A musty aircon smell almost always means mould or bacteria are growing somewhere damp inside the unit: the indoor coil, the drain pan, or a clogged filter. A filter clean sometimes fixes it. If the smell is back within days, it needs a proper hygiene clean.
Key Takeaways
HealthyWA lists aircon condensate drain pans as a hidden spot for mould.
Mitsubishi Electric recommends cleaning split-system filters every two weeks in heavy use.
A smell that returns after a filter clean means the coil, fan, or drain needs a professional clean
What Causes a Musty Aircon Smell?
When your aircon cools, moisture condenses on the cold indoor coil and drips into a drain pan. Add dust and a dark, enclosed box, and you’ve got the damp, dark, poorly ventilated conditions mould needs, according to HealthyWA’s guide to mould and dampness. The fan blows that growth’s smell into the room. Not every bad smell is mould, though.
| Smell | Likely cause | What to do |
| Musty or earthy | Mould on the coil, fan, drain pan or filter | Clean the filter; book a clean if it returns |
| Burning or electrical | An overheating motor or wire | Switch off at the isolator; call a technician |
| Rotten egg | Usually a gas leak, not the aircon | Get outside; call the WA gas emergency line, 13 13 52 |
| Sharp or chemical | A possible electrical or refrigerant fault | Switch off; call a licensed technician |
What Can You Check Yourself?
Start with the filter. Mitsubishi Electric’s filter care guide says to clean it every two weeks in heavy use and dry it in the shade; if you put it back damp, it grows mould of its own. Here’s how to clean your air conditioner filter.
Before switching off, run fan mode for more than 10 minutes to dry the coil, as LG Australia’s advice on aircon odours suggests. What shouldn’t you do? Spray cleaners or water inside: Mitsubishi warns against solvents, insecticides, and water on the inner workings.
When Does It Need a Professional Clean?
If the smell is back within days, the mould is on the coil, fan, or drain pan, out of safe reach. HealthyWA advises that recurring or hidden mould may need professional help, and it’s one of the signs your air conditioner needs a professional service.
Our Hydro service for split systems includes a full hygiene hydro clean of the indoor coil and fan; Platinum adds an anti-bacterial drain pan tablet. Compare them to see what an air conditioning service includes. Drips too? A blocked drain often causes both; see why your aircon is leaking water.
Frequently Asked Questions
The first rush of air pushes built-up odour into the room. If it fades within minutes, a filter clean may do. If it’s there every time, the coil or drain pan needs cleaning.
Yes. Its return air filter, coil and drain pan can all hold mould. Clean the return air filter first; if the smell stays, have a technician inspect the indoor unit and drain.
Clean filters and run fan mode before switching off. The US EPA’s guide to mould and moisture recommends keeping indoor humidity below 60 percent. An annual service catches growth before you smell it.
Musty Smell That Won’t Shift? We’ll Clean It Out
WestOz Trades’ refrigeration mechanics clean split and ducted systems across Perth, with a photo condition report after every service. Book an air conditioning service in Perth or talk to our team.
