HVAC Mould in Commercial Buildings: Prevention, Detection & Remediation

If your commercial building smells musty when the air conditioning kicks in, there's a good chance mould has colonised your HVAC system. It's one of the most common — and most overlooked — indoor air quality issues in Australian commercial buildings.

The problem isn't just cosmetic. Mould in HVAC systems distributes spores throughout every room the system serves. A single contaminated air handling unit can affect an entire floor. For building managers and facility teams, this creates health risks, compliance exposure, and tenant complaints that escalate fast.

How mould gets into HVAC systems

Air conditioning systems create exactly the conditions mould needs: moisture, darkness, and organic material to feed on.

Condensation is the primary driver. Every time your system cools warm air, moisture condenses on coils, drip trays, and ductwork surfaces. If this moisture doesn't drain properly — or if the system cycles off before surfaces dry — mould colonises within 24–48 hours.

Filters trap dust, pollen, and skin cells — all organic material that mould feeds on. Wet or damp filters become breeding grounds. Many buildings run filters well past their replacement schedule, compounding the problem.

Ductwork provides kilometres of dark, undisturbed surface area. Insulation lining inside ducts absorbs moisture and is almost impossible to dry once wet. Flex duct connections sag and trap condensate, creating permanent wet spots.

Fresh air intakes near cooling towers, exhaust vents, or vegetation can introduce external mould spores directly into the system.

Health and compliance risks

Mould-contaminated HVAC systems are a serious occupational health issue. Building occupants inhale mould spores continuously while the system runs — often without knowing it.

Common complaints include:

• Persistent coughing, sneezing, and respiratory irritation
• Headaches and fatigue (often attributed to "sick building syndrome")
• Aggravated asthma and allergic reactions
• Eye and throat irritation

Under Work Health and Safety (WHS) legislation, building owners and employers have a duty to provide a safe working environment. Indoor air quality is explicitly covered. If occupants report health symptoms and mould is found in the HVAC system, the building owner faces potential liability.

For aged care facilities, hospitals, and childcare centres, the stakes are higher. Vulnerable populations are more susceptible to mould-related illness, and regulatory scrutiny is tighter.

Signs your HVAC system has mould

Watch for these indicators:

• Musty odour when the system starts — the most common early sign
• Visible mould on diffusers, grilles, or around vent openings
• Dark staining on ceiling tiles near supply vents
• Increased sick leave or occupant complaints about air quality
• Condensation visible on ductwork, diffusers, or ceiling panels
• Standing water in drip trays or drain pans

If you suspect contamination, don't wait for complaints to escalate. Professional air quality testing can confirm mould presence and species within days.

Professional remediation approach

HVAC mould remediation requires specialist equipment and methodology. Surface cleaning alone doesn't work — mould colonies inside coils, drain pans, and lined ductwork need targeted treatment.

1. Assessment and air quality testing

Professional air sampling establishes baseline spore counts inside the building versus outside. Surface swabs from coils, drip trays, and duct interiors identify species and concentration. This data drives the treatment plan.

2. Containment and isolation

Affected sections of the HVAC system are isolated to prevent cross-contamination during treatment. Negative air pressure prevents spores from spreading to occupied areas.

3. Mechanical cleaning

Coils, drain pans, filters, and accessible ductwork are physically cleaned to remove visible mould colonies and accumulated biofilm.

4. Treatment

At Pureairo, we use SAN-AIR technology — a 100% natural, TGA-accredited antimicrobial that kills mould spores on contact. The fogging process disperses treatment throughout the entire duct system, reaching concealed surfaces that mechanical cleaning can't access.

Unlike chemical biocides, SAN-AIR leaves no toxic residues in the airstream. The building can be reoccupied the same day.

5. Ongoing protection

SAN-AIR Aerosperse gel systems installed in air handling units provide continuous antimicrobial protection for up to 3 months, preventing recolonisation between maintenance cycles.

Prevention strategies

The best approach is preventing mould from establishing in the first place:

• Maintain drain pans and condensate lines — check monthly for standing water and blockages
• Replace filters on schedule — never run wet or visibly contaminated filters
• Run the system to dry — after cooling cycles, run fans for 15–20 minutes to dry internal surfaces
• Monitor humidity — maintain indoor relative humidity below 60%
• Schedule regular HVAC inspections — include coil and ductwork checks, not just filter changes
• Consider UV-C treatment for coils in high-risk environments

What building managers should do now

If you manage a commercial property and haven't had your HVAC system inspected for mould in the past 12 months, it's time. Early detection and treatment is significantly cheaper than remediation after contamination has spread through the entire system.

Contact us for a commercial HVAC assessment — we work with building managers, strata committees, and facility teams across Sydney, Brisbane, Melbourne, and Canberra. Our assessments include air quality testing, system inspection, and a detailed report with recommendations.

For portfolio-scale solutions, see our commercial and strata services.