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 filter cleaning misses it entirely, 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 roughly five percent of the 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 microscopic. A single square centimetre of mould-affected coil can release thousands of spores into the airstream every time the system operates.
According to NSW Health, inhaling mould spores can cause respiratory symptoms including coughing, wheezing, nasal congestion and eye irritation.
- People with asthma may experience more frequent attacks triggered by airborne spores
- Children and elderly household members are more vulnerable to prolonged exposure
- Symptoms often worsen gradually, making it harder to identify the air conditioner as the source
- Mould spores circulated by the aircon can settle on walls, ceilings and soft furnishings, spreading the contamination beyond the unit itself
A musty smell from your air conditioner is not a maintenance issue. It is a sign that mould spores are being actively distributed through your home every time the system runs. Professional mould testing can confirm what is growing and how far it has spread.
A 2022 review in the International Journal of Environmental Research found that poor indoor air quality from mould-affected HVAC systems is linked to increased rates of respiratory illness in Australian households.
Wiping the filter clean does not reduce the spore count in your air. The contaminated components behind the filter continue pushing spores into every room connected to the system.
Why DIY Aircon Mould Cleaning Spreads the Problem
Some homeowners go further than filter cleaning and attempt to spray the coil with vinegar, bleach or retail mould sprays. This approach creates more problems than it solves.
Spraying liquid onto the evaporator coil without proper containment sends mould spores airborne. Disturbing a mould colony without sealing the area first spreads contamination to parts of your home that were previously unaffected.
- Bleach kills surface mould on contact but cannot reach the hyphae (root structure) embedded in porous materials inside the unit
- Vinegar and tea tree oil solutions are too weak to eliminate established colonies on HVAC components
- 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
The result is a unit that smells slightly better for a week and then returns to the same state. Meanwhile, the spore disturbance during cleaning may have seeded new mould growth on nearby walls or ceiling surfaces.
What Actually Stops Aircon Mould From Coming Back
Eliminating mould from an air conditioning system requires three things that no filter clean or DIY spray can deliver: full access to internal components, a treatment that kills the root structure, and control over the moisture source.
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.
Signs Your Aircon Has a Mould Problem Beyond the Filter
Not every musty aircon needs a full mould treatment. But these signs indicate the problem goes deeper than what a filter wash can fix.
- 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. Properties across the Hunter Valley and Port Stephens are particularly prone to aircon mould due to coastal humidity.
Stop Cleaning the Filter and Start Treating the Source
Cleaning your aircon filter is worth doing for airflow and efficiency. It is not a mould treatment and was never designed to be one.
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 air conditioner smells like mould, the colony is on the coil, the drain pan and the internal components behind the filter. No amount of filter washing changes that.
Mould and Hygiene Solutions offers free mould inspections across Newcastle, the Central Coast, Lake Macquarie, Hunter Valley and Port Stephens. Book yours today and find out exactly what is growing in your system and what it takes to stop it.
