The Hidden Truth About Mould in Split and Ducted Systems
That musty, damp smell coming from your air conditioner isn’t just unpleasant—it’s a red flag. Whether it’s a wall-mounted split system or a ducted air conditioner, foul odours usually mean one thing: mould and bacteria growth inside your system.
But why does this happen? And more importantly, how can you prevent it?
At A-Grade Air Conditioning, we’ve cleaned and restored hundreds of systems across Sydney, and we can tell you—it’s more common than you think.
Why Air Conditioners Grow Mould
Air conditioners work by removing heat and moisture from the air. That moisture collects on cold surfaces inside the system—like coils, drain pans, and ducts—creating the perfect environment for mould to thrive.
Here’s what typically causes the issue:
- Moisture Build-Up: Every time your AC runs, it generates condensation. If this moisture doesn’t drain properly, it becomes a breeding ground for mould and bacteria.
- Dust & Organic Matter: Dust, pet hair, pollen, and even skin cells collect inside the unit. Mould feeds on this organic matter.
- Poor Maintenance: When filters, coils, and drain lines aren’t regularly cleaned, the system becomes dirty and damp—creating ideal conditions for microbial growth.
- Infrequent Use: If your air conditioner isn’t used for long periods, moisture can sit stagnant in the system. Mould loves a dark, humid, untouched space.
Split System Units – Why They Smell
Split systems tend to grow mould behind the indoor fan coil unit (the bit on the wall). This is where cold air is produced and pushed into the room.
Common issues include:
- Mould growing on the blower fan and behind the fascia
- Slimy, black patches behind the filters or coil
- Water not draining correctly, leading to damp smells
- Units that are never serviced or cleaned for years (check out the whoppers on our instagram: @agradeairconsydney)
- This is especially noticeable when the system kicks on—blowing out a burst of warm, stale, mouldy air.
Ducted Systems – A Hidden Problem
Ducted systems are even trickier because most of the problem areas are hidden in the roof space or inside the ductwork.
What we typically find:
- Mould growing inside flexible ductwork (especially in humid zones or unused rooms)
- Dirty or clogged return air filters
- Dust and debris collecting around vents
- Mould crawling on the indoor coil and contaminating the air flow being pushed through the duct
- Moisture building up inside the main indoor unit due to poor drainage
In some homes, ducted systems haven’t been properly cleaned in over 10 years—leading to major air quality concerns for occupants.
What Does It Smell Like?
If your air conditioner smells like any of these, mould is likely the culprit:
- Wet socks or damp clothes
- Strong urine smell in severe cases
- Rotting wood or musty basements
- Dirty dishcloth or stale water
These aren’t just unpleasant—they can indicate bacterial growth and poor indoor air quality.
How to Prevent a Mouldy Air Conditioner
At A-Grade, we recommend the following for every home:
- Clean filters every 6-12 months or apply San Air Gel (monthly in high-use seasons).
- Book a professional deep clean annually, especially before summer
- Ensure proper drainage—no standing water in or around the system
- Use the ‘dry’ or ‘fan’ mode after cooling to help evaporate leftover moisture
- Treat units with anti-mould products—we use lab-tested treatments from San-Air and other trusted Australian brands
- Use a dehumidifier & air purifier (we sell Daikins air purifiers – contact for a quote).
Technician Tip
We always tell our clients: If it smells, it’s growing something. Don’t just cover it up with sprays or air fresheners. Mould spores can poison and infect your home environment, make you sick, and reduce your system’s efficiency.
Need a Proper Clean?
Whether it’s a split or ducted system, our team specialises in detailed air conditioner cleaning and mould treatment services across Sydney. We don’t just do basic results —we treat the root cause and help you keep your system cleaner, longer.
Book a service with A-Grade Air Conditioning today and breathe cleaner, fresher air in every room.