A musty smell when the air conditioner starts is not something to mask with a diffuser or spray. It is often the first clue that moisture and build-up are sitting inside the system. For Brisbane homes, offices and commercial sites, understanding what causes mould inside air conditioners helps prevent poor indoor air quality, unpleasant odours and avoidable system faults.
Mould needs three things to grow: moisture, warmth and a food source. Air conditioners can provide all three if condensate is not draining properly, filters are dirty or regular servicing has been missed. Queensland’s humidity makes the issue more common, but mould is not an unavoidable part of owning an air conditioner.
Why air conditioners create the right conditions for mould
Your air conditioner removes heat and humidity from indoor air. As warm air passes over the cold indoor coil, water vapour condenses into liquid water. Under normal operation, that water collects in the drain pan and flows safely through the condensate drain line.
The problem starts when water remains in the unit instead of draining away. A damp drain pan, coil or fan barrel can stay wet long after the system has stopped. Add airborne dust, skin cells, pet dander and other organic material caught by the filter, and mould has a place to establish itself.
This is why mould can develop in a split system even when it is still cooling. Cooling performance alone is not proof that the internal components are clean or draining correctly.
Blocked or poorly draining condensate lines
A blocked drain is one of the most common causes. Sludge, dust, algae and biological growth can restrict the drain line over time. Water then pools in the drain pan or backs up into the indoor unit.
In a residential split system, you may notice dripping from the wall-mounted head, staining below the unit, or water running where it should not. In a commercial system, the signs may be less obvious until ceiling damage, odours or a fault alarm appears. A drain issue needs attention promptly, as ongoing water exposure can damage finishes and electrical components as well as encourage mould.
Drainage can also be affected by an incorrectly installed drain line, insufficient fall, a disconnected line or a failed condensate pump. Cleaning the visible filter will not solve these faults.
Dirty filters and restricted airflow
Filters capture dust before it reaches the indoor coil. When they become heavily loaded, they restrict airflow and allow more dust to settle inside the unit. That dust becomes food for mould when it meets condensation.
Restricted airflow can also make the coil run colder than intended. In some cases, this contributes to coil icing and excess water when the ice melts. The result is a damp internal environment that is harder for the system to dry out between cycles.
For most households, filters should be checked regularly during heavy-use periods. Homes with pets, open windows, nearby construction or high occupancy often need more frequent attention. For businesses, filter intervals should reflect the building’s use, operating hours and occupant needs rather than a one-size-fits-all schedule.
Dust and grime on coils, fan barrels and drain pans
A filter is only the first line of defence. Fine particles can still pass through it and collect on the evaporator coil, fan barrel, louvres and drain pan. Over time, this build-up can hold moisture against surfaces and reduce airflow.
The fan barrel is a frequent source of the musty smell people notice when starting a split system. It is tucked behind the coil and can develop a layer of grime that is not reachable with a quick wipe. If mould is established there, running the system can circulate odours and particles through the room.
This is where a proper hygiene clean is different from a basic filter rinse. It involves accessing and cleaning the relevant internal components safely, checking drainage and confirming the system is operating as it should.
High humidity and long periods of low use
Brisbane’s humid weather gives mould a head start, particularly in rooms that are closed up for days or weeks. Air conditioners in holiday properties, spare rooms, vacant tenancies and seasonal business areas can sit in warm, humid conditions without enough operation to manage moisture.
The opposite situation can also create problems. A system that runs continuously but is oversized for the room may cool the space quickly and shut off before it has removed sufficient humidity. The room may feel cool but clammy, and moisture can remain on internal surfaces.
Correct system selection matters. A unit that is too small may struggle and run excessively, while an oversized system can short-cycle. Both situations can affect comfort, moisture control and long-term cleanliness.
Poor maintenance or incomplete cleaning
A spray applied through the front of a unit may freshen the smell briefly, but it will not necessarily remove growth from the coil, fan barrel or drain system. Likewise, rinsing filters is worthwhile but does not replace a full service.
Mould often returns when the underlying cause has not been corrected. If the drain remains partially blocked, airflow is still restricted or the system has a refrigerant or control issue affecting coil temperature, surface cleaning is only a short-term fix.
For commercial properties, incomplete maintenance can create a bigger operational risk. Buildings with high occupancy, food service, education, health-related operations or aged care environments need maintenance programs that consider hygiene, asset condition, drainage, filtration and reliability together.
Signs mould may be inside your air conditioner
A visible black mark near the outlet is a clear warning, but mould is not always easy to see. Watch for these signs, especially after the unit has been off overnight or for several days:
- A musty, damp or sour smell when cooling starts
- Black, grey or green spotting on louvres, vents or around the indoor head
- Water leaks, dripping or unexplained staining below the unit
- Reduced airflow, rattling, gurgling or a system that struggles to cool
These symptoms do not always mean mould is the only fault. A musty odour can also come from stagnant water in the drain pan, while poor airflow may be caused by a blocked filter or fan issue. Either way, it is worth having the unit assessed rather than guessing.
What to do if you find mould in an air conditioner
If you can see mould around the air outlet, switch the system off until it can be inspected, particularly if the smell is strong or there is active water leakage. Do not poke tools into the unit or flood it with household cleaner. Moisture near electrical components, damage to delicate coil fins and chemical residue can create more problems than they solve.
You can remove and wash reusable filters according to the manufacturer’s instructions, then allow them to dry fully before reinstalling them. Wipe accessible external louvres with a clean, lightly damp cloth. This is useful routine care, but it is not a substitute for cleaning internal components where mould commonly develops.
A qualified air conditioning technician can inspect the coil, fan barrel, drain pan and condensate line, clear drainage issues, complete a hygiene clean where required and check airflow and system operation. If water is leaking, the unit is tripping power, or mould keeps returning, arrange service sooner rather than later.
How to prevent mould from coming back
The most effective approach is to keep the system clean, draining and correctly matched to the space. Regular filter cleaning reduces dust entering the unit, while scheduled professional servicing deals with the components you cannot safely reach.
After running cooling on a humid day, using a fan-only or dry-out function, where available, can help reduce residual moisture inside the indoor unit. This is helpful, but it is not a cure for a blocked drain or dirty fan barrel. Some systems perform an automatic coil-drying cycle after shutdown, and it is generally best not to interrupt it.
Keep doors and windows closed while cooling where practical, particularly during very humid weather. This reduces the moisture load on the system. For buildings with recurring humidity issues, a technician may need to assess unit sizing, run times, ventilation, zoning or the condition of the broader HVAC system.
Big Dog Mechanical provides hygiene cleaning, repairs and preventative maintenance for split, ducted and commercial air conditioning systems across Brisbane. A tailored maintenance plan can be particularly valuable for businesses that cannot afford odours, leaks or unexpected downtime.
A clean, well-draining air conditioner should quietly manage comfort in the background. If yours smells damp, leaks water or sends a stale odour through the room, treat it as an early maintenance signal and get the cause sorted before it becomes a larger repair.
