Why Mould in Your Aircon Is a Problem You Can't Ignore

That musty smell when your aircon first kicks on is not just unpleasant. It is usually a sign that mould in your aircon has already taken hold inside the unit. Mould growth inside air conditioners is one of the most common maintenance problems in Australian homes, and it is especially widespread in coastal cities like Sydney and Brisbane where humidity stays high for months at a time. This article covers the four things you need to know: what causes mould to grow, the health risks it creates, how to remove it safely and how to stop it coming back.

What Causes Mould to Grow Inside an Air Conditioner?

Mould needs three things to thrive: moisture, darkness and organic matter. An air conditioner's indoor unit provides all three in abundance, which is why mould is such a persistent problem even in well-maintained homes.

Moisture is the primary driver. Every time your aircon runs in cooling mode, the evaporator coil drops to a temperature well below the ambient air, causing water vapour to condense on its surface. That condensation drips into the drain pan and flows out through the drain line. If the drain pan sits with residual water between cycles, or if the drain line becomes partially blocked, standing moisture creates the perfect breeding ground for mould spores to germinate.

Darkness is the second factor. The interior of your indoor unit is sealed and almost completely dark when the system is off. Mould spores that land on damp surfaces inside the unit face no UV exposure and very little airflow to dry them out, so they establish quickly.

Organic matter is the third ingredient. Dust, skin cells, pet dander and airborne debris accumulate on the air filter and evaporator coil over time. This organic layer acts as a food source for mould colonies, allowing them to grow even when moisture levels are relatively low.

Australian homes are particularly susceptible because of the climate. Coastal and subtropical regions, including Sydney, Brisbane and the Gold Coast, regularly see relative humidity above 70% for extended periods. That ambient moisture makes it harder for internal components to dry out between uses, and it means more condensation forms on the coil during every cooling cycle.

Infrequent servicing accelerates the problem significantly. A clogged filter restricts airflow across the evaporator coil, causing the coil to run colder than it should and produce more condensation than normal. That extra moisture, combined with the thick layer of dust already on the filter, creates conditions where mould can spread from the filter to the coil and into the drain pan within a single season. For a practical walkthrough of what a proper clean involves, see our guide on how to clean your air conditioner.

The Most Common Places Mould Hides in Your Unit

Mould rarely stays in one spot. Once it establishes in a damp area, spores spread through the airflow to other surfaces inside the unit. These are the four locations where our technicians find mould most often:

• The air filter. The filter is the first point of contact for airborne dust and spores. A dirty filter holds moisture and provides a ready food source, making it the most common starting point for mould growth. Filters should be rinsed every two to four weeks during heavy use.
• The evaporator coil. Sitting directly behind the filter, the coil is cold and wet during operation. Dust that bypasses a clogged filter coats the coil fins, and mould colonies establish in the gaps between fins where cleaning is difficult without professional tools.
• The drain pan and drain line. The drain pan collects condensation from the coil. If the drain line is partially blocked by algae or debris, water pools in the pan and mould spreads across the pan surface and up into the coil above it.
• The internal fan blades (barrel fan or blower wheel). The blower wheel sits downstream of the coil and circulates air through the room. Mould on the blower wheel is particularly problematic because every time the fan runs, it blasts spores directly into your living space.

Ducted systems add another layer of complexity. Mould can establish not just in the indoor unit but throughout the ductwork itself, particularly in flexible ducts that sag and collect condensation. Because ductwork is largely hidden inside ceilings and walls, a professional inspection is the only reliable way to assess the full extent of any mould problem in a ducted system.

Health Risks of Mould in Aircon Units

Mould growing inside an air conditioner is a genuine health concern because the unit actively blows air across the contaminated surfaces and distributes spores throughout your home every time it runs. The most common symptoms are respiratory irritation including coughing, wheezing and shortness of breath, along with allergic reactions such as sneezing, itchy or watering eyes and skin rashes. For most healthy adults these symptoms are unpleasant but manageable, and they typically ease once the mould source is removed. The good news is that the risks are entirely preventable with regular maintenance. For a broader look at how your home's air quality affects your health, see our article on indoor air quality.

Vulnerable groups face a higher level of risk. Children, the elderly and people with asthma, hay fever or compromised immune systems are more likely to experience severe reactions to airborne mould spores. For someone with asthma, a mould-contaminated aircon can trigger frequent attacks that seem to have no obvious cause, particularly because the connection between the unit and the symptoms is not always obvious.

