Mould, smells and aircon cleaning
A musty smell from a split system is usually a sign that moisture and buildup are sitting somewhere inside the unit or nearby. Dirty filters can contribute, but the source is often deeper in the fan barrel, indoor coil, drain tray, drain line or internal airflow path.
Black spots or dark grime inside an air conditioner are not automatically proof of dangerous black mould. Dust, ordinary dirt, microbial growth, staining and degraded material can look very similar from the room, especially when you are inspecting the unit with a torch.
I provide professional air conditioner cleaning in North Brisbane, Moreton Bay and the Redcliffe Peninsula, and I regularly find units that look clean around the filters but have heavy buildup underneath the coil and along the fan barrel. In this guide, I will explain why the smell develops, what the black material may be, what you can safely check yourself, why deodorising sprays often fail and when a professional internal clean may solve the problem.
Why does my air conditioner smell musty?
The most common explanation is a combination of moisture, dust and organic buildup somewhere inside the indoor unit.
A split system creates condensation while it cools. That water should collect in the drain tray and leave through the drain line, but the coil, tray and nearby internal surfaces can remain damp after the system stops. When airborne dust and other particles collect on those damp areas, the unit can develop a stale, earthy or musty odour.
The smell may come from:
- Dusty or damp filters
- Buildup on the fan barrel
- Dirt or microbial growth on the indoor coil
- Residue in the drain tray
- A dirty, restricted or poorly draining drain line
- Moisture remaining inside the unit between uses
- Mould or dampness somewhere else in the room
- An unrelated fault, pest or damaged component
Why split system air conditioners develop musty smells
The indoor part of a split system is designed to move warm room air across a cold evaporator coil. As the air cools, moisture condenses on the coil, drips into the drain tray and exits through a drain line.
That process is normal. The problem starts when dust and fine airborne material gradually collect on surfaces that regularly become damp.
Moisture is part of normal cooling
Even when the drain is working correctly, the coil and nearby plastic surfaces do not become instantly dry when the unit switches off. In humid conditions, they may remain damp for longer. This is one reason musty odours can be more noticeable in Brisbane and Moreton Bay homes during periods of warm, humid weather.
Queensland Health notes that excess moisture, heat and humidity create conditions that favour mould growth, and that a musty odour can sometimes reveal mould that is not easy to see.
Dust supplies the buildup
Filters catch a portion of the dust entering the system, but they do not create a perfectly sealed or sterile airflow path. Fine particles can pass through or move around the filter edges and settle on the coil, fan barrel, drain tray and internal plastic channels.
Over time, that buildup can become sticky and difficult to remove with a dry cloth or quick surface spray. The fan barrel is particularly prone to collecting grime because every narrow blade repeatedly moves damp, dusty air.
The unit repeatedly becomes wet and dry
An air conditioner can cycle between damp operation, partial drying and long periods sitting unused. That repeated pattern can allow odours to become noticeable when the fan starts moving air again.
This is why a system can smell strongest during the first few minutes after startup and then seem to improve. The smell has not necessarily disappeared. It may simply become less noticeable as more room air passes through the unit.
The most common causes of a musty air conditioner smell
1. Dirty or damp filters
Filters are the first thing to check because they are designed to be removed and maintained by the homeowner. Heavy dust can hold odours, restrict airflow and allow more contamination to collect deeper inside the unit.
Washable filters should be cleaned according to the air conditioner manufacturer's instructions and allowed to dry completely before being reinstalled. Putting wet filters back into the system can add more moisture to an already damp environment.
However, clean filters do not prove the rest of the unit is clean. I regularly remove spotless filters and then find a heavily coated fan barrel underneath.
2. Buildup on the fan barrel
The fan barrel, also called the cross-flow fan or blower wheel, is the long cylindrical fan behind the air outlet. Its narrow blades move air through the room, and buildup across those blades can contribute to stale smells and reduced airflow.
The fan barrel is easy to miss because it sits deeper inside the unit. A quick glance at the filters or visible coil does not show its condition. You normally need the system switched off, the louvres open and a torch angled upward through the outlet to see it properly.
