A split system or ducted air conditioner in a mould-affected building does not just fail to help the problem, it actively makes it worse. Every time the system runs, it draws air (and the mould spores suspended in it) through the return air grille, circulates them through ductwork or across the evaporator coil, and distributes them through every supply air register in the building. In a home with a localised mould problem, bathroom ceiling, under-sink cabinet, behind a wardrobe, an operating ducted system can turn that contained issue into whole-of-house contamination within days. This is one of the most underappreciated vectors of mould spread in Illawarra homes. For specific guidance on split system units, see our black mould in air conditioner guide.
Quick Answer (BLUF)
Ducted HVAC systems spread mould spores throughout the building every time they operate. If you have active mould in your home and run ducted air conditioning, you are distributing spores to clean rooms. The system must be turned off during professional remediation and inspected and cleaned as part of any whole-home mould remediation project.
How Ducted Systems Spread Mould
Understanding the mechanism helps explain why ducted systems are such an effective mould distribution channel:
1. Return air pathway Ducted systems draw air from the building through return air grilles (typically in hallways or central living areas). This air, along with any mould spores it carries, passes through the filter and into the air handler. If the filter is dirty or underspecified (a MERV 6 filter will not capture mould spores; a MERV 11-13 will capture most), spores reach the evaporator coil.
2. The evaporator coil, mould’s favourite location The evaporator coil operates at a cold surface temperature, causing moisture from the air to condense on the coil fins. This condensate is a perfect growth medium for mould, cold, continuously wet, dark, and with a supply of organic material from the airstream. Aspergillus, Cladosporium, and Penicillium species all thrive here. Once established, the coil becomes a continuous mould spore source within the system.
3. Supply air distribution Air passing the coil (or the blower wheel in a split system) picks up spores and carries them through ductwork or directly out of the indoor unit, depositing spores in every room the system serves. In a typical Wollongong home with a 5-7kW ducted system, this means bedrooms, living areas, and kitchen all receive continuous spore loading when the system runs.
4. The drain pan Below the evaporator coil sits a condensate drain pan. If the drain line blocks (algae, debris), this pan fills with standing water, a petri dish for mould and bacterial growth that then gets picked up by the airstream.
Signs Your HVAC System Has a Mould Problem
- Musty or earthy odour when the system first turns on (the “dirty sock” phenomenon)
- Visible dark streaks or deposits around supply air registers, these are spore deposits being accelerated out of the grille
- Mould recurring in rooms far from any obvious moisture source
- Members of the household noticing symptom improvement when the system is off and worsening when it is on
- A dirty or blocked filter in a system that was serviced recently
The Risk in Illawarra Conditions
Wollongong and the broader Illawarra coast present specific risk factors for HVAC mould:
- High ambient humidity means evaporator coils are almost always wet, providing an ideal mould growth surface
- Many ducted systems in Illawarra homes are 10-15+ years old and have never had duct cleaning
- Coastal homes in Thirroul, Austinmer, and Wollongong CBD have fine salt particles in outdoor air that coat coil fins, reducing airflow and trapping organic material
- Homes in the Escarpment foothills (Figtree, Keira, Keiraville) draw in cool, very humid air that saturates return air ducting with condensation, especially in summer
What Remediation Involves for HVAC Systems
A ducted system implicated in whole-home mould spread requires more than a standard duct clean:
1. System shutdown during whole-home remediation Turn the system off at the start of remediation and do not run it until the HVAC component has been inspected and cleaned. Running the system while mould is disturbed in other parts of the home dramatically increases cross-contamination.
2. HVAC-specific inspection A qualified assessor should inspect the return air plenum, evaporator coil, drain pan, supply air plenum, and accessible ductwork with a borescope camera. This is separate from a general mould inspection and ideally performed by someone with HVAC technical knowledge as well as mould assessment credentials.
3. Coil cleaning The evaporator coil should be cleaned with a biocidal foaming coil cleaner. This is an HVAC technician’s job, the process involves discharging the condensate properly, applying the cleaner, and ensuring it drains correctly through the drain pan and condensate line.
4. Drain pan treatment The drain pan should be cleaned and treated with a biocidal tablet or drain pan treatment to prevent algae and mould regrowth.
5. Duct cleaning Accessible ductwork should be cleaned with a rotary brush system and HEPA vacuum extraction. Flexible ducting in poor condition (torn, collapsed) should be replaced rather than cleaned.
6. Filter upgrade Upgrade the return air filter to at least MERV 11 (or the equivalent, check the system’s design airflow before uprating, as high-MERV filters increase resistance and can reduce airflow if the system is not designed for them). Change the filter every 1-3 months in coastal Illawarra conditions.
7. Post-cleaning air testing A post-remediation air quality test confirms that spore counts in supply air have returned to acceptable levels.
FAQs
Can I run my ducted air conditioning to dry out the house after mould remediation?
No, not until the system itself has been inspected and cleaned. Running a contaminated system over a freshly remediated building re-contaminates it. Once the system is confirmed clean, it can then be used for drying assistance with the understanding that outdoor air humidity may limit its effectiveness in Illawarra summer conditions.
How much does it cost to have ducted HVAC cleaned for mould in Wollongong?
Costs vary depending on system size, accessibility, and the extent of contamination. A combined duct clean and coil treatment for a standard residential system typically costs $400 to $900. This is in addition to, not a substitute for, general building mould remediation if mould is present throughout the home.
Can split system air conditioners spread mould too?
Yes, the principle is the same. The evaporator coil and blower wheel in a wall-mounted split system can both become mould-contaminated. The difference is that a split system only distributes spores in the room it is installed in, whereas a ducted system distributes them throughout the entire building.
Will a fogging treatment clean mould inside ducts?
Antimicrobial fogging can reach inside ductwork via the supply registers and provides a useful supplementary treatment. However, fogging alone is not a substitute for physical cleaning of the coil, drain pan, and ductwork, it does not remove the biofilm or the physical debris that supports mould growth.