That characteristic sour smell at switch-on comes from a damp biofilm on the evaporator and from dust in the ducts.
In cooling mode the evaporator in the indoor unit is the coldest object in the room. Water condenses out of the warm room air onto the cold metal — exactly as it does on a cold bottle in summer. That is condensation, and it is normal: it usually runs off through a tray and a drain hose to the outside.
The problem is that all the room's air passes through that same evaporator, carrying dust, skin particles, textile fibres and kitchen grease. The filter holds back most of it, but not all. The fine particles settle on the damp fins and in the tray.
The same happens in a ventilation system. A layer of dust builds up in the ducts over the years, and where the air is damp — in a bathroom extract, a kitchen duct, near a heat recovery unit — it becomes damp too and starts to smell. The grilles are only the visible part; the layer continues further into the duct.
Split systems with a long drain run and ducted units built into ceilings deserve a separate mention. There moisture can also build up outside the evaporator itself — in a sag in the run or in the insulation — and the smell then comes not from the grille but from the whole ceiling zone. In a case like that the source cannot be found without opening up the equipment.
Contributing factors that speed it up: a blocked drain hose leaving water standing in the tray; filters not washed for a long time; smoking in the room, which leaves a sticky deposit on the fins; and a unit that has not been used for a whole season and has stood all winter with damp deposits inside.
This is the most typical complaint: you switch it on, and for the first few minutes there is a sharp wave of smell in the room, after which it almost disappears.
Sometimes people describe the opposite variant too: the smell appears not at switch-on but a few minutes after switch-off. That points to the same cause from the other side — when the fan stops, the evaporator stays wet and moisture starts evaporating off it along with everything that has built up on the fins. The solution is the same in both cases.
The unit has to be dry when it is standing idle.
The explanation is simple. While the unit is switched off, the biofilm on the fins slowly dries out and accumulates volatile compounds. At switch-on the fan immediately blows a large volume of air through that layer, and everything accumulated enters the room in one go. After a few minutes the concentration drops and the smell weakens — but the source remains.
A very useful habit follows from this. Before switching the air conditioner off, run it for five to fifteen minutes in fan-only mode, without cooling. The evaporator has time to dry, and the damp environment that the biofilm grows in no longer exists all night. Many newer units have this function built in, called a dry or self-clean mode. Done regularly, it stops the smell arising in the first place in a large proportion of cases.
Let us start with what we do not do, so there is no misunderstanding. We do not dismantle air conditioners, we do not take off the evaporator housing, we do not clean the drainage system and we do not service or recharge units. That is the job of a climate technician, and if the biofilm is thick or the drain is blocked, that is who you need, not us.
Our part is the room and whatever is accessible without dismantling equipment. That means surfaces, textiles, soft furniture, ventilation grilles and the length of duct you can reach by taking off a grille — and the room air itself.
| Task | Who does it |
|---|---|
| Washing the filters | Owner, once a month in season |
| Dry mode before switching off | Owner, every time |
| Cleaning the evaporator with dismantling | Climate technician |
| Cleaning the drainage system | Climate technician |
| Treating the ventilation grilles | Us |
| Treating the accessible part of the duct | Us |
| Ozone treatment of the room and textiles | Us |
| Cleaning the shared riser | Property manager |
In practice this means: if the smell is strong and comes directly from the indoor unit, the correct order is the technician's cleaning first, our treatment afterwards. Ozone treatment before that will remove the smell from the room, but the evaporator stays as it is and within a few days it all starts again. We say so up front.
If the smell comes from the ventilation grilles rather than from the air conditioner, the situation is different — then treating the grilles and the accessible part of the duct along with ozone-treating the room is often enough. But first you have to know whether the duct is drawing at all: read Ventilation in a flat on that. If there is no draught, the treatment will be short-lived.
If the smell comes directly from the indoor unit, the correct order is the technician's cleaning first, our treatment afterwards — otherwise the evaporator stays as it is and within a few days it all starts again.
With ventilation ducts there is one more limit that is fair to state up front. All that is accessible inside a flat is the grille and a short section of duct behind it. The rest of the riser runs through the building structure and can only be reached through cleaning organised by the property manager. If the layer of dust and grease runs the length of the riser, treating the grille will improve things only partly and only for a while.
Another situation worth separating out: sometimes people think the air conditioner smells when in fact it is only spreading a smell that is already in the room. The unit draws air in from the room and blows it back at high speed, so a faint background smell — from a damp spot behind furniture, from an old carpet or from the ventilation — suddenly becomes pronounced.
The sign that points to this: the smell does not fade after the first few minutes but continues evenly the whole time the unit runs. In that case cleaning by a technician will change nothing, and it is the room itself that has to be dealt with.
If the smell does not fade after the first few minutes but continues evenly the whole time the unit runs, the equipment is only spreading it — and it is the room itself that has to be dealt with.
In offices and commercial premises the question is often broader, because there is centralised ventilation with ducts shared between several rooms. One room can then smell because of something happening at a completely different end of the floor — in a kitchen or a toilet area, for instance. In properties like that we first establish the layout of the system and only then plan the treatment, because treating one room while the source is in another makes no sense.
What to do yourself regularly:
We establish whether the smell comes from the unit, from the grilles or from the room itself.
If cleaning by a technician is needed, we say so before the treatment.
We treat the ventilation grilles and the accessible part of the duct.
We treat the room, the textiles and the soft furniture the smell has soaked into.
We air the premises and tell you what to do so the smell does not come back.
For more on the methods themselves, read Ozone treatment and Air disinfection. If the smell is distinctly musty and there are also signs of damp in the room, see Musty mould smell, and for the other types of smell, the Odour removal section.
Ozone treatment €1.20/m², minimum €60. If surface treatment is needed — after an inspection.
No. We do not dismantle equipment and we do not service it — that is a climate technician's job. We treat the room, the ventilation grilles and the accessible part of the duct, and that usually happens after the unit has been cleaned.
While the unit stands idle, volatile compounds build up on the evaporator. At switch-on the fan blows them into the room in one go. After that the concentration drops, but the source remains.
The smell in the room will go, but if there is a thick damp layer of deposits on the evaporator it will come back within a few days. In that case cleaning by a technician is needed first, and only then does treatment make sense.
Describe the situation — we will tell you whether treatment is needed at all, and which one.
Fill in the form — we will call you on the number you provide, ask a few clarifying questions and propose a solution.
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