When the barrel is dirty, dark material may appear as:
- Small black or grey dots on individual blades
- A continuous dark coating along the barrel
- Patchy clumps around the blade edges
- Uneven buildup that is heavier at one end
Spraying the front of the coil does not automatically clean the barrel behind it.
3. Contamination on the indoor coil
The evaporator coil is the finned metal section visible behind the filters. It becomes cold and wet during cooling, so dust can stick between the fins and on less visible areas underneath or behind the front face.
A light layer of surface dust may not create a strong smell by itself. A persistent odour is more likely when buildup extends through the coil, remains damp or combines with contamination elsewhere in the airflow path.
The fins are delicate. Hard brushing, pressure washing or pushing tools into the coil can flatten them and restrict airflow.
4. Residue in the drain tray
The drain tray catches condensation below the coil. Fine dust, slime, residue and other debris can collect in it over time, particularly around corners, channels and the drain outlet.
If water is not leaving efficiently, part of the tray may stay wet for longer than it should. The unit can then smell damp even when the filters and visible coil look reasonably clean.
The tray is only partly visible in many wall-mounted systems. Cleaning what you can see from the front may leave the rear section and drain outlet untouched.
5. A dirty or restricted drain line
The drain line carries condensation outside. Sludge, debris, poor fall, a kink, insect activity or an installation problem can restrict the flow.
A restricted drain does not always create an obvious leak straight away. It may first cause slow drainage, stagnant residue or a smell near the indoor unit. If the restriction worsens, water may overflow into the room.
A smell or leak should not automatically be blamed on dirt. Cracked trays, damaged insulation, frozen coils and installation faults can produce similar symptoms and may require an air-conditioning technician.
6. Moisture remaining after the unit switches off
Some air conditioners include a coil-dry, self-clean or fan-dry function that keeps air moving briefly after cooling. When used according to the manufacturer instructions, this can help the inside of the unit dry more thoroughly.
If your model does not have that function, check the manual before changing how you operate it. Do not assume that every unit should be left running in fan mode, because control logic and manufacturer recommendations differ.
7. The smell is coming from the room, not the air conditioner
An air conditioner constantly recirculates room air. It can pick up an existing odour and make it seem as though the smell started inside the unit.
Possible external sources include:
- Damp carpet, curtains or furniture
- Mould behind cupboards or along external walls
- A roof, plumbing or window leak
- Wet clothing or towels
- Pet bedding
- Smoke, cooking oils, incense or candles
- A pest or dead animal in the wall or ceiling
If the smell remains when the air conditioner has been off for several hours, inspect the room as well as the unit.
Is the black buildup inside your air conditioner mould or dirt?
You usually cannot identify black material accurately from colour alone.
Dark marks inside a split system may be mould or another form of microbial growth, but they may also be ordinary dust, sticky grime, smoke residue, staining, degraded foam or a combination of several materials.
The term "black mould" is often used as though it describes one specific dangerous substance. In reality, mould can appear in many colours, and a dark appearance does not tell you the species, the amount present or whether it is producing toxins.
The US Environmental Protection Agency explains that mould comes in many colours and that moisture control is the key to preventing indoor growth. The visual colour of a patch is not a reliable danger rating.
What black material commonly looks like inside a split system
| What you can see | What it may be | What it tells you |
|---|---|---|
| Dry grey dust on filters | Ordinary household dust and fibres | The filters need maintenance, but deeper areas may still be clean or dirty |
| Black dots on fan blades | Dust, grime, microbial growth or mixed buildup | The fan barrel needs closer inspection and likely cleaning |
| Slimy dark residue in the tray | Wet dust, sludge or biological buildup | Moisture and drainage conditions should be addressed |
| Flat black staining on plastic | Old contamination, staining or material discolouration | The surface may remain marked even after it is physically clean |
| Fuzzy or spreading patches | Possible mould growth | A visual inspection cannot identify the species or health risk |
| Crumbly black foam | Degraded insulation or sealing material | Cleaning may not fix it and a technician may need to replace the material |
Do you need to test the black material?
Laboratory testing is the only reliable way to identify a particular mould species, but most homeowners do not need to know the exact species before addressing visible contamination and moisture.
The practical questions are usually:
- Is there visible buildup inside the airflow path?
