It is one of the few maintenance jobs anyone can do without tools and without risk. It takes ten minutes per unit, should be done every two months during the season of use, and extends the life of the unit.
It is also the most overrated maintenance job there is. Let us go through the steps first and then explain why.
Step by step
1. Switch the unit off at the mains. The remote is not enough. Switch it off at the consumer unit or the plug. It is the only part of this process where there is real risk.
2. Open the front cover. Practically all wall-mounted units have a hinged cover that lifts from the bottom upwards, with two side catches. Lift slowly until you feel the stop. Do not force it.
3. Remove the filters. They are two plates of plastic mesh that slide up and then out. Note the position before taking them out — there are models where the left and right filters are not the same.
4. Vacuum the loose dust first. With the vacuum cleaner, on the side the air comes in. If you go straight to water, you turn the dust into mud and it gets much harder.
5. Wash with lukewarm water. From back to front, to push the dirt out instead of working it into the mesh. Lukewarm, not hot — above 40 °C some plastics deform. A neutral detergent helps if there is grease.
6. Dry in the shade, completely. Hours, not minutes. Never in direct sun, which yellows and makes the plastic brittle, nor with a hairdryer. A filter refitted damp is the quickest way to create the smell you are trying to avoid.
7. Refit and close. Each filter in its position. If the cover will not close first time, the filter is not seated properly — do not force it.
How often
| Situation | Wash the filters |
|---|---|
| Normal summer use | Every 2 months during the season |
| Continuous use, home or office | Once a month |
| Home with pets | Once a month |
| Kitchen or restaurant | Every 15 days |
| After building work in the property | Immediately, and repeat after a week |
Three common mistakes
Scrubbing with a hard brush. The filter mesh is fine and tears easily. A torn filter lets through everything it should hold back, straight onto the heat exchanger.
Using bleach or a strong degreaser. It attacks the plastic and leaves a residue that is then blown into the room.
Refitting before it is dry. The most common cause of the damp smell that appears straight after a "clean".
Where this stops being enough
Here is the part almost nobody explains.
The filter is there to hold back coarse dust. It is not a fine filtration system — it is a plastic mesh with a wide weave, designed to protect the unit from leaves, hair and larger particles. Everything finer than that passes through. And a lot passes through.
What passes through ends up in the three places behind: the heat exchanger fins, the fan barrel blades and the drain tray. Because those three components are permanently damp during use, the dust that reaches them does not stay loose — it binds into biofilm, a dark, sticky layer that does not come off with running water and is the real origin of the smell.
You can wash the filters every month for ten years and the biofilm will keep forming at exactly the same rate. Clean filters do not prevent it, because the problem is not on their side.
How to tell you have reached that point
Four signs, and one is enough:
- A damp smell in the first seconds after switching on
- Less air coming out of the grille than you remember
- Having to lower the thermostat more than before for the same cooling
- Water dripping from the unit
None of these is solved with filters, because none of them comes from the filters.
What a proper clean is
It is opening the unit. Removing the fan barrel, washing the heat exchanger fin by fin, cleaning the drain tray, clearing the drainage, applying a sanitising product made for air conditioning and testing operation before leaving.
It is one to two hours' work per unit, with water, collection equipment and protection of the space. It is not the same service, and no amount of filter washing replaces it.
Do the filters at home, every two months. It is worth it and it is easy. For the rest, once a year is enough — and we take care of that in Almancil, Quinta do Lago, Vale do Lobo, Vilamoura, Quarteira, Loulé, Albufeira and Faro.
