluntari, new 4-storey block, 12,000 BTU Panasonic, 3 years old, living room. On Cool the air is fine. On Dry, in the evening, to pull the humidity out, within 10 minutes a stale smell arrives, like 'laundry dried indoors'. I read Dry is healthier for the unit. So why does it smell precisely on Dry?

Editorial explanation · Revizie AC

What is really happening

Dry mode cools the evaporator just as much but drops the fan roller to minimum speed so the air passes slowly and leaves as much water as possible on the fins. Low airflow means long contact time with the biofilm-covered surface and less room ventilation, so the volatile compounds (acids and alcohols produced by bacteria and fungi) reach your nose at a higher concentration. Same unit, same layer, different 'dose'. The indoor-laundry smell is typically bacterial – Dry is not bad for the unit, it is a very sensitive smell test.

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

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]
Dense bacterial biofilm (~11,000–47,000 CFU/cm²) dominated by Methylobacterium forms on aluminium heat exchangers. [Current Microbiology / PubMed Central, 2012]
Mould odour is treated by WHO and epidemiological studies as an indicator of active microbial growth, even when nothing is visible. [PLOS ONE, 2012]
The relative humidity range with the fewest adverse health effects is roughly 40–60%. [Environmental Health Perspectives / PubM, 1986]

What we do on site

We open and check the roller and evaporator – at 3 years, in a living room, usually a thin but continuous layer.
Collection cover; cleaning solution; pressure wash in place.
Sanitising solution.
Pan and drain cleaned – Dry produces a lot of condensate, the pan must be clean.
Outdoor unit, washed in place.
Test on Dry, with you, 10 minutes – if it does not smell on Dry, it is clean. See current rates on the Prices page.

What you can do yourself

Keep using Dry, it is useful on humid evenings; after Dry, 20 minutes on Fan.
Keep the living-room humidity between 40 and 60% (a cheap hygrometer helps).
Filters every two weeks.

What NOT to do

Do not avoid Dry 'so it does not smell' – the smell is a signal, not the cause.
Do not spray anything on the evaporator while it runs on Dry.

Prevention

Annual sanitising; Dry becomes your control test – when it starts to smell on Dry again, it is time.

When to call us

0764 012 943 or WhatsApp, Monday–Saturday 08:00–20:00, Voluntari.

Woman on the sofa noticing the unpleasant smell when switching the air conditioner on

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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.