A musty or sour smell when you turn on the AC is one of the most common complaints we get, especially during Pekanbaru's rainy season when humidity stays high for weeks at a time.
Mold and mildew on the evaporator
The evaporator coil stays cold and wet during operation — a perfect environment for mold to grow if it isn't cleaned regularly. This is the single most common cause of a musty smell, and it typically requires a proper indoor unit cleaning (not just a filter wipe) to fully resolve.
Dust and dirt trapped in the filter
A filter that hasn't been cleaned in months collects dust, skin cells, and in some cases pet dander, all of which can smell stale once air passes through it repeatedly.
A clogged drain line
Standing water in a blocked drain pan or line can develop a sour smell over time, similar to a stagnant puddle. If the odor is more "swampy" than "musty," this is often the culprit — and it usually comes with a slow drip as a second symptom.
Cigarette smoke or cooking odors
AC units recirculate room air, so smoke and strong cooking smells can get absorbed into the filter and internal components over time, then released again every time the unit runs.
How we fix it
A proper AC cleaning service covers both the indoor evaporator and outdoor unit, removing mold buildup and clearing the drain line — not just wiping the visible filter. This is exactly the "Cuci AC" service most customers with this complaint end up needing, and it typically resolves the smell completely in one visit.