A clean aircon filter does not stop mould from growing inside your air conditioning system.
If you have noticed a musty smell coming from your split system or wall unit, your first instinct is probably to pull out the filter and give it a wash. That removes dust and surface debris, but the mould colony sitting on the evaporator coil, drain pan and internal ducting is completely untouched.
This article explains where mould actually grows inside an air conditioner, why it keeps spreading through the system, and what needs to happen to stop mould spores circulating through your home.
If your aircon smells musty or you can see dark spots on the vents, book a free mould inspection with Mould and Hygiene Solutions across Newcastle, the Central Coast, Lake Macquarie, Hunter Valley and Port Stephens.
Where Mould Actually Grows Inside Your Air Conditioner
The filter sits at the front of your indoor unit and catches airborne particles before they reach the coil. It is the easiest part to access, which is why most people assume it is the main source of the problem.
Mould does not grow on the filter because the filter dries out between uses. It grows on the components behind it where moisture lingers for hours after the unit shuts off.
- Evaporator coil collects condensation every time the system runs, creating a permanently damp surface for mould to colonise
- Drain pan sits beneath the coil and holds water before it drains outside, often staying wet for hours
- Drain line can develop biofilm and partial blockages that trap moisture inside the unit
- Fan barrel and blower wheel accumulate organic matter that feeds mould growth in humid conditions
- Internal ducting in ducted systems provides dark, undisturbed surfaces where colonies spread unchecked
By the time you see mould on the filter or around the vent louvres, the colony behind those components is well established. Cleaning the filter addresses a small fraction of the total surface area where mould actually lives.
Why Filter Cleaning Fails as a Mould Solution
Washing your aircon filter is good maintenance. It keeps airflow efficient and reduces dust buildup.
It fails as a mould solution for the same reason wiping a kitchen bench does not fix a leaking pipe. The filter is a symptom, not the source.
| What filter cleaning does | What filter cleaning misses |
|---|---|
| Removes dust and lint from the mesh | Mould colonies on the evaporator coil |
| Improves airflow through the unit | Biofilm in the drain pan and drain line |
| Reduces visible discolouration on the filter | Spores embedded in the fan barrel |
| Makes the unit smell slightly better temporarily | The moisture cycle that drives regrowth |
Homeowners across Newcastle and the Central Coast often report the same pattern. They clean the filter, the smell disappears for a few days, and then it comes back stronger than before.
That cycle repeats because the mould source is untouched. Every time the unit runs, it draws air across the contaminated coil and blows spores directly into the room.
The Health Risk You Cannot See From the Filter
Mould spores are too small to see. A patch of mould on the coil that looks minor is releasing spores into the air every time your system kicks in.
NSW Health warns that breathing in mould spores can cause coughing, wheezing, blocked nose and eye irritation.
- Asthma sufferers tend to get more frequent attacks when spores are in the air
- Kids and older people are hit harder by ongoing exposure
- Symptoms creep up slowly, so most people do not connect them to the aircon
- Spores blowing out of the aircon land on walls, ceilings and furniture, so the problem spreads well past the unit
A musty smell coming from your aircon means mould spores are blowing through your house every time it runs. That is not a maintenance issue. Professional mould testing can confirm what is growing and how far it has spread.
A 2022 review published in the International Journal of Environmental Research and Public Health linked indoor mould exposure in Australian homes to higher rates of respiratory illness, calling out damp air conditioning systems and poor ventilation as contributing factors.
Wiping the filter does nothing about the spore count in your air. The contaminated coil and fan barrel behind it keep pushing spores into every room the system feeds.
Why DIY Aircon Mould Cleaning Spreads the Problem
Some people go further and spray the coil with vinegar, bleach or a can of mould spray from Bunnings. This usually makes things worse.
Spraying liquid onto the coil without containing the area sends spores airborne. You disturb the colony, and now you have mould spreading to parts of the house that were clean before.
- Bleach kills what it touches on the surface but cannot get to the root structure (hyphae) embedded in porous materials inside the unit
- Vinegar will not kill an established colony. Tea tree oil will not either.
- Retail aircon cleaning sprays foam the coil surface but do not treat the drain pan, fan barrel or internal ducting
- None of these products address the moisture conditions that caused the mould in the first place
You get a week of slightly better smell, then it comes back. Worse, the spores you kicked up during cleaning may have started new mould on walls or ceiling near the unit.
What Actually Stops Aircon Mould From Coming Back
Getting rid of aircon mould properly means getting inside the unit, killing the root structure rather than just the surface, and fixing the moisture conditions that let mould grow. No filter clean or spray from a hardware store does all three.
