In short: the clearest signs of mould in an air conditioner are a musty smell when it first starts up, dark specks or patches on the blades and vents, and symptoms that ease when you leave the house. Mould grows on the evaporator coil and in the drain pan, where moisture sits — not on the parts you can see from the front.
The five signs worth acting on
1. A musty smell in the first minute
This is the most reliable one. The unit sits idle, growth develops on the damp coil, and the first blast of air pushes that smell into the room. If it fades after a few minutes, that does not mean it has gone — it means you have stopped noticing it.
2. Dark specks on the blades or louvres
Look into the outlet with a torch. Black or dark green speckling on the barrel fan blades or around the louvres is growth, not dust. Dust is grey and even; mould is patchy and dark.
3. Symptoms that follow the aircon
A blocked nose, itchy eyes, a cough or a headache that starts when the unit runs and eases when you are out of the house is worth paying attention to. If anyone in the home has asthma or an allergy, take it more seriously rather than less.
4. Water where it should not be
Drips from the indoor unit or damp on the wall below usually mean the drain pan or drain line is blocked. A blocked drain means standing water, and standing water is exactly what mould needs.
5. Weaker airflow than it used to have
A coil coated in build-up restricts air. If the room takes longer to cool than it did last summer and the filters are clean, the problem is further inside.
Can I clean it myself?
You can and should rinse the filters — warm water, dried fully before they go back. Beyond that, the parts that hold the growth are the evaporator coil, the barrel fan and the drain line, and reaching them means taking the unit apart. Spraying supermarket cleaner through the front vents mostly moves the problem around and can push moisture into places you do not want it.
What a professional clean removes
The unit comes apart, and the filters, evaporator coil, barrel fan, drain pan and drain line are all cleaned with eco-friendly, non-toxic products, then sanitised, dried and tested. That is what takes the smell away for good rather than for a fortnight.
Why mould grows inside a split system in the first place
A split system cools by pulling room air across a coil that is colder than the air. Water condenses out of that air onto the coil, runs down into the drain pan and out through the drain line. That is the unit working correctly.
The problem is what happens when it stops. The coil stays damp, the pan holds a film of water, and the dust that got past the filters is sitting on both. Dark, damp and organic is the combination mould needs, and a wall unit provides all three every time you switch it off.
Where it actually grows
Almost every homeowner looks at the wrong part. Working from the front of the unit inwards:
- The louvres and front cover. What you can see. Usually the least affected.
- The filters. Catch dust, easy to rinse, and the part people do clean.
- The evaporator coil. Behind the filters, permanently damp in summer. This is where most of it is.
- The barrel fan. The long cylinder that pushes the air out. Growth here goes straight into the room every time the unit runs.
- The drain pan and drain line. Standing water. Blockages here cause both the smell and the drips.
Nothing you can reach through the front vents touches the last three.
Mould, dust or something else
Not every dark mark is mould, and it is worth knowing the difference before you book anything.
- Dust is grey, even, and sits in a soft layer. It wipes off dry.
- Mould is patchy and dark, often black or dark green, and it clusters where moisture sits rather than spreading evenly.
- Nicotine and cooking residue is a yellow-brown film with a stale rather than musty smell, common in units near a kitchen.
- Rust on the coil fins is orange and structural. It is a different conversation, and worth mentioning when you call.
What makes it worse in Sydney homes
- Humidity. Sydney summers are humid, so there is more water condensing on the coil than in a dry climate.
- Seasonal use. A unit used only in summer spends most of the year switched off with a damp coil inside it.
- Coastal air. Closer to the water, things stay damp longer and dry out more slowly.
- Closed-up homes. Running the aircon means the windows are shut, so the same air keeps circulating past the coil.
What not to do
- Do not spray bleach into the vents. It damages the coil fins and the smell comes back once it evaporates.
- Do not run it on high to blow it out. That pushes more of it into the room, not less.
- Do not put wet filters back in. They need to be fully dry, or you have added water to the problem.
- Do not ignore drips. Water from the indoor unit means a blocked drain, and a blocked drain means the water has nowhere to go.
How to stop it coming back
Once the unit has been properly cleaned, three habits keep it that way:
- Run the fan for ten minutes before switching off. Most remotes have a fan or dry mode. It dries the coil instead of leaving it wet for months.
- Rinse the filters every few weeks. Warm water, dried fully.
- Book the deep clean once a year. Twice if you have pets, if the unit runs year-round, or if anyone in the house has asthma or allergies.
A note on health
We clean air conditioners, we are not medical professionals. If someone in the house has ongoing symptoms, that is a conversation for a GP rather than something to diagnose from a wall unit. What we can say is that a coil coated in growth is not something you want the air of your home passing through, and cleaning it is straightforward.
Getting it sorted
A split system deep clean is $249 + GST, fixed before we start. We cover Sydney metro, seven days a week, 7:00am to 7:00pm, and can usually get to you the same week. Call 0483 928 714 or request a quote on our aircon cleaning page. Not sure how often yours needs doing? See how often to clean an air conditioner.
Dealing with mould in your aircon? See aircon cleaning in North Shore, Northern Beaches, Eastern Suburbs, Sydney CBD & Inner South, Inner West, St George & Bankstown, Sutherland Shire, Ryde, Parramatta, Hills District, Blacktown, Penrith, South West Sydney and Campbelltown & Camden.
Frequently asked questions
What are the signs of mould in an air conditioner?
A musty smell in the first minute after it starts, dark patchy specks on the barrel fan blades or louvres, water dripping from the indoor unit, weaker airflow than it used to have, and symptoms that ease when you are out of the house.
Where does mould grow inside a split system?
On the evaporator coil, the barrel fan and in the drain pan. All three stay damp, and none of them can be reached through the front vents.
Can I clean the mould out myself?
You can and should rinse the filters in warm water and dry them fully. Beyond that the growth sits on parts that need the unit taken apart. Spraying cleaner through the front vents mostly moves the problem around and can push moisture where you do not want it.
Is it mould or just dust?
Dust is grey, even, and wipes off dry. Mould is patchy and dark, often black or dark green, and clusters where moisture sits. A yellow-brown film with a stale smell is usually cooking or nicotine residue rather than mould.
How do I stop it coming back?
Run the fan or dry mode for ten minutes before switching the unit off so the coil dries, rinse the filters every few weeks, and book a deep clean once a year, or every six months with pets or year-round use.
