Air Conditioner Smells Like Mould When You Turn It On — What Is Growing in Your Unit

A musty, damp, or mouldy smell from your air conditioner when it first starts up is one of the most common complaints we receive from homeowners across Sydney and Melbourne. It is not a smell you should get used to. It is mould, and it is being distributed through your home every time the system runs.

Here is where the mould is growing, why it grows, and what the fix involves.

Why Air Conditioners Grow Mould

The indoor unit of your air conditioner is a near-perfect environment for mould growth during periods of non-use. The evaporator coil is wet from condensation when the unit is running in cooling mode. The drain tray holds residual moisture. The air filter captures organic particles from your indoor air. When the system is turned off, that moisture sits on surfaces in a warm, dark, enclosed space.

In Sydney’s humid coastal climate and Melbourne’s variable autumn and spring conditions, this is particularly pronounced. Units that are run heavily in summer and then left off over winter accumulate the most mould growth in the off-season.

The Evaporator Coil

The most significant mould source in a split system or ducted unit is the evaporator coil. Mould spores from the indoor air attach to the damp coil surfaces and colonise between the fin rows. When you start the unit and air flows across the coil, you are blowing mould spores directly into your living space.

Cleaning the coil requires removing the front panel of the indoor unit and applying a specialist coil cleaning solution. This is not the same as cleaning the filter. Most homeowners clean the filter but never address the coil. A professional air conditioning service includes coil cleaning as standard.

The Drain Tray

Stagnant water in the condensate tray is a primary mould breeding site. Even when the drain line is flowing correctly, residual water sits in the tray between cooling cycles. In a unit that has not been serviced recently, this tray can accumulate a significant mould and bacteria colony.

Treating the drain tray with a diluted bleach solution and ensuring full drainage of the tray during each service visit addresses this issue. Some technicians install slow-release biocide tablets in the condensate tray to inhibit regrowth between services.

Ducted System Return Air Paths

In ducted systems, mould can grow not just in the indoor unit but in the return air plenum and the ductwork surfaces, particularly in areas of the duct where moisture has accumulated from condensation or a previous leak. Mould in ductwork is more problematic to address because the affected section needs to be physically cleaned or replaced.

For Sydney homes with ducted systems, our Sydney air conditioning team can carry out a full system inspection including duct assessment. Melbourne residents can book through our Melbourne air conditioning page.

Health Implications

Mould spores distributed through an air conditioning system are a genuine health concern for household members with asthma, allergies, or compromised immune systems. Stachybotrys and Cladosporium, common mould species found in air conditioning systems, are known respiratory irritants. Children and elderly residents are particularly vulnerable.

If household members are experiencing increased allergy symptoms, frequent coughing, or respiratory irritation that correlates with the air conditioning running, mould in the system is a plausible cause that should be investigated and addressed.

The Fix Is Straightforward

A professional air conditioning service that includes coil cleaning, drain tray disinfection, and filter replacement will address the mould in a single visit in most cases. For severely affected units with visible mould colonisation on the coil fins, a deeper clean using specialist products may require a second pass.

Running the system in fan-only mode for fifteen to twenty minutes after the last cooling use of the day allows the coil to dry slightly before the system is shut down. This simple habit significantly reduces mould growth between services.

Frequently Asked Questions

Q1: Is it safe to keep running an air conditioner that smells mouldy?
Running it briefly to confirm the smell is reasonable. Continuing to use a mouldy unit for weeks or months without servicing it means continuously distributing mould spores through your home. For households with asthmatic or allergy-prone members, this is a genuine health risk. Book a service and have the unit cleaned.

Q2: Can I clean the mould out of my air conditioner myself?
You can clean the filter and wipe down accessible surfaces inside the front panel. However, the evaporator coil between the fins is not accessible for effective manual cleaning without specialist equipment and chemical treatment. DIY attempts with household spray products can push mould deeper into the coil or damage the fins. A professional clean with the appropriate coil cleaning product and rinse is the correct approach.

Q3: How often should I service my air conditioner in Sydney or Melbourne to prevent mould?
Annual servicing is the minimum recommended frequency. In Sydney’s humid coastal suburbs, or in Melbourne homes where the unit is used heavily in summer and left off through winter, a service at the start of each cooling season is strongly recommended. Units in high-humidity areas such as near the coast or with poor indoor ventilation may benefit from six-monthly attention.

Q4: My unit smells like dirty socks, not mould. Is that different?
Dirty sock syndrome is a distinct but related phenomenon, particularly associated with heat pump systems. It occurs when certain bacteria, specifically Bacillus subtilis, colonise the damp evaporator coil. The smell is more pungent than typical mould and tends to be strongest when the unit first starts up. The cause and treatment are similar: coil cleaning with an appropriate antimicrobial agent. Some manufacturers also have updated firmware or operating cycles to help prevent it in affected models.

Q5: I just had my unit serviced but it still smells. Why?
If the unit was heavily colonised with mould, a single service may not eliminate all of it. A follow-up clean within four to six weeks may be needed. It is also worth asking specifically what the service included. Some basic services clean only the filter and check pressures without addressing the coil. Confirm with your technician that coil cleaning was part of the service before assuming the problem has been fully addressed.