Mould and Hygiene Solutions treats mould at the source using an Australian-made, non-hazardous, non-corrosive and environmentally friendly antimicrobial solution that penetrates surfaces to destroy the colony, not just the visible layer.
- Inspection identifies where mould is growing inside the unit and whether it has spread to surrounding surfaces
- Containment prevents spore dispersal during treatment
- Treatment applies the antimicrobial solution to all affected surfaces including the coil, drain pan, fan barrel and any accessible ducting
- Moisture assessment identifies why the system is holding excess moisture and what needs to change to prevent regrowth
The treatment is backed by a 12-month unconditional mould-free guarantee on all treated areas. If mould returns within that period, it is retreated at no charge.
No retail product offers that because no retail product reaches the root structure. A professional mould removal treatment does.
How to Reduce Aircon Mould Risk Between Services
Professional treatment eliminates the colony, but what you do between services affects how quickly mould tries to return. These habits reduce moisture inside the unit and make it less hospitable to regrowth.
- Run the fan for 15 to 20 minutes after cooling. When you switch off cooling mode, moisture sits on the coil and evaporates slowly. Running the fan alone dries the coil faster and removes the standing moisture mould needs.
- Keep indoor humidity below 60 percent. Mould struggles to colonise surfaces when relative humidity stays below 60 percent. A basic hygrometer on the wall tells you whether your home is sitting in the danger zone.
- Clean filters every four to six weeks. This does not stop mould on its own, but clogged filters restrict airflow. Restricted airflow keeps the coil wetter for longer and feeds the conditions mould needs.
- Check the drain line is flowing. A blocked or slow drain line holds water in the drain pan for hours. If you see water dripping from the indoor unit or pooling around the base, the line needs clearing before mould takes hold.
- Improve room ventilation. Good ventilation reduces the humidity load your aircon has to manage. Less humidity means less condensation on the coil and a drier system overall.
If you already have mould, these habits will not fix it. But they slow regrowth after the system has been properly treated.
Signs Your Aircon Has a Mould Problem Beyond the Filter
Not every musty aircon needs a full treatment. But if you are seeing any of these, the problem goes deeper than the filter.
- A musty or sour smell that returns within days of cleaning the filter
- Visible dark spots on the vent louvres, barrel or around the indoor unit housing
- Respiratory symptoms that worsen when the aircon runs and improve when it is off
- Water dripping from the indoor unit or visible moisture around the drain line
- Black residue on walls or ceilings near the air conditioning vents
If you recognise two or more of these in your home, a free mould inspection is the fastest way to confirm whether the mould is limited to the unit or has spread further. Coastal humidity across the Hunter Valley and Port Stephens means air conditioners in these areas are more likely to circulate mould spores.
Stop Cleaning the Filter and Start Treating the Source
Keep cleaning your filter. It helps with airflow. But it was never designed to deal with mould, and it will not.
The Victorian Department of Health recommends addressing indoor mould promptly. An air conditioning unit blowing spores through your home is the opposite of addressing it.
If your aircon smells like mould, the colony is behind the filter, on the coil, the drain pan, the fan barrel. Washing the filter a hundred times will not change that.
Mould and Hygiene Solutions treats aircon mould at the source across Newcastle, the Central Coast, Lake Macquarie, Hunter Valley and Port Stephens. Find out what is growing in your system and what it takes to prevent it from coming back.
Frequently Asked Questions
Can a mouldy aircon filter make you sick?
If your filter has mould on it, the coil and drain pan behind it are almost certainly worse. Every time the system runs, it pushes those spores into the room. NSW Health links inhaled mould spores to coughing, wheezing, blocked nose and worsened asthma symptoms.
How often should you clean your aircon filter?
Every four to six weeks during regular use. More often if you have pets or live near the coast. Clean filters improve airflow but do not treat mould growing on the coil, drain pan or internal ducting.
Does running your aircon on dry mode prevent mould?
Dry mode pulls humidity out of the room but leaves the internal components wet. Running the fan for 15 to 20 minutes after you finish cooling does more. It actually dries the coil.
Can you see mould on an aircon filter?
Sometimes. Dark spots or a fuzzy coating on the filter mesh indicate mould has reached the filter. But the filter is usually the last place mould appears, and by the time it is visible there, the coil and drain pan behind it are already heavily contaminated.
Should you replace a mouldy aircon filter or clean it?
Most split system filters are reusable, so just wash it. But whether you wash or replace the filter, it does nothing about the mould on the coil, drain pan and fan barrel. A new filter will grow mould again within weeks if the source is not treated.
Is aircon mould worse in coastal areas?
Homes across Newcastle, the Central Coast, Lake Macquarie and Port Stephens deal with higher outdoor humidity. Air conditioning systems in these areas work harder and produce more condensation, which accelerates mould growth inside the unit.