- Is the unit producing a recurring musty smell?
- Are internal surfaces staying damp or draining poorly?
- Can the affected areas be cleaned safely and completely?
- Is there a larger moisture problem elsewhere in the home?
The US Centers for Disease Control and Prevention does not generally recommend home mould testing because the type does not change the basic need to remove visible growth safely and correct the moisture source.
Where to look when your air conditioner smells musty
You can perform a basic visual check without dismantling the system. Turn the air conditioner off first and follow the owner's manual for any area you open.
1. The filters
Lift the front cover and inspect the mesh filters. Look for heavy dust, damp patches, discolouration or a stale smell directly on the filter material.
2. The visible face of the coil
With the filters removed, inspect the visible fins without touching them. Look for dust packed between the fins, patchy residue or sections that appear much darker than the rest.
3. The fan barrel
With the system off, carefully look upward through the air outlet using a torch. Do not put your fingers, tools or cleaning brushes through the louvres. Rotate nothing by hand unless the manufacturer specifically permits it.
4. The louvres and discharge opening
Black spotting around the outlet may be a visible sign of deeper contamination, especially when similar buildup is present on the fan blades behind it.
5. The drain area
Check for water stains on the wall, dripping from the indoor head, a wet smell below the unit or unusually slow drainage outside while the system is cooling.
6. The room around the unit
Inspect nearby walls, the ceiling line, curtains, cupboards and soft furnishings. A split system may simply be drawing in an odour from another damp area.
What different air conditioner smells may mean
Smell descriptions are subjective, so this table is a guide rather than a diagnosis.
| Smell | Possible cause | What to do |
|---|---|---|
| Musty, earthy or damp | Moisture, dirty filters, fan buildup, coil contamination, drain tray residue or room mould | Check filters and visible internals. Arrange a proper clean if the smell returns or buildup is visible |
| Sour or "dirty sock" smell | Microbial buildup on damp internal surfaces, especially around the coil, fan or drain area | Do not mask it with fragrance. Have the internal airflow path and drainage inspected |
| Stagnant water smell | Slow drainage, residue in the tray, drain restriction or another moisture source | Check for leaking and arrange inspection before water damages the wall or unit |
| Dusty smell after months unused | Loose dust on filters and internal surfaces | Clean the filters and monitor. Persistent odour suggests deeper buildup |
| Burning, hot plastic or electrical | Overheating, wiring, motor, control board or another electrical fault | Turn the system off and arrange assessment by a suitably qualified technician |
| Dead animal or decaying smell | Pest inside or near the unit, wall cavity, roof space or drain | Stop using the unit until the source has been located and removed safely |
| Rotten egg or gas-like smell | Potential gas leak or another serious external source, not normal split system operation | Leave the area, avoid electrical switches and contact your gas supplier or emergency services |
Why spraying deodoriser does not fix a musty air conditioner
Fragrance sprays can temporarily overpower an odour without removing the material producing it. Once the fragrance fades, the musty smell returns because the buildup, moisture or drainage issue is still present.
It may only reach the visible surface
A spray aimed through the filters usually lands on the front of the coil. It may not reach the fan barrel, rear of the drain tray, deeper coil surfaces or drain outlet where the smell is actually developing.
It can leave residue
Products not designed for indoor air-conditioning components may leave sticky residue that attracts more dust. Strong fragrances and household cleaners can also create unpleasant fumes when the unit runs.
It does not correct moisture or drainage
Even a product that reduces surface bacteria will not repair a restricted drain, remove stagnant sludge, correct a cracked tray or solve high humidity elsewhere in the home.
Some products should not be sprayed into an air conditioner
Do not use bleach, household disinfectant, degreaser, essential oils or general mould spray inside a split system unless the product and air conditioner manufacturers specifically approve that use. Electrical components, protective coil coatings, plastics and indoor air exposure all need to be considered.
For a broader breakdown of surface sprays and wash bags, read my guide: Are air conditioner cleaning kits worth it?
Will professional cleaning remove the musty smell?
Often, yes, when the odour is coming from dirt, grime, microbial growth or stagnant residue inside the parts being cleaned.
