Mould in Your Aircon: How to Spot It, Remove It, and Prevent It

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

That stale, musty smell that hits you the moment your aircon kicks on is not just unpleasant. It is usually a sign that mould in your aircon has taken hold, and it is actively blowing spores into the air you breathe. For households with asthma sufferers, allergy-prone kids or anyone with a sensitive respiratory system, that is a real health concern, not just a housekeeping issue.

Air conditioners are practically purpose-built for mould growth. They pull warm, humid air across cold evaporator coils, which creates condensation. That moisture sits in a dark, enclosed space alongside a steady supply of dust and organic debris. Mould needs moisture, warmth, darkness and food to thrive, and your aircon provides all four.

The good news is that mould in an aircon is fixable, and with the right habits it is largely preventable. This article covers how to spot the signs, how to clean it out safely, and what you can do to stop it coming back.

How to Tell If Your Aircon Has Mould

Mould in a split system or ducted unit often announces itself before you ever see it. A musty or earthy smell when the unit first starts running is the most common early warning sign. If that smell fades after a few minutes, the mould is likely on a surface the airflow passes over directly, such as the filter or louvres.

Here are the four main signs to watch for:

• Visible dark spots or fuzzy growth on the louvres, around the air outlet, or on the surface of the filter. Mould typically appears black, green or dark grey and may look powdery or slightly raised.
• A musty or earthy smell when the unit runs. This is caused by microbial volatile organic compounds released by active mould colonies. If your room smells like a damp basement when the aircon is on, treat it as a red flag.
• Increased allergy or asthma symptoms in household members, particularly when spending time in the room where the unit operates. Sneezing, itchy eyes, a runny nose or worsening asthma that improves when you leave the room can all point to airborne mould spores.
• Black or green residue on the air outlet or around the edges of the indoor unit. This residue is often a mix of mould and dust that has been carried out by the airflow and deposited on surrounding surfaces.

Keep in mind that mould can be well established inside the unit long before any of these signs appear. Regular visual checks every few months are worth building into your routine, especially heading into summer when the unit works hardest.

Where Mould Hides Inside a Split System

Mould in a split system tends to concentrate in three specific spots, each progressively harder to access without tools.

The air filter is the most accessible hiding spot and the first place to check. Pop the front panel off your indoor unit and slide the filter out. Hold it up to the light. Black or grey patches, a fuzzy texture or a musty smell from the filter itself all confirm mould is present. Filters should be cleaned every four to six weeks during heavy use.

The evaporator coil and drip tray sit directly behind the filter and are less accessible. You will need a torch to get a proper look. The coil fins can harbour mould colonies that are invisible from the front of the unit, and the drip tray below the coil is a prime spot because it holds standing water whenever the drain is slow or blocked. A blocked drain pan is one of the most common triggers for mould growth inside a split system. For more on what causes water to pool inside your unit, see our guide to air conditioner leaking water.

The louvres and fan barrel are the third common location. The fan barrel, in particular, is a notorious mould trap because it stays damp after the unit shuts off and sits in the dark between uses. Mould on the fan barrel gets flung directly into the room air every time the unit runs.

Ducted systems add another layer of complexity. Mould can establish itself inside the ductwork, particularly near supply vents where condensation forms. Duct interiors are not something you can inspect yourself without specialist equipment, so if you suspect mould in a ducted system, a professional inspection is the right call.

How to Remove Mould from Your Aircon: Step-by-Step

To remove mould from a split system indoor unit, turn the unit off, put on protective gear, wash the filters with warm soapy water or a diluted white vinegar solution, wipe down the louvres and accessible surfaces with a mild anti-mould solution, allow everything to dry fully before reassembling, then run fan-only mode for 20 to 30 minutes to dry out the interior.

1. Turn off and unplug the unit. Switch the aircon off at the remote and at the wall switch or circuit breaker. If your indoor unit has a power cord you can reach safely, unplug it. Never work on a running unit or one that is still live at the wall.
2. Put on gloves, a mask and eye protection. Mould spores become airborne the moment you disturb them. A P2 respirator mask is ideal. Disposable nitrile gloves and safety glasses will protect your skin and eyes from both the mould and any cleaning solution you use.
3. Remove and wash the filters. Slide the filters out of the indoor unit and take them outside or to a laundry sink. Wash them with warm soapy water or a solution of one part white vinegar to three parts water. Scrub gently with a soft brush, rinse thoroughly and set aside to air dry. For a full walkthrough of the filter cleaning process, see our guide to how to clean your air conditioner.
4. Wipe down the louvres and accessible surfaces. Use a damp microfibre cloth with a mild anti-mould solution, such as a diluted white vinegar mix or a purpose-made coil and surface cleaner. Work carefully around the air outlet and the inside edges of the front panel. Avoid soaking any electrical components.
5. Allow all parts to dry completely before reassembling. Putting damp filters back into the unit is one of the fastest ways to restart a mould problem. Give the filters at least two hours to air dry in a well-ventilated spot, or longer on a humid day. Do not use a hair dryer or direct heat source on the filter mesh.
6. Run fan-only mode for 20 to 30 minutes after reassembly. Once everything is back in place, set the unit to fan-only mode at a medium or high fan speed. This circulates air through the interior and helps dry out any residual moisture on the evaporator coil and fan barrel before you return to normal cooling use.

Do not attempt to clean the evaporator coil or the internal fan barrel yourself using harsh chemicals or high-pressure sprays. These components are delicate, and the wrong product can corrode the coil fins, damage the fan barrel coating or strip the protective layer from internal parts. DIY attempts on internal components can also void your manufacturer warranty. Leave those parts to a licensed technician.

