Smell & air quality problems

A musty or sour smell when the aircon starts is the most common air quality complaint. It usually means mould or bacteria have colonised the evaporator coil or drain tray — Singapore's humidity makes this almost inevitable without regular cleaning. The smell is strongest at startup because stale moisture from the last cycle gets blown into the room.

If the smell persists even after a standard servicing, the coil may need a chemical wash rather than a water rinse. Standard servicing cleans the filters and drain but does not deep-clean the evaporator fins where mould embeds itself. A chemical wash strips the biofilm and restores fresh airflow.

Mist or fog blowing from the indoor unit is usually harmless — it happens when very humid air meets the cold evaporator, creating visible condensation. But if the room stays humid even while the aircon runs, the unit may be undersized, the fan speed too high for dehumidification, or there is an air leak in the room letting humid air in continuously.