A proper internal clean can physically remove buildup from areas such as the filters, covers, coil, fan barrel, drain tray and accessible airflow path. If that buildup is the source, the smell should improve substantially or disappear.
Cleaning is most likely to help when:
- The smell starts when the air conditioner starts
- The odour is strongest directly below the outlet
- The fan barrel or louvres have visible dark buildup
- The smell returns soon after filter cleaning
- The drain tray contains residue or sludge
- The system has not had a proper internal clean for a long time
- Airflow has gradually reduced alongside the smell
Cleaning may not fully solve it when:
- The smell comes from damp carpet, walls, curtains or furniture
- There is a roof, plumbing or wall leak
- The drain is incorrectly installed or damaged
- The tray is cracked
- Insulation or foam inside the unit has degraded
- A pest is inside the wall, roof or unit
- The unit has an electrical or mechanical fault
- The air conditioner is very old and contaminated material cannot be safely restored
A responsible cleaning service should not promise that every odour is guaranteed to disappear. The source needs to be identified first, and unrelated faults should be referred to an appropriately licensed air-conditioning or electrical technician.
Safe checks you can do before booking a clean
- Notice exactly when the smell begins. Does it start immediately, only during cooling, after the unit stops or whenever the room becomes humid?
- Check whether it is strongest at the outlet. Compare the smell directly below the air conditioner with the rest of the room.
- Turn the system off. Do not inspect moving parts while the fan is operating.
- Inspect and clean the filters. Follow the manufacturer's directions and allow washable filters to dry completely.
- Use a torch for a visual inspection. Look through the outlet without putting fingers or tools into the fan area.
- Look for water evidence. Check the wall, floor and drain outlet for staining, dripping or unusually slow drainage.
- Check the room for another damp source. Smell curtains, carpet and cupboards near the unit and inspect external walls.
- Run the system only if it otherwise operates normally. Stop if there is burning smell, tripping power, unusual noise, visible electrical damage or water near electrical parts.
When a smelly air conditioner needs urgent attention
A musty odour alone is usually a cleaning or moisture warning, but some smells and symptoms point to a more serious problem.
Turn the system off and arrange professional assessment if:
- You smell burning, hot plastic or electrical insulation
- The breaker trips or the unit repeatedly loses power
- Water is leaking near wiring, controls or the power side of the unit
- The unit has an error code or will not operate normally
- You hear grinding, buzzing, arcing or harsh mechanical noise
- The indoor head is heavily contaminated after a flood or major water event
- There is widespread mould elsewhere in the room or wall cavity
- Someone in the home experiences significant symptoms around the unit
Cleaning is not an electrical, refrigeration or mechanical repair. A cleaner should stop and refer the job when the unit cannot be safely tested, isolated or returned to normal operation.
A careful note about mould and health
Mould exposure affects people differently. Queensland Health states that people with asthma or lung disease, mould allergies, chronic disease and some older people may be more likely to experience health problems. Anyone concerned about symptoms should speak with a doctor.
Cleaning an air conditioner can remove dust, grime and visible buildup from the system's airflow path, but it is not medical treatment. I do not diagnose mould species from photographs or guarantee a particular health result.
If a person in the home is immunocompromised, has serious respiratory illness or reacts strongly to mould, get medical advice and consider whether broader building inspection or remediation is required rather than treating the split system as the only possible source.
How to reduce the chance of the musty smell returning
Clean the filters regularly
Check the filters more often during high-use periods, in homes with pets and in dusty environments. Follow the frequency and method in the unit manual rather than waiting until airflow is obviously restricted.
Allow washable filters to dry fully
Reinstalling wet filters adds avoidable moisture and can create a damp smell at startup.
Use the manufacturer's drying function
If your air conditioner includes coil-dry, self-clean, internal clean or a similar feature, use it as directed. These functions vary between brands and models, so check the manual instead of guessing.
Keep the room itself dry
Repair roof, plumbing and window leaks. Use bathroom and kitchen extraction correctly and address condensation on walls or windows. Cleaning the air conditioner will not stop a damp wall from smelling.
Do not block airflow
Keep curtains, furniture and stored items away from the indoor intake and outlet. Restricted room airflow can reduce comfort and make moisture management more difficult.
