A musty smell from an air conditioner often raises concern about mould. Moisture and organic build-up can be involved, especially in humid conditions, but an odour alone cannot identify the material or its source.
Why moisture is present
During cooling, the indoor coil becomes cold and moisture condenses from the air. The system is designed to collect and drain that water. Filters, coil surfaces, fan components and the drain area can also collect dust. Moisture plus organic material can support microbial growth.
Odours may also come from a dry drain trap, room furnishings, stored items, pests or air drawn from another space. Investigation should remain open to more than one cause.
Start with low-risk steps
Follow the manual to inspect and clean owner-accessible filters. Check for visible water, staining or a blocked drain outlet without dismantling the unit. Run fan or self-clean functions only as the manufacturer recommends; these features vary and are not substitutes for resolving persistent moisture.
Avoid spraying bleach, fragrance, household disinfectant or unapproved foam through an assembled unit. Products can affect coatings, plastics, electrics and indoor exposure. Never mix cleaning chemicals.
When a detailed clean may help
If odour returns quickly after filter care, or visible build-up exists beyond owner-accessible areas, a scoped internal clean may be appropriate. Ask which components will be accessed, how the room and electronics will be protected, what products are used and how wastewater will be managed.
Cleaning may need to include drainage checks. Treating a surface without correcting retained water can allow the issue to recur.
Think about the whole room
Indoor air quality is influenced by ventilation, outdoor air, humidity, cooking, cleaning products, furnishings and building moisture—not only the air conditioner. Keep the conditioned space reasonably dry, address leaks and use exhaust ventilation where appropriate.
People with health symptoms should seek guidance from a qualified health professional. An HVAC inspection can report equipment condition, but it should not make medical claims or identify a substance without suitable testing.
Last reviewed by Sunshine Coast Air Care editorial team.