You switch the unit on after months of standing idle and, in the first few seconds, a musty smell comes out. Forgotten laundry in the basket. A closed-up cellar. After a while it eases, but it comes back every time the unit starts.
It is not the house or the air outside. It comes from the unit itself, and the cause is almost always the same.
Where the smell comes from
An air conditioner that is cooling does two things at once: it lowers the temperature of the air and removes moisture from it. That moisture condenses on the heat exchanger fins, runs down into the drain tray and leaves through the drain pipe.
The problem is what gets left behind on the way. The air entering carries dust, pollen, fabric fibres, kitchen grease and spores that float in the air of any home. That material settles on the fins and the fan barrel, and finds exactly the conditions it needs to take hold: a damp surface, permanent darkness and a mild temperature.
The result is a dark, sticky layer — biofilm — where fungi such as Aspergillus, Penicillium and Cladosporium thrive. They are what produce the volatile compounds responsible for the smell. When the fan starts, it pushes them into the room.
Why a spray does not shift it
The disinfectant sprays sold in supermarkets act on the surface they can reach — the grille, the front fins, perhaps the first few centimetres of the heat exchanger. The biofilm is not there. It is deep between the fins, on the fan barrel and in the tray, and no aerosol reaches that far.
The same goes for washing the filters. Filters should be washed, and regularly, but they are there to catch the coarse dust. Everything that gets past them — and a lot does — ends up on the components behind.
The only way to remove biofilm is mechanical: open the unit, take apart what has to be taken apart and wash each component with water and a specific product.
The three places the smell hides
The heat exchanger. A block of tightly packed aluminium fins. Dirt gets in between them and stays trapped. It is washed fin by fin, with enough pressure to carry the dirt away and low enough not to bend the aluminium.
The fan barrel. The cylinder that pushes the air. It builds up a crust on the blades that, besides smelling, reduces airflow. It is removed and washed completely.
The drain tray and drainage. This is where the standing water is. When the drainage blocks, the tray fills and becomes the most active source of all. It is also the reason some units start to drip.
When the smell is a sign of something else
Not every smell comes from dirt. It is worth telling them apart:
| What you smell | Likely cause |
|---|---|
| Damp, cellar, forgotten laundry | Biofilm on the heat exchanger or standing water in the tray |
| Burning or hot plastic | Electrical component. Switch the unit off and call an engineer |
| Rotten egg or gas | Nothing to do with the equipment. Leave and contact the gas provider |
| Cigarettes or old grease | Particles held in the unit, common in rented homes and restaurants |
The first case is solved by cleaning. The second and third are not cleaning jobs and need another kind of intervention.
What to do in the meantime
There are two things that help and that anyone can do.
Wash the filters every two months during the season of use. Warm water, no harsh detergent, and dried in the shade before putting them back.
And use the drying function, if your unit has one — many models call it dry, or have a fan symbol that keeps running for a few minutes after switching off. It is there precisely to dry the heat exchanger and reduce the moisture left inside.
Neither removes biofilm that is already established. They slow the formation of the next layer.
After a deep clean
The smell goes the same day. Airflow increases noticeably — it is one of the things that most surprises anyone who has never had it done. And the unit goes back to cooling with the same intensity it had when new, without having to lower the thermostat by two or three degrees to compensate.
If your air conditioning smells musty, no diagnosis is needed. The unit needs to be opened.
We deep clean air conditioning in Almancil, Quinta do Lago, Vale do Lobo, Vilamoura, Quarteira, Loulé, Albufeira and Faro, with a price per unit and a 15-day guarantee.
