Why Mould in Your Air Conditioner Keeps Spreading

Mould in your air conditioner sends spores into every room each time the unit runs.

Most people notice a musty smell when they switch on the air con and assume the unit just needs a filter clean. The reality is that mould colonies growing inside the unit have already spread through the coil, drain pan and ductwork before you smell anything.

This article covers how mould takes hold in air conditioning systems, what it does to your indoor air quality, and why the only permanent fix is professional treatment that reaches the parts you cannot see.

How Mould Gets Inside Your Air Conditioner

Air conditioners create the three conditions mould needs to grow: moisture, warmth and a food source. Condensation forms on the evaporator coil every time the unit runs, and dust particles settling on wet surfaces give mould spores exactly the organic matter they feed on.

The drain pan that catches this condensation is the most common starting point. If the drain line is partially blocked, even by a thin film of biofilm, water sits in the pan long enough for spore germination to begin within 24 to 48 hours.

Component	Why mould grows there	How quickly
Evaporator coil	Constant condensation mixed with dust	24 to 48 hours
Drain pan	Standing water collects organic debris	24 to 48 hours
Ductwork	Warm air carries moisture and dust buildup	1 to 2 weeks
Filter housing	Trapped dust absorbs ambient moisture	1 to 2 weeks
Blower fan	Organic buildup coats blade surfaces	2 to 4 weeks

Newcastle and Central Coast homes are particularly vulnerable. Coastal humidity regularly sits above 60%, and older split systems without adequate drainage create the perfect environment for rapid mould colonisation.

Signs Your Air Conditioner Has a Mould Problem

The first sign is usually a damp, musty smell when the system starts. By the time you notice it, the colony is already established inside the unit and potentially through the ductwork.

Watch for these warning signs:

• A musty or earthy smell that appears within minutes of turning on the unit
• Visible dark spots around the vents, louvres or inside the filter housing
• Increased sneezing, coughing or itchy eyes when the system runs
• A thin film of black or green residue on the air filter
• Unexplained condensation or water marks around the indoor unit

According to NSW Health, mould exposure can cause nasal congestion, eye irritation, skin rashes and wheezing. People with asthma or weakened immune systems are at higher risk of serious respiratory episodes.

Why Cleaning Your Air Conditioner Yourself Does Not Fix the Problem

Retail air conditioner cleaning sprays and foam products treat the surfaces you can reach. The evaporator coil fins, the inside of the drain pan and the ductwork behind the indoor unit are not accessible without disassembling the system.

Even when you remove the filters and spray into the unit, the product only contacts a fraction of the contaminated surface area. Mould root structures (hyphae) embedded in dust buildup and biofilm on internal components survive surface-level treatment and regrow within two to six weeks.

Here is why DIY air conditioner mould removal fails every time:

• Retail sprays reach less than 20% of the internal surface area
• Mould hyphae grow beneath biofilm that surface sprays cannot penetrate
• Wiping visible mould without containment releases thousands of spores into the room
• The moisture source (condensation) is never addressed by cleaning products
• Without a residual antimicrobial barrier, recolonisation begins immediately

There is no consumer product available in Australia that treats the root structure of mould inside an air conditioning system. Surface cleaning removes what you can see and leaves everything you cannot.

What Happens When You Keep Running a Mouldy Air Conditioner

Every time the unit cycles, the blower fan pushes air across contaminated coils and through mouldy ductwork. Those spores are distributed to every room connected to the system, including bedrooms where people spend eight hours breathing.

The Better Health Channel lists respiratory irritation, allergic reactions and worsened asthma among the health effects of mould exposure. An air conditioner actively distributing spores amplifies this across every room in the home.

The health effects escalate with ongoing exposure:

• Short-term: sneezing, coughing, nasal congestion, throat irritation, itchy or watery eyes
• Medium-term: persistent headaches, fatigue, skin rashes, worsening asthma symptoms
• Long-term: chronic respiratory conditions, sensitisation to mould allergens, recurring sinus infections

The other problem is structural. Mould that starts in the air conditioner migrates into wall cavities and ceiling spaces through the ductwork.

Homeowners across Lake Macquarie and the Hunter Valley regularly discover mould behind walls that originated from an untreated split system. A mouldy air conditioner is not a comfort issue. It is an active contamination source that gets worse each day it runs.

How Professional Mould Treatment Stops the Cycle

Professional treatment works differently because it addresses the three things DIY cannot: full internal access, root-level treatment and residual protection.

Mould and Hygiene Solutions starts every job with a full inspection of the air conditioning system and surrounding areas. This identifies the extent of contamination, including secondary colonies in ductwork and wall cavities that are invisible from outside the unit.

The treatment uses an Australian-made, non-hazardous, non-corrosive and environmentally friendly antimicrobial solution that penetrates biofilm and kills mould at the root structure. This is not a surface spray.

It reaches the hyphae embedded in the substrate and eliminates the colony entirely. Retail products cannot do this because they are not formulated to penetrate beyond the visible surface layer.

Here is what professional treatment includes:

• Full disassembly and inspection of the indoor unit
• Treatment of evaporator coil, drain pan, blower fan and accessible ductwork
• Application of the antimicrobial solution to all contaminated surfaces
• Moisture source identification and recommendations to prevent recurrence
• A 12-month mould-free guarantee on all treated areas

The 12-month guarantee matters because it proves the treatment works at the root level. If surface cleaning were enough, no company could stand behind that commitment.

Keeping Your Air Conditioner Mould-Free After Treatment

Professional treatment eliminates the existing colony, but preventing recolonisation requires ongoing attention to the conditions that caused the problem in the first place.

These steps reduce the chance of mould returning:

1. Run fan-only mode for 15 to 20 minutes after cooling to dry the internal components
2. Replace or clean filters every four to six weeks during heavy use periods
3. Keep indoor humidity below 60% using exhaust fans and ventilation
4. Schedule a professional mould prevention inspection every 12 months
5. Address any water leaks or condensation issues around the indoor unit immediately

Even with good maintenance habits, air conditioners in coastal areas like Newcastle, the Central Coast, Lake Macquarie, Hunter Valley and Port Stephens face higher humidity year-round. Annual professional inspections catch early colonisation before it spreads through the system again.