Why does the AC still smell mouldy after the filter has been cleaned?
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Hong Kong's weather is humid and AC use is frequent, so mouldy smells are not uncommon. Some people have already cleaned the filter, but still smell damp, mouldy or stuffy air when the AC is on. This does not necessarily mean the AC is broken, and it is not always because the cleaning was not done properly. The filter is only one part of the AC, and the source of the smell may be hidden deeper inside.
Only surface dust on the filter is handled
Filters mainly block dust, hair and particles in the air. Regular filter cleaning helps maintain basic airflow and reduce surface dust. However, the filter cannot completely remove dirt inside the air conditioner. If the AC has been used for some time, moisture and dust may build up on the evaporator, blower wheel, air outlet and drainage areas. Even if the filter looks clean, odour may still come out when the unit is switched on.
Internal moisture may cause mouldy odours
Air conditioners produce condensation during operation, and some parts inside the unit can easily stay damp. In Hong Kong, where humidity is high, if moisture cannot disperse after shutdown and dust and dirt remain, musty odours may gradually appear. These odours are usually more obvious when the unit is first switched on, and may fade after a while, but return the next time it is started. If this keeps happening, it is worth checking whether there are dampness and dirt buildup issues inside the unit.
Black marks at the air outlet are worth noting
If the filter has been cleaned but black spots, mould stains or dust remain on the air outlet, fan blades or edges, the mouldy smell may be related to these areas. The air outlet is where cool air is blown out; if dirt builds up nearby, odours can easily be carried into the room with the cold air. Surface black spots can be wiped off first, but if the dirt has spread deeper, cleaning the outer layer alone may not be enough.
Dirt build-up on the fan drum may be one possible cause
The fan drum sends cool air out of the room and is one of the parts inside the AC that most easily collects dust. After long-term use, dust, moisture, and grime may stick to its surface. When the fan drum rotates, odours may be carried out with the airflow, so even if the filter has already been cleaned, a musty smell may still remain. If there is also weak airflow, a noticeable dusty smell, or black spots at the outlet, you may wish to consider whether deeper cleaning is needed.
Hidden dirt on the evaporator is not easy to notice
The evaporator is an important internal part of the air conditioner and is exposed for long periods to condensation water and airborne particles. If dirt builds up on the surface, it may cause a damp or mouldy smell. This area is usually not easy to clean yourself, and external visual inspection alone may not be enough to judge the condition. If the filter has already been cleaned but the smell persists, the evaporator is one possible area to check.
The drain outlet may have an odour
Condensation from the AC needs to be drained through the drain pan and drain pipe. If there is dirt, standing water or poor drainage near the drain outlet, drain pan or drain pipe, it may cause a damp smell. This kind of smell can sometimes mix with a mouldy smell, especially when the unit is first switched on or on more humid days. If the AC is also dripping, making unusual water sounds, or leaving water marks near the unit, it should not be treated as just a normal odour issue.
The room environment can also affect odors
Musty AC smell does not always come from the air conditioner itself. If a room stays damp for a long time, is poorly ventilated, or nearby curtains, carpets, mattresses or fabric furniture absorb odours, the smell may become more noticeable when the AC is running. In bedrooms, studies, or spaces with little window opening, poor air circulation makes musty smells linger more easily. When dealing with AC odours, also check room humidity and the cleanliness of nearby items.
If there is still a smell after filter cleaning, observe first
If there is still a slight smell for a short time after cleaning the filter, first check whether the airflow, dripping, black spots and sound of the air conditioner are normal. If a mouldy smell continues to appear, or if you smell it every time the unit is switched on, the filter may not be the only issue. In that case, continuing to clean only the filter may not improve deep-seated dirt or a moisture source, and further inspection of the inside of the air conditioner may be needed.
Routine maintenance can reduce smell build-up
You can clean the filter regularly, keep the area around the air outlet clean, and avoid piling up clutter near the air conditioner. After use, if the room is still humid, keep it moderately ventilated to reduce the damp feeling. Routine maintenance may not completely remove dirt inside the unit, but it can reduce long-term buildup of dust and moisture and keep the air conditioner feeling fresher in use.
Seek help from a professional team
If the filter has been cleaned but there is still a mouldy smell, or dripping and increased noise are also present, it is recommended to seek help from a professional team for inspection. A professional cleaning team will check the filter, blower wheel, evaporator, drip tray and drainage points to identify the source of the dirt.
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