Ducted air conditioning systems are one of the most effective ways to distribute mould spores throughout an entire home. When mould grows inside the air handler unit, on evaporator coils or in ductwork, the airflow that normally cools and heats the home simultaneously carries spores into every room the system serves. A mould problem in the HVAC system is not a room-specific problem, it’s a whole-house problem.
Mould in ducted systems is common in Wollongong and Illawarra, the coastal humidity that drives mould growth generally is particularly concentrated in air conditioning systems where condensation forms on cold components during operation.
How Mould Gets Into Ducted Air Conditioning
Evaporator Coil Condensation
The evaporator coil (the cold coil inside the indoor air handler unit) operates at temperatures well below the dew point of ambient air. Moisture from the air stream condenses on the coil during every cooling cycle. This condensate collects in a drain pan and should drain out through a condensate drain line, but if the drain line is blocked, cracked or too flat to drain by gravity, water accumulates in the drain pan.
A stagnant condensate drain pan covered in warm, humid, nutrient-rich water is a perfect mould cultivation environment. Mould in the drain pan is then drawn into the airstream and distributed throughout the duct system.
Duct Moisture Infiltration
Poorly insulated ductwork running through unconditioned spaces (roof voids, subfloor voids) develops condensation on the exterior duct face in humid conditions. If there are any cracks, disconnected joints or penetrations in the ductwork, moist outside air infiltrates the duct interior, raising humidity inside the ducts and allowing mould to grow on dust accumulations inside.
Air Handler Cabinet Leaks
Many domestic air handler cabinets are not perfectly sealed, there are small gaps around filter housings, cable entries and service panels. When the system operates at negative pressure (drawing air through the return), unfiltered, humid roof void or subfloor air can be drawn in. This air brings both moisture and spores.
Poor Maintenance
Air conditioning filters that are never cleaned accumulate a thick layer of dust, skin cells and organic debris. This biofilm retains moisture from the airstream and becomes a mould growth medium. Neglected filters are the most common single cause of mould in domestic air conditioning systems in Illawarra.
Signs That Your Ducted System Has Mould
Musty smell when the system starts. The classic sign, particularly noticeable at first startup in spring after the system hasn’t run in months. The smell may reduce as the system runs (as spores are distributed and diluted) or may persist.
Visible mould at supply grilles. Look at the air supply vents in each room. Dark staining or visible mould growth at or around the grille indicates mould in the duct close to that outlet.
Visible mould on or around the return air grille. The large return air inlet, often in a ceiling corridor or central location, draws air from the room and back to the air handler. Mould around this grille indicates mould in the ductwork near the air handler or in the air handler itself.
Black dust around supply outlets. Not always mould, can be dust and carbon particulate, but black dusty buildup at supply outlets warrants investigation.
Household members with symptoms that improve away from home. Mould distributed through an air conditioning system produces uniform exposure throughout the home. If respiratory symptoms improve consistently when outside the home and worsen when the system runs, HVAC mould is a likely contributor.
Water stains on ceiling near supply outlets. Indicates condensation on undersized or poorly insulated ductwork. Wet ductwork interiors develop mould.
Assessment and Investigation
A mould inspection for suspected HVAC mould typically includes:
- Visual inspection of accessible air handler components
- Inspection of return air and supply grilles
- Air sampling from within the ducted system (specialist sampling head)
- Air sampling in living spaces during system operation
- Condensate drain check and drain pan inspection
- Filter condition assessment
For ductwork in roof voids, a flexible camera inspection is the most effective way to see inside ducts without destructive access.
Treatment Options
Drain Pan and Coil Treatment
Algae and mould in the drain pan and on the evaporator coil is treated with HVAC-specific antimicrobial agents applied by a qualified air conditioning technician. The drain line is cleaned and flushed. Condensate treatment tablets that slowly release antimicrobials can be placed in the drain pan to prevent recurrence.
This scope of work typically runs $300-$700 as part of an HVAC service call.
