Why does my air conditioning smell?

The mechanism is worth understanding because it explains both the fix and the prevention. Cooling condenses water onto the indoor coil, so the coil and the tray beneath it finish every session wet, in a dark enclosed space, at room temperature. Add the fine dust that gets past filters and you have a growth medium. The smell you notice at start-up is the fan pushing air across that colony.

Left alone it progresses: from a faint mustiness in the first minutes, to a persistent smell whenever the unit runs, to visible dark spotting on the louvres and, in bad cases, dripping from a tray whose drain the sludge has blocked. None of this harms the refrigeration circuit, but you are breathing the room's air through it, so it is worth taking seriously.

The fix is the deep clean that is central to a proper annual service: filters washed, the coil cleaned with the right treatment, the tray and drain line cleared and flow-tested. A service-grade clean removes the smell rather than masking it; the supermarket freshener sprays mask it briefly and add their own residue to the coil.

Prevention is a habit: after a long cooling session, especially in humid weather, run fan-only mode for twenty minutes to dry the coil before shutdown. Some modern units automate this as a self-drying or coil-dry function worth enabling. Different smells tell different stories, by the way: burning means switch off and call us; a drainy smell points at the condensate trap rather than the coil.

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