Air conditioning is the most commonly used tool for managing Northern Rivers’ subtropical heat, and, as it happens, one of the most effective dehumidification tools in a high-humidity climate. But there’s an irony at the core of every air conditioning unit: the same process that removes moisture from the air creates an internal environment, wet, dark, and with a ready food source of organic dust, that is ideal for mould growth.
Quick answer (BLUF)
Mould growing inside air conditioning units is extremely common in Northern Rivers’ humid subtropical climate, particularly in split systems that are not regularly serviced. Mould inside a running AC unit circulates spores throughout the room every time it operates. Signs include musty odour when the unit runs, visible dark staining on the louvres or inside the unit, and unexplained respiratory symptoms that worsen at home.
How mould develops inside AC units
All refrigerant air conditioning systems remove moisture from the air as a byproduct of the cooling process. As warm, humid air passes over the cold evaporator coil inside the indoor unit, moisture condenses on the coil surface, exactly as it does on a cold drink can on a humid day. This condensate drains away through the condensate line. But the evaporator coil, the fan blower wheel, and the surrounding internal surfaces of the unit remain wet between cycles.
In a Northern Rivers home, where indoor humidity is often 65-80 percent even with the unit running, the residual moisture on internal surfaces never fully dries. Organic dust, skin cells, fibres, dirt, builds up on the wet coil and blower wheel surfaces. Add warmth when the unit switches to heating mode or simply sits idle in a warm room, and mould spores find a near-perfect growth environment.
The result is often visible as dark staining on the coil fins, fan blades, or the louvres inside the unit. More concerning is the mould you can’t see, deep within the coil assembly, on the rear surfaces of the blower wheel, and inside the drain pan. When the unit runs, the fan circulates spore-laden air directly into the room.
Signs your air conditioner has mould
Musty or earthy smell: The most common sign. If your AC unit produces a musty or damp smell in the first few minutes after starting, this strongly suggests mould inside the unit. The smell often fades as the unit runs because spores disperse, but they’re still in the air.
Dark staining on the louvres: Look at the air outlet louvres on the front of the unit. Dark staining, green, black, or grey, is visible mould. This is usually just the exposed surface of a more extensive internal colony.
Worsening respiratory symptoms: If household members experience increased cough, sneezing, or asthma symptoms in rooms where the AC runs frequently, mould in the unit may be contributing to the symptom load.
Visible growth on the drain pan or coil: If you remove the front panel of a split system unit (or have a technician do so), dark growth on the drain pan, coil fins, or blower wheel is direct confirmation.
Testing your air conditioning unit for mould
There are several approaches to confirming mould in an AC unit:
Visual inspection: Remove or open the front panel and examine the coil, blower wheel, and drain pan. Dark staining anywhere on these surfaces suggests mould. Note that many modern split systems have visible mould that is obvious upon inspection, you don’t need a test to confirm what you can see.
Air sampling while the unit runs: An indoor air quality test, either a DIY settling plate test or professional air sampling, conducted while the unit is running provides evidence of elevated spore counts being introduced by the unit’s operation. Compare results from a room with the unit running to the same room with the unit off to isolate its contribution.
Swab testing: A professional swab of the unit’s internal surfaces, coil, blower, drain pan, can identify the mould species present. This is particularly useful if you need to know whether toxigenic species are present before allowing vulnerable occupants (children, people with asthma) to be exposed to the running unit.
Professional mould inspection services can conduct appropriate testing for AC-related mould as part of a whole-home assessment.
Treatment options for mould in AC units
Professional AC cleaning: An air conditioning service that includes chemical cleaning of the coil, blower wheel, and drain pan using appropriate antimicrobial agents is the standard starting point. In Northern Rivers’ climate, this should be done at least annually for any unit in regular use.
Antimicrobial fogging: For units with established mould colonies, mould fogging treatment either within the unit or throughout the room (with the unit running to distribute the treatment through its internal surfaces) can be effective.
Coil replacement: Where the evaporator coil has extensive mould that cannot be adequately cleaned due to fin density or coil damage, replacement is sometimes the most cost-effective option, particularly for older units.
Ducted system ductwork: Ducted air conditioning systems have an additional challenge: mould can establish inside the ductwork itself. Signs include differential mould levels in rooms on the same duct run, visible staining at register faces, and persistent musty smell from the ducted system despite cleaning of the air handling unit. Ductwork cleaning and treatment is a specialist service that should be considered for ducted systems with confirmed mould issues.
Prevention in Northern Rivers’ climate
Given the climate, proactive AC maintenance is essential:
- Professional clean every 6-12 months for split systems in regular use
- Ensure condensate drain is clear and flowing freely, a blocked drain allows water to pool in the pan and is a primary mould source
- Run the unit on a “fan only” mode or leave the louvres open after extended use to allow the coil to dry before the unit is closed up
- Ensure the unit is correctly sized for the room, an oversized unit that short-cycles never runs long enough to properly dehumidify, leaving residual moisture
FAQs
Can mould in my air conditioner make me sick?
Yes. Operating an AC unit with mould inside it disperses spores and, in the case of toxigenic species, mycotoxins directly into the room air. Prolonged exposure can cause respiratory symptoms, allergic reactions, and in susceptible individuals (children, people with asthma) can trigger or worsen illness. Treat mould in an AC unit as a health priority.
How do I clean mould out of a split system air conditioner myself?
Minor surface mould on the louvres and front filter can be carefully cleaned with an appropriate antifungal cleaner. However, cleaning the internal coil, blower wheel, and drain pan without disassembling the unit requires specialist pressure-wash and chemical-clean equipment. Attempting to clean the coil yourself with household sprays is rarely effective and risks damaging delicate coil fins. Professional cleaning is strongly recommended for any mould beyond the surface louvres.
My rental property has mould in the air conditioner. Is this the landlord’s problem?
In NSW, maintaining installed appliances (including air conditioning) in working order is generally the landlord’s responsibility unless the failure results from tenant misuse. A poorly maintained AC unit developing mould from lack of servicing is a landlord maintenance issue. Request repairs in writing and escalate to NCAT if the landlord fails to act.
How do I know if mould from my AC is causing my asthma to worsen?
A useful self-test: monitor your symptoms in rooms with and without the AC running over several days. If symptoms (cough, wheeze, runny nose) consistently worsen when the unit is in operation and improve when it’s off, this strongly suggests the AC is the environmental trigger. Discuss with your GP, allergy testing for common mould species can confirm sensitisation, and an indoor air quality test while the unit runs can provide objective evidence.