Some mould species produce mycotoxins, which are toxic compounds that can cause more serious symptoms with prolonged exposure. These include persistent headaches, fatigue, sinus congestion and in severe cases neurological effects. The risk of mycotoxin exposure is highest when mould has been present for an extended period and the unit is running regularly, as this maximises the volume of contaminated air circulating through the home.

The key point is proportion. A small amount of surface mould on a filter that gets cleaned promptly is a minor issue. Mould that has spread to the evaporator coil or blower wheel and gone unaddressed for months is a more serious problem, particularly for anyone in the household with a respiratory condition.

How to Get Rid of Mould in Your Aircon

Removing mould from your aircon starts with understanding what you can safely handle yourself and what needs a professional. DIY cleaning is effective for surface-level mould on accessible parts like the filter, louvres and front panel. Anything deeper inside the unit, particularly the evaporator coil, drain pan or blower wheel, requires specialised equipment and should not be attempted without proper training. Follow these steps in order.

1. Turn off the unit and isolate power. Switch the aircon off at the wall controller and turn off the circuit breaker or isolator switch for the indoor unit. Never clean any part of the unit while it is powered on.
2. Remove and inspect the air filter. Most split system filters slide out from behind the front panel without tools. Hold the filter up to the light. If it is grey, dusty or shows visible dark spots, it needs cleaning. Rinse it under warm running water, allow it to dry completely in the shade and reinstall it. If the filter is damaged or heavily discoloured, replace it rather than cleaning it.
3. Wipe down accessible surfaces. Mix a solution of equal parts white vinegar and water, or use a mild detergent diluted in water. Use a soft cloth or sponge to wipe the louvres, front panel and any plastic surfaces you can reach without disassembling the unit. Avoid spraying liquid directly into the unit. Dry all surfaces with a clean cloth when finished.
4. Run fan-only mode for 20 to 30 minutes. After cleaning, set the unit to fan mode (not cooling) and run it for at least 20 minutes. This circulates air through the interior and helps dry out any residual moisture from your cleaning, reducing the chance of mould returning quickly.
5. Assess whether the problem goes deeper. After completing the steps above, run the unit in cooling mode for 10 minutes. If a musty smell persists, or if you noticed visible mould inside the unit during your inspection, the mould has spread beyond what DIY cleaning can address. At this point, book a professional service.

When to Call a Professional Instead of DIYing It

There are four clear signs that mould in your aircon has gone beyond a DIY fix. If any of these apply, stop cleaning and call a technician.

• A musty smell persists after cleaning. If the odour returns within a day or two of cleaning the filter and surfaces, mould is present deeper in the unit, most likely on the evaporator coil or in the drain pan.
• Visible mould on the coil or inside the casing. If you can see dark spots on the coil fins or inside the unit body, the contamination is beyond what household products can treat effectively.
• Mould returns within weeks of cleaning. Recurring mould after a surface clean usually means the root colony is on the coil or blower wheel and is continuously re-seeding the filter and louvres.
• You have a ducted system with a musty smell. Mould in ductwork cannot be addressed without professional inspection equipment. Contaminated flexible ducts may need to be replaced entirely.

Professional cleaning uses high-pressure coil cleaning equipment, anti-microbial treatments and purpose-built blower wheel brushes that are simply not available to homeowners. These treatments reach the full surface area of the coil fins and blower wheel, kill the mould colony at its source and leave a protective residue that slows regrowth. Household vinegar and detergent solutions, while useful for surface cleaning, cannot penetrate the coil fin stack or reach the back of the blower wheel.

Frozone Air's professional air conditioning cleaning service covers a full internal clean of the evaporator coil, blower wheel, drain pan and drain line, along with an anti-microbial treatment to address mould at the source. If your unit has a persistent mould problem or has not been professionally cleaned in the past 12 months, this is the most effective way to resolve it properly.

How to Prevent Mould in Your Aircon Coming Back

Preventing mould from returning is mostly about managing moisture. Once you have cleaned the unit, a few consistent habits will keep conditions inside the aircon dry enough that mould spores cannot establish between services. Here is what makes the biggest difference.

