Why does my split AC smell musty/sour when it first starts, and how do I get rid of it?

Short answer: that musty start-up smell is almost always biofilm and mould growing on the cold evaporator coil and in the drain pan inside the indoor unit — the damp, dark, cool interior is a perfect breeding ground in the humid UAE. Filters are only the surface; the smell lives on the coil, so a filter wash alone won't clear it. A proper coil-and-drain clean does.

Why it smells worst at start-up: while off, moisture sits on the coil and in the pan and the growth releases odour; when the fan first kicks in it blows that smell into the room, then it fades as things dry and air moves.

How to actually fix it (source, not spray): 1. Clean/replace the filters — necessary but not sufficient. 2. Deep-clean the evaporator coil and disinfect the drain pan and condensate drain — this is where the biofilm is; it's the step that removes the smell. 3. Let the coil dry after cooling — running the fan-only mode for a few minutes before shutdown helps dry the coil so mould has less to grow on.

Health-wise: it's mould/bacteria you're breathing, so worth clearing rather than living with — especially for anyone with allergies or asthma.

Common mistake: masking it with sprays/tablets — they perfume the mould, they don't remove it, and it returns within days. This is a straightforward job for professional AC repair and maintenance — a proper coil-and-drain clean, and it usually stays gone with regular servicing (it's the same reason a dirty filter matters, just deeper). Anyone found the fan-dry-before-off trick actually helps keep it away?