As the fierce Melbourne summer fades and crisp autumn air settles in, you switch your reverse-cycle unit from cooling to heating — and the first time it runs in weeks, a heavy, musty odour blows into the living room. That smell is the classic sign of mould in split system air conditioner units, and it means the machine is doing more than warming your home.
If your air conditioner smells like a damp basement, it is actively blowing microscopic fungi into your breathing zone. Many homeowners are surprised that their reliable unit is, in fact, a near-perfect incubator for fungal growth. Here is the professional guide to how mould forms inside a split system, the health risks involved, and exactly when to schedule a deep clean.
How Does Mould Form in a Split System Air Conditioner?
To understand why your unit harbours fungi, look at how it works in summer. When set to cool, the split system draws warm room air over a set of freezing-cold evaporator coils. As warm air meets the cold metal it forms condensation, and that liquid water drips down the coils into a drain pan before flowing outside. In other words, throughout the entire summer the inside of your air conditioner is dark and soaking wet.
At the same time, the unit pulls in airborne dust, dead skin cells and pet dander from the room. Even with standard dust filters fitted, microscopic organic particles slip through and stick to the wet coils and blower wheel (the spinning barrel fan that pushes air into the room). Now you have the complete recipe for an infestation: darkness, moisture and an organic food source. As autumn arrives and the unit sits idle for a few weeks, dormant spores rapidly colonise the internal plastic casing and components — and the moment you switch it on, it can spread those spores through every room. This is exactly why a musty smell from the air conditioner in autumn is such a reliable warning sign.
What makes this so easy to miss is that none of it happens where you can see it. The growth takes hold on the back of the coils and inside the barrel of the fan, hidden behind the plastic fascia, so the unit can look perfectly clean from the outside while harbouring a thriving colony within. By the time a visible black speckling appears on the louvres — the angled vents that direct the airflow — the contamination inside is usually well established. The cooling season effectively runs a months-long incubation that you only notice once the seasons turn and the heating kicks in.
Is Mould in a Split System Air Conditioner Dangerous?
Yes — it is a real concern for both your health and the efficiency of the unit. Unlike mould sitting statically on a wall, an infested split system acts as a distribution mechanism. When the fan runs, it disperses spores and microbial volatile organic compounds (MVOCs) — the gases responsible for that musty smell — throughout your whole home.
The National Asthma Council notes that mould is a common allergic trigger for asthma and is often found in areas with poor air circulation. If you or your children suffer unexplained coughing, sneezing or irritated eyes whenever the heater runs, the air conditioner is a prime suspect.
There is a financial cost too. A heavy build-up on the blower wheel restricts airflow, and the Australian Government’s energy advice confirms that keeping filters and the unit clean is essential for it to run efficiently — a clogged, mouldy system has to work harder, quietly pushing up your power bill.
Filter Cleaning vs Deep Cleaning: What’s the Difference?
The biggest mistake homeowners make is assuming that rinsing the removable plastic dust filters in the sink is enough to prevent mould. Washing those filters monthly is great for airflow, but the filters only catch large debris — they do not stop moisture, and they cannot prevent mould growing deep inside the machine.
True professional air conditioner cleaning goes far beyond the filters. It involves dismantling the plastic fascia, applying antimicrobial treatments to the evaporator coils, and pressure-washing the barrel fan where the heaviest, most stubborn mould growth hides out of sight.
The distinction matters because the two jobs target completely different problems. Filter washing manages airflow — it keeps large dust from choking the intake. A deep clean manages contamination — it removes the biological growth embedded on surfaces a filter never touches. Treating one as a substitute for the other is why so many homeowners scrub their filters religiously yet still get a musty blast every autumn: the source of the smell was never in the filter to begin with.
How Often Should You Deep-Clean a Split System?
For healthy indoor air, a professional deep clean should be carried out annually. The ideal time is during a seasonal transition — early autumn before you lean on heating, or spring before the intensive summer cooling period begins.
Homes with multiple pets, heavy smokers, or occupants with serious respiratory sensitivities may need a deep clean every six months. And if you are unsure whether spores have already spread beyond the unit, professional mould testing can analyse your indoor air and reveal the true extent of the contamination.
Conclusion
Your reverse-cycle split system is meant to keep your family comfortable, not to make them unwell. A musty smell, black spots on the louvres, or a fit of sneezing whenever the heater starts are all signs the unit needs attention rather than another season of being ignored.
If those signs sound familiar heading into the cooler months, clearing the unit before you rely on it is a sensible move, and booking a professional air conditioner deep clean is the most direct way to make sure the air it circulates is clean rather than contaminated.
Frequently Asked Questions (FAQs)
How often should I service my air-conditioning system?
Do basic upkeep monthly by removing and washing the plastic dust filters in warm soapy water. A comprehensive professional service — checking refrigerant levels, electrical components and internal mechanics — should be scheduled once a year so the unit runs efficiently and safely.
How often should I deep clean my split system?
At least once a year. A professional deep clean dismantles the unit to pressure-wash the evaporator coils, drain pan and blower wheel with antimicrobial treatments, which is the only effective way to remove the hidden mould and bacteria that filter washing cannot reach.
How does mould form in a split system air conditioner?
It forms from the condensation created when the unit runs in cooling mode. As warm room air passes over the freezing evaporator coils, water drips inside the unit, and when that moisture meets the organic dust and skin cells pulled in from the room, it creates a dark, damp, nutrient-rich environment where spores multiply rapidly.
Why does my air conditioner smell musty when I turn on the heat?
That musty smell is usually MVOCs — gases released by mould growing inside the unit over summer. After weeks sitting idle, the fungal growth has matured, so the first blast of air carries those odours and spores straight into the room.