Watch for drainage changes
New dripping, water stains, bubbling sounds or stagnant odours should be investigated early before they become a larger leak.
Arrange internal cleaning based on condition
There is no single schedule that suits every home. Usage, pets, smoke, humidity, room type and filter maintenance all affect how quickly the system becomes dirty. The right time to clean is when inspection and symptoms show that internal buildup is developing, not simply because a calendar says so.
What I clean during a professional split system service
My service is designed to clean the internal areas that commonly contribute to odours, not just wash the filters and spray the visible coil.
The exact process depends on the unit and its condition, but it generally includes:
- Inspecting and testing the air conditioner before cleaning
- Safely isolating the power
- Protecting the wall, floor and nearby furniture
- Removing filters, the outer case and accessible removable parts
- Cleaning filters, covers and louvres separately
- Cleaning the indoor coil with controlled water and containment
- Cleaning the fan barrel along the internal airflow path
- Cleaning the drain tray and checking the drain path where appropriate
- Removing residue from accessible internal channels and discharge areas
- Reassembling and testing the system
- Providing before-and-after photos of the internal result
I am owner-operated and fully insured, so the person you book with is the person who arrives at your home. There is no upfront payment, and you can see the result before paying.
Frequently asked questions
Does a musty smell mean my air conditioner has mould?
It can, but not always. A musty smell may come from mould or other microbial growth, wet dust, grime in the fan barrel, residue in the drain tray, a restricted drain or dampness elsewhere in the room. The unit needs to be inspected rather than diagnosed from smell alone.
Is every black mark inside an air conditioner black mould?
No. Dark material may be mould, but it can also be dust, ordinary dirt, smoke residue, staining, degraded foam or mixed buildup. Colour alone cannot identify the material or tell you whether it is dangerous.
Can I tell what type of mould it is from a photo?
No. A photograph can show that buildup is present, but it cannot reliably identify a mould species or its health significance. Laboratory analysis is required for species identification, although exact testing is often unnecessary before safely removing visible contamination and fixing moisture.
Will cleaning the filters get rid of the smell?
It may help if the filters are the main source. If the smell returns, the buildup is likely deeper in the coil, fan barrel, drain tray, drain line or another part of the room.
Why does the smell only last for a few minutes?
Air sitting around damp internal surfaces can smell strongest when the fan first starts. The odour may become less noticeable as it mixes with more room air, even though the source remains inside the unit.
Why does my air conditioner smell worse after being unused?
Dust and moisture can remain inside during long periods without operation. When the fan starts again, it moves air across that buildup and releases the trapped odour into the room.
Can I spray mould killer into my split system?
I would not use a household mould product inside an air conditioner. It may damage coatings or plastics, leave residue, create fumes or reach electrical components. Only use products specifically approved for the relevant component and follow both the product and unit manufacturer instructions.
Does professional cleaning guarantee the smell will disappear?
No responsible service should guarantee every odour without identifying the source. Cleaning is often effective when the smell comes from internal buildup, but it will not repair a cracked tray, damaged insulation, poor installation, electrical fault, pest problem or damp wall.
Can a dirty air conditioner affect airflow?
Yes. Heavy dust on the filters, coil or fan barrel can restrict the amount of air moving through the unit. Cleaning may improve airflow when dirt is the cause, but it will not fix refrigerant, motor, sensor or other mechanical problems.
Should I stop using an air conditioner that smells musty?
A mild musty smell is usually a sign to inspect and clean the system rather than an immediate electrical emergency. Stop using it if there is burning smell, tripping power, unusual noise, leaking near electrical components, an error code or serious health concern.
How can I stop mould coming back after cleaning?
Keep filters maintained, address room humidity and leaks, ensure drainage is working and use any manufacturer-approved internal drying function. Mould and odours are more likely to return when the moisture source remains.
Professional split system cleaning
Still getting a musty smell from your air conditioner?
I provide professional split system air conditioner cleaning across North Brisbane, Moreton Bay and the Redcliffe Peninsula. I clean the coil, fan barrel, drain tray and internal airflow path, then show you before-and-after photos so you can see exactly what was removed.
You can check the price and choose an available appointment online. No card is required, and payment is due after the service.
