Mould growth in evaporative coolers is a common Melbourne problem — the combination of moisture, organic material from pads and pollen, and Melbourne’s warm spring conditions creates an ideal environment for several mould species. The first sign for most Melbourne homeowners is a musty or earthy odour from ceiling vents, typically when the system is first switched on in September or October. This guide explains why mould grows, the health implications, how to treat it, and how to prevent it returning.
Why Mould Grows in Evaporative Coolers
Evaporative coolers provide three of the four conditions mould needs to grow: moisture, a porous surface (Chillcel cellulose pads), and moderate warmth (15 to 35°C). The fourth condition — organic material as a nutrient source — arrives with Melbourne’s notable pollen load (Melbourne has one of the world’s highest grass pollen concentrations), dust, and dead insects drawn through the roof unit intake.
Peak mould risk periods in Melbourne
The highest mould risk period in Melbourne evaporative coolers is the spring shoulder season — October and November. This is when the system has been dormant since the previous autumn (often with residual moisture in the pads and sump if not properly winterised), Melbourne’s spring warmth brings temperatures into the optimal mould growth range, and the system starts drawing in high concentrations of outdoor pollen. Systems that are not run regularly during this period — switched on occasionally as temperatures dictate — experience more mould than those used continuously on hot days.
Common mould species in Melbourne evaporative coolers
The most frequently identified mould species in Melbourne evaporative coolers are Cladosporium (dark green-black spots), Aspergillus (varied colours, common across surfaces), and Penicillium (blue-green growth). These are all widespread environmental moulds found in Melbourne’s outdoor air; the evaporative cooler concentrates and distributes them when they establish on the moist pad and sump surfaces.
Health Risks of Mould in Evaporative Cooling
Mould in an evaporative cooler distributes spores and volatile organic compounds (VOCs) through the home’s main air supply at typical airflow rates of 1,500 to 3,000 litres per second — enough to affect every room connected to the duct system. The health effects depend on the species, concentration, and the sensitivity of household members.
Respiratory symptoms
Mould exposure from evaporative coolers most commonly manifests as worsening of existing respiratory conditions — increased asthma frequency or severity, hay fever-like symptoms (runny nose, sneezing, itchy eyes), or a persistent cough that is worse when the cooling system is running. These symptoms typically improve within hours of the system being turned off and the home being ventilated — this symptom pattern is a strong indicator of system contamination.
At-risk household members
Households with elderly members, infants, asthma sufferers, or people on immunosuppressive therapy (chemotherapy, organ transplant medications) face higher health risk from mould exposure. For these households, a pre-season professional inspection and cleaning — rather than self-inspection — is the recommended approach.
Identifying Mould in Your System
Visual inspection on the roof
From the roof (with the system off and power isolated), remove one pad frame and look for: black, dark green, or grey surface discolouration of the pad; slippery or slimy coating on the sump interior walls; or dark growth on the pad frames and distribution troughs. Note that mineral scale appears white or grey and is distinct from mould — mould growth has a softer, more irregular texture and is often associated with the musty smell.
At the ceiling outlet grilles
Check the ceiling outlet grilles for dusty growth on the inner louver surfaces — this is a sign that mould spores have been distributed through the ducts. Remove a grille cover and shine a torch into the duct — any visible growth on the duct interior walls within the first metre indicates duct contamination.
Smell test
With the system running, stand beneath each ceiling outlet and check for a musty, earthy, or mildew-like odour distinct from the normal slightly mineral-scented cool air. A musty smell that is present at all outlets simultaneously points to pad or sump mould (a centralised source). Musty smell at only one or two outlets may indicate localised duct contamination near those outlets.
Cleaning and Treating Mould Safely
Pad replacement (always required for mouldy pads)
Mouldy pads must be replaced, not cleaned. Remove pads wearing gloves and dispose of them in sealed bags — disturbing mouldy pads releases spores, so work outdoors or with good ventilation. Do not shake or scrub pads before disposal.
Sump and frame disinfection
After removing the pads, clean the sump interior and pad frames mechanically (scrubbing brush, clean water) to remove scale and biofilm. Apply a registered dilute biocide solution — a 1:50 dilution of household bleach (sodium hypochlorite) is effective — to the sump interior and allow 20 minutes contact time, then rinse thoroughly with clean water. Do not apply bleach directly to installed pads or into the air stream.
Water distribution clean
Clear the dripper holes and distribution troughs with a pin or small brush. Scale-blocked drippers cause dry spots on pads — and dry spots reduce evaporation efficiency while mould-colonised spots remain moist, accelerating growth.
When to call a professional
Call FreshDuct for professional mould treatment if: the sump has visible heavy biological growth (slippery biofilm covering most surfaces); there is any evidence of duct contamination; or household members are experiencing respiratory symptoms. Professional treatment includes a full sump descale and biocide treatment, new pad installation, duct outlet inspection, and a post-treatment airflow test. Call 0431 918 137 or book online.
Preventing Mould Return in Melbourne
Mould prevention in Melbourne evaporative coolers comes down to two things: eliminating the moisture-stagnation periods that allow mould to establish, and removing the nutrient substrate (organic pad material) regularly enough that mould cannot accumulate.
- Winterise properly each April/May: drain the sump fully so no residual moisture supports winter mould growth. See our winterising guide for the full procedure.
- Replace pads every one to two seasons: fresh pads have less accumulated organic material for mould to colonise. See our pad replacement guide.
- Annual sump clean at the spring service: clean sump surfaces reduce the biofilm layer that mould attaches to.
- Run the system regularly during the season: regular high-airflow use dries the pads between operating periods and reduces the moist stagnation that mould favours.
- Use the bleed-off function: modern Brivis and Breezair controllers include a bleed-off cycle that periodically refreshes sump water, diluting organic and mineral accumulation.