When to Call a Professional Instead

A DIY clean handles surface mould on filters and louvres, but there are three situations where you should call a professional rather than push further. First, if you can see mould on the evaporator coil itself, a technician is needed to clean it safely without damaging the fins. Second, if the musty smell persists after you have completed a thorough DIY clean, the mould source is almost certainly somewhere you cannot reach, such as the fan barrel or the drain pan. Third, if you have a ducted system, ductwork mould requires specialist equipment to locate and treat properly. It is not a job for a vacuum cleaner and a cloth.

A professional service for a split system typically includes a chemical coil clean, a drain pan flush and a check of the condensate drain line. These are the areas most likely to harbour mould that a DIY clean misses. To understand what a service call is likely to cost, our article on air conditioner service prices breaks down typical rates across Australia so you know what to budget for before you book.

How to Prevent Mould in Your Aircon for Good

Preventing mould in your aircon comes down to controlling moisture. Clean filters regularly, dry out the unit after each cooling session, keep airflow unobstructed, book an annual professional service and consider a unit with a built-in auto-dry function. These five habits, done consistently, make mould growth very difficult to establish.

Here is what each habit looks like in practice:

1. Clean or replace filters every four to six weeks during heavy use. A clogged filter restricts airflow, which causes the evaporator coil to run colder and produce more condensation than it should. That extra moisture is exactly what mould needs. During summer, when the unit runs daily, a monthly filter clean is a reasonable minimum. Outside of peak season, every six weeks is fine for most households.
2. Run fan-only mode for 15 to 20 minutes after each cooling session. This is one of the most effective mould prevention habits you can build. After you finish cooling a room, switch the unit to fan-only mode for 15 to 20 minutes before turning it off completely. The fan circulates air across the evaporator coil and fan barrel, drying out the residual moisture that would otherwise sit in the dark and feed mould growth overnight.
3. Keep the area around the indoor unit well-ventilated. Avoid pushing furniture, curtains or shelving up against the indoor unit. Restricted airflow around the unit means the air it draws in is more stagnant and humid, which raises the moisture load on the coil. Good clearance on all sides helps the unit breathe and reduces condensation.
4. Book a professional service once a year, ideally before summer. An annual service gives a technician the chance to flush the drain line, clean the coil and check the drip tray before the heavy cooling season begins. Catching a slow drain or early mould growth in October is far cheaper than dealing with a full mould problem in January.
5. Consider a split system with a self-cleaning or auto-dry function. Some units handle the drying step automatically, which removes the reliance on you remembering to run fan mode. The Daikin Cora 2.5kW (from $1,054) runs an internal drying cycle after each cooling session, which inhibits mould and keeps the coil cleaner between services. The Mitsubishi Electric AP 2.5kW (from $1,040) includes a similar post-operation drying function. Both are solid choices for households that have had recurring mould problems. Browse the full range of split system air conditioners to compare models and capacities.

For households where mould has already triggered allergy symptoms, it is worth addressing the air quality in the room while you work on the unit itself. The Daikin Air Purifier MC40YPVM ($492) uses a HEPA filter to capture airborne mould spores, dust and allergens, which can make a real difference while the aircon recovers from a clean or while you wait for a service appointment.

Auto-dry functions add to the upfront cost of a unit, and they do not eliminate the need for regular filter cleaning. They reduce the moisture that mould feeds on, but a dirty filter will still restrict airflow and create problems regardless of what the unit does after shutdown. The habits and the hardware work best together.

Ready for a Fresh Start? Browse Our Range of Split Systems

Mould in an aircon is common, but it is not something you have to live with. Clean the filters and accessible surfaces yourself, build a habit of running fan mode after each cooling session, and book a professional service when the mould goes deeper than you can reach. Those three steps cover the vast majority of cases.

If your unit is older, running less efficiently than it used to, or has had recurring mould problems despite regular cleaning, it may be worth considering an upgrade rather than another service call. Modern split systems are better sealed, drain more effectively and many include auto-dry functions that actively work against mould between cleans. The Daikin Cora 2.5kW (from $1,054) and the Mitsubishi Electric AP 2.5kW (from $1,040) are both good examples of units built with this in mind.

Whatever you decide, the goal is the same. Clean air, a unit that runs well and no more musty surprises when you reach for the remote on a hot day.

Frequently Asked Questions

Is mold in an air conditioner harmful?

Yes, mould in an air conditioner is harmful, posing two main risks. Health effects include respiratory irritation, allergic reactions and worsened asthma symptoms from airborne spores. The severity depends on the extent of the growth and the vulnerability of your household, with children, elderly people and those with existing respiratory conditions at greatest risk. A mouldy unit also restricts airflow and forces the system to work harder, pushing up your running costs.

What should I do if my AC has mold in it?

If your AC has mould in it, stop using the unit until it has been properly cleaned, as running it spreads spores through the room. Start by cleaning or replacing the filters, then wipe down any accessible surfaces with a diluted white vinegar or anti-mould solution. If the mould has reached the evaporator coil or keeps coming back after cleaning, book a professional service to treat the problem at its source.

What is the 3 minute rule for air conditioners?

The 3-minute rule refers to waiting at least three minutes before restarting your air conditioner after switching it off, giving the system time to equalise its refrigerant pressure and protecting the compressor from unnecessary strain. This is a general operational tip rather than a mould-specific guideline, but following it consistently is good practice for extending the life of your unit.

Can I use my AC if it has mold?

It is not recommended to use your AC if it has mould, because running the unit circulates spores into your living space and can worsen allergy and asthma symptoms for anyone in the home. The safest approach is to clean the unit thoroughly before switching it back on. If you need airflow in the meantime, switching to fan-only mode produces less condensation than cooling mode, which can slow further mould growth while you arrange a proper clean.