Mould in Air Conditioning Units: Causes, Signs & Solutions

If your split system smells musty when it kicks in, there's a good chance mould is growing inside it. This is one of the most common — and most overlooked — indoor air quality problems in Sydney homes. Your air conditioner is supposed to cool and filter your air. When it's contaminated with mould, it does the opposite: it actively distributes spores throughout every room it services.

Why air conditioners grow mould

Air conditioning units create nearly perfect conditions for mould:

• Condensation — the cooling process generates moisture on the evaporator coils and in the drip tray. Every time your unit runs, water forms inside it.
• Darkness — the interior of a split system or ducted unit sees no sunlight, removing one of nature's best mould inhibitors.
• Dust and organic matter — filters trap dust, skin cells, and other organic particles. These become food for mould once moisture is present.
• Stagnant air when off — when the unit isn't running, the moist, dark interior sits undisturbed. In Sydney's humid climate, especially during the warmer months from October to March, this is prime growing time.
• Temperature cycling — the shift between running (cold) and off (warm) creates condensation cycles that keep surfaces perpetually damp.

Split systems are particularly vulnerable because the indoor unit sits in a warm room and contains all the condensation-producing components right there on your wall. Ducted systems have similar issues but spread across longer duct runs, making the problem harder to detect and treat.

Signs of mould in your air conditioner

Watch for these indicators:

• Musty or stale smell when the unit starts — this is the most common first sign. If the air smells "off" for the first few minutes of operation, mould is almost certainly present.
• Visible dark spots around the air outlet vents or on the louvres
• Black dust or debris on walls or surfaces near the unit
• Increased allergy symptoms — sneezing, itchy eyes, or congestion that worsens when the aircon is running
• Respiratory irritation — coughing, wheezing, or a tight chest, particularly at night if the unit runs in your bedroom
• Worsening asthma in household members, especially children

If multiple people in the household experience symptoms that improve when they leave the house or when the aircon is off, the unit is a prime suspect.

The health risk is real

A contaminated air conditioner doesn't just passively harbour mould — it actively blows spores into your living space at volume. The fan pushes air across mould-covered coils and through contaminated filters, dispersing millions of spores directly into the room. You're essentially breathing mould every time the unit runs.

For healthy adults, this causes irritation and discomfort. For children, the elderly, asthmatics, and anyone with compromised immunity, it can trigger serious respiratory episodes. If you've noticed signs of mould exposure that correlate with aircon use, get the unit inspected.

Can you clean it yourself?

You can and should maintain the basics:

Regular filter cleaning (every 2–4 weeks)

1. Turn the unit off at the wall
2. Open the front panel and remove the filters
3. Wash with warm water and mild detergent
4. Rinse thoroughly and dry completely before reinstalling
5. Never run the unit without filters in place

Louvre and housing wipe-down (monthly)

• Wipe the visible surfaces with a damp cloth and mild detergent
• Dry thoroughly

These maintenance steps help prevent mould but won't fix an existing problem inside the unit. The evaporator coil, fan barrel, drip tray, and internal housing — where mould does most of its growing — are not accessible without disassembly.

What you should not do:

• Spray bleach or chemical cleaners into the unit — this damages components, voids warranties, and creates toxic fumes when the unit runs
• Attempt to disassemble the indoor unit yourself — split systems contain electrical components and refrigerant lines
• Ignore the problem and hope it goes away — mould in an enclosed, moist environment only gets worse

Professional aircon mould treatment

A proper treatment involves:

1. Inspection — checking the evaporator coil, fan barrel, drip tray, and drain line for mould contamination
2. Deep clean — professional disassembly and cleaning of all internal components
3. Antimicrobial treatment — application of an antimicrobial product to eliminate remaining mould and inhibit regrowth
4. Air treatment — addressing any airborne spores that have been distributed through the property
5. Drain line clearance — ensuring condensate drains freely (a blocked drain is a common cause of excessive moisture)

SAN-AIR gels for split systems

This is where SAN-AIR technology really shines. SAN-AIR gels are specifically designed for use inside air conditioning units. A small gel container placed inside the indoor unit provides continuous antimicrobial action through evaporative release:

• Treats the air as it passes through — neutralising spores before they reach your room
• Reaches surfaces sprays can't — the vapour penetrates the coil fins, fan barrel, and drip tray
• 100% natural and non-toxic — no chemical residues in your air stream
• Lasts 8–12 weeks — ongoing protection between professional services
• Won't damage components — safe for use with all major split system brands

The gel sits passively inside the unit and works whenever the fan runs. It's a simple, effective way to maintain clean air between full professional cleans.

Pureairo's air purification service includes full aircon treatment and the installation of SAN-AIR gels for ongoing protection.

Preventing mould in your air conditioner

Prevention is straightforward once you know what creates the problem:

• Run the fan-only mode for 15–20 minutes after cooling — this dries out the evaporator coil and internal surfaces before the unit sits idle
• Clean filters regularly — dirty filters restrict airflow and trap more organic matter for mould to feed on
• Schedule annual professional servicing — a yearly deep clean keeps internal components free of buildup
• Use the "dry" mode if your unit has one — this runs the fan with mild dehumidification to dry the interior
• Don't let the unit sit unused for long periods during humid months — run it briefly every few days to cycle air through
• Maintain indoor humidity below 60% — a standalone dehumidifier helps in particularly damp homes
• Keep the area around the outdoor unit clear — restricted airflow reduces efficiency and increases internal condensation

Ducted systems: a bigger challenge

Ducted air conditioning adds complexity because mould can grow anywhere along the duct runs — and you can't see inside them without specialist equipment. Signs of mould in ducted systems include:

• Musty air from specific vents (not all — which tells you the contamination is localised)
• Visible mould around ceiling vents or return air grilles
• Discolouration on ceiling near vents (moisture leaking from duct joints)

Ducted systems require professional duct cleaning and treatment. If you're in a strata building with shared ducted HVAC, the body corporate is typically responsible for maintenance of common systems.

Need help?

If your air conditioner smells musty or you suspect mould contamination, Pureairo can inspect, test, and treat both split and ducted systems across Sydney. Our air purification service includes full internal treatment and SAN-AIR gel installation for ongoing protection. Contact us for a free assessment — we'll check your unit and give you straight advice on what it needs.