Duct Cleaning with Antimicrobial Treatment
Duct cleaning removes dust, debris and surface mould from within ductwork using high-velocity agitation and HEPA vacuum extraction. Significant equipment is required, this is not a DIY job and not a job for a company using leaf blowers and shop vacuums.
After cleaning, antimicrobial fogging or application of an EPA-registered duct treatment product inhibits regrowth. Prices for residential duct cleaning in Wollongong: $800-$2,500 depending on system size and number of outlets.
HEPA Air Handler Unit Remediation
If mould is present in the air handler cabinet itself, on insulated surfaces, in the cabinet lining, this requires specialist remediation. Some insulated cabinet liners that have absorbed mould growth cannot be adequately cleaned and must be replaced.
Duct Replacement
Heavily mould-contaminated flexible ductwork is sometimes more cost-effective to replace than to clean, particularly in older systems where flexible duct insulation has deteriorated. New flexible duct with quality insulation provides clean start for both mould and thermal performance.
Prevention After Treatment
Clean filters monthly. For ducted systems in coastal Illawarra, monthly filter cleaning rather than the manufacturer’s quarterly recommendation is appropriate. This is the single highest-impact maintenance item.
Annual HVAC service including coil and drain pan cleaning. Schedule an annual air conditioning service that specifically includes evaporator coil cleaning and drain pan check. This prevents the drain pan mould scenario from developing.
Upgrade to antimicrobial filter media. Activated carbon or antimicrobial-coated filters reduce the biofilm accumulation on filter surfaces that promotes mould.
Dehumidify when the system is not running. In Illawarra’s humid winters, the air conditioning system may not run for months. Running a dehumidifier during this period keeps indoor humidity below mould thresholds without relying on the A/C system.
UV light installation in air handler. A UV-C germicidal lamp installed in the air handler near the evaporator coil kills mould, bacteria and viruses as air passes through. Effective at preventing mould on the coil and in the drain pan. Installation cost: $400-$800 from an A/C technician.
Mould in Split System Air Conditioners
Split system (reverse cycle) units without ductwork develop mould in the indoor unit’s fan housing and heat exchanger fins. The treatment approach is simpler than ducted systems but the same principles apply, coil cleaning, drain pan treatment, and filter maintenance. See our guide on black mould in air conditioners.
Frequently Asked Questions
Can I test for mould in my air conditioning system without a professional? You can take air samples during system operation using a home test kit, but these kits are unreliable for quantitative assessment. Professional testing with calibrated equipment, including air sampling at supply and return grilles with outdoor baselines, gives you meaningful data. A building inspector or mould specialist with HVAC assessment experience is the right call.
Is it safe to run ducted air conditioning if I suspect mould? If you have strong evidence of HVAC mould (visible growth at outlets, strong musty smell, health effects), running the system spreads spores throughout the home. Turn it off and arrange assessment. If you have mild symptoms or slight musty smell, running the system while arranging inspection is your call, but be aware it may be worsening the distribution.
My builder installed a new ducted system last year and it already smells musty, why? New systems can develop mould quickly if installed with inadequate drain pan slope (standing water from day one), using ductwork that had moisture exposure during the build, or in a home with high ambient humidity and no humidity control. Installation quality matters significantly for HVAC mould outcomes. Ask for a service call under warranty including drain pan inspection.
How long does duct cleaning take? A typical 3-4 bedroom home with a ducted system: 3-5 hours for thorough cleaning with HEPA extraction. Larger homes or systems with more outlets take longer. Don’t accept quotes for duct cleaning that claim to do 10+ outlets in under 2 hours, this isn’t enough time for thorough work.
Can the air conditioning technician do both the system service and the mould treatment? Many air conditioning technicians perform coil cleaning and drain pan treatment as part of a service, and some work with mould remediation specialists for duct treatment. For whole-house HVAC mould, ductwork, air handler, and supply outlets, a specialist mould fogging and antimicrobial treatment team working alongside the A/C technician is the most effective approach.
Mould smell coming from your ducted air conditioning in Wollongong? Don’t spread it further. Contact Illawarra Mould Removal for a professional HVAC mould assessment.