AC smells musty/sour for the first few minutes when it starts — what causes that and how do we stop it?

Short answer: that musty, sour, "dirty sock" smell on start-up is mould and bacteria growing on the AC's EVAPORATOR COIL and in the condensate/drain pan inside the indoor unit — not a mechanical fault, and it won't clear on its own. Here's why it happens: the cold evaporator coil constantly pulls moisture out of the air (that's condensation), and if the coil surface and drain pan are dirty, that moisture lingers on a layer of dust and biofilm and lets mould/bacteria grow. When the AC starts and air first blows across that damp, colonised coil, you smell it — then it fades as the coil dries a little in the airflow. In our climate the units run hard and humid, so it's common. The REAL fix is cleaning at source: a fresh filter helps and sometimes reduces it noticeably, but if it's established you need the COIL professionally cleaned and disinfected and the CONDENSATE DRAIN LINE/PAN flushed out (a clogged, standing-water drain pan is a mould factory). Masking it with sprays does nothing. To stop it coming BACK: regular servicing/coil cleaning, keep the filter clean, make sure the drain runs freely, and a UV-C light fitted at the coil is a genuinely effective preventer because it kills the microbes before they colonise. It's also worth sorting for air quality, not just the smell.

What's actually going on: - The smell = mould/bacteria on the evaporator coil + drain pan, thriving on condensation + dust/biofilm. - Why on start-up: first airflow across the damp, colonised coil releases the odour; it fades as it dries. - Why here: hard-running units + humidity = ideal conditions.

The real fix (at source): - Filter first: fit a clean filter — sometimes cuts it noticeably. But rarely the whole answer if it's established. - Professional coil clean + disinfect: clean the evaporator coil with proper HVAC-safe cleaner — this removes the odour at source. - Flush the condensate drain + pan: clear standing water/sludge (a blocked pan breeds mould — see condensate drain issues).

Stop it recurring: - Regular servicing/coil cleaning + clean filters + a free-running drain. - A UV-C light at the coil kills microbes before they grow back — a proven preventer. - Health-wise it's worth clearing (mould/bacteria in the airstream), not just for the smell.

Common mistake: masking it with air fresheners or just changing the filter and hoping — if it's on the coil/drain, only cleaning at source fixes it. Get the coil cleaned and drain flushed, then keep it up: an AC service and maintenance specialist can do the coil clean and set up prevention. Anyone fitted a UV light and had the smell stay gone?