Musty smell for a few minutes after switching the AC on, then it goes away – is that normal?

What is really happening

Inside the unit, the evaporator (the cold fins behind the filters) stays wet for hours while cooling, then remains damp and warm after shutdown. On that surface a living layer of bacteria and fungi – biofilm – grows and gives off volatile compounds that smell of mould or 'wet socks'. At start-up the cross-flow fan (the long roller that pushes the air) suddenly blows the stale overnight air, spores and biofilm fragments into the room, which is why the smell peaks in the first minutes. Clean filters do not help here: they stop dust, but the biofilm lives behind them.

Explanatory diagramOdours can have several sources

What we do on site

1. We open the cover, take out the filters and inspect the evaporator, fan and drain pan with a torch – the grey-black film on the roller is usually obvious.
2. We fit the collection cover (bag) under the indoor unit, sealed against the wall, so all the dirty water runs into a bucket, not onto your furniture.
3. We apply the alkaline cleaning solution to the evaporator and fan and let it work for a few minutes to loosen the biofilm.
4. We wash with controlled pressure, in place, no dismantling – fins, fan roller, drain pan and drain path.
5. We apply the second, sanitising solution to the now-clean surfaces; only now does it make sense, because there is no layer of dirt shielding what lies beneath.
6. We go out to the outdoor unit on the balcony or facade and wash the condenser free of poplar fluff and dust, also in place.
7. We dry, reassemble, start the unit and check the smell is gone and the water drains properly. See current rates on the Prices page.

What you can do yourself

• After each evening of cooling, leave the unit on 'Fan' (ventilation only) for 20–30 minutes before switching it off, to dry the evaporator.
• Keep washing the filters every two weeks – it will not fix the smell, but it slows down evaporator fouling.
• Air the room for 10 minutes in the morning with the unit off.

What NOT to do

Prevention

A unit used daily in summer rebuilds its biofilm within 1–2 seasons, so an annual sanitising in spring, before the heat, is the realistic rhythm. Between visits, drying on 'Fan' and clean filters keep the smell away for much longer.

When to call us

If the smell comes back at every start-up, the unit already has biofilm and will not clean itself – call 0764 012 943 (also WhatsApp), Monday–Saturday 08:00–20:00; in Titan we usually come within the same week.