• Run fan-only mode for 10 to 15 minutes after cooling. When you finish a cooling session, switch the unit to fan mode before turning it off. This circulates dry air across the evaporator coil and drain pan, evaporating residual condensation before it has a chance to sit overnight. Many people skip this step, and it is one of the simplest things you can do to reduce moisture build-up.
• Clean or replace filters every four to six weeks during heavy use. A clogged filter restricts airflow, forces the coil to run colder and produces more condensation than normal. During summer, check your filter at least once a month. A quick rinse under warm water and a full dry in the shade takes less than 20 minutes.
• Book a professional service at least once a year. Ideally, schedule this before summer so the unit is clean and ready for its heaviest workload. A professional service reaches the coil fins, blower wheel and drain pan in ways that DIY cleaning cannot.
• Keep the area around the indoor unit well-ventilated. Avoid stacking furniture or curtains directly against the unit. Restricted airflow around the return air grille means the unit draws in more stagnant, humid air, which accelerates moisture build-up inside.
• Consider a unit with a self-cleaning or auto-dry function. Some modern split systems automate the drying process for you. Units like the Daikin Cora (for example the FTXM25Y, 2.5kW, from approx. $1,099) include an auto-dry mode that runs the fan after each cooling cycle to dry out the coil. The Mitsubishi Electric MSZ-AP series (for example the MSZ-AP25VGD, 2.5kW, from approx. $1,149) offers a similar self-clean function that helps reduce moisture build-up between services. The Fujitsu ASTG series (for example the ASTG09KMCA, 2.5kW, from approx. $999) also includes a filter-cleaning indicator, which takes the guesswork out of knowing when maintenance is due.

One cause of recurring mould that often gets overlooked is a partially blocked drain line. If the drain line is slow or obstructed, water pools in the drain pan rather than draining away, and mould re-establishes within weeks of a clean. If you notice water dripping from your indoor unit or pooling around the base, that is a strong sign the drain line needs attention. Our article on blocked drain line causing water leaks covers the causes and fixes in detail and is worth reading alongside this one.

Frequently Asked Questions About Mould in Aircon Units

Is mould in aircon dangerous?

Mould in an aircon is a genuine health risk because the unit blows air directly across the contaminated surfaces and distributes spores throughout your home every time it runs. For most healthy adults the effects are respiratory irritation and allergy symptoms. For children, the elderly or anyone with asthma or a compromised immune system, the risks are more serious and can include frequent asthma attacks and prolonged sinus problems.

Can I clean mould from my aircon myself?

You can safely clean surface mould from the filter, louvres and front panel yourself using a diluted vinegar solution or mild detergent. Anything deeper, including the evaporator coil, drain pan or blower wheel, requires professional equipment to clean properly. If a musty smell persists after you have cleaned the accessible parts, the mould has spread beyond what DIY methods can reach.

Why does my aircon smell musty even after cleaning?

A musty smell that returns after cleaning almost always means mould is present on the evaporator coil or blower wheel, not just on the filter and surfaces you can reach. These components sit deep inside the unit and require high-pressure coil cleaning equipment and anti-microbial treatments to address properly. A professional service is the only reliable fix at this point.

How often should I have my aircon cleaned to prevent mould?

A professional clean once a year is the minimum for most Australian homes, with the ideal time being just before summer. In high-humidity regions like coastal Queensland, New South Wales or anywhere with extended periods above 70% relative humidity, twice-yearly servicing is worth considering. Between professional cleans, rinse your filter every four to six weeks during heavy use and run fan-only mode for 10 to 15 minutes after each cooling session.

Mould in Your Aircon Is Fixable. Here's Your Next Step

Mould thrives inside air conditioners because the conditions are ideal: moisture from condensation, darkness between cycles and organic dust as a food source. Left unchecked, it poses real health risks for everyone in the home, particularly children, the elderly and anyone with asthma. Surface cleaning buys you time, but professional servicing is the only reliable way to clear mould from the evaporator coil, blower wheel and drain pan where it actually lives.

The good news is that mould is entirely preventable with consistent habits. Clean your filter every four to six weeks, run fan-only mode after cooling sessions and book a professional service at least once a year before summer. If you are upgrading, units like the Fujitsu ASTG series include filter-cleaning indicators that make it easier to stay on top of maintenance between services.

If your unit has a persistent musty smell or has not been professionally cleaned in the past 12 months, the next step is straightforward. Call Frozone Air on 1300 801 839 to book a service, or if you are in the Sydney area, visit our air conditioning service in Sydney page to get started.