Key Takeaways

• Air conditioners naturally produce condensation, which can become a breeding ground for mould without regular maintenance.

• A musty smell when the AC starts is one of the earliest warning signs.

• Mouldy units can reduce airflow, increase energy bills, and worsen allergy symptoms.

• Always disconnect power before cleaning, and treat coils, filters and the drain pan separately.

• A Power Crystals gel on the intake is the step most cleaning guides leave out entirely.

Air conditioners can keep homes lovely and comfortable in hot weather, but they naturally produce moisture as a result of condensation. Without regular maintenance, this can become a breeding ground for mould.

Homeowners and businesses should check AC units periodically for any signs of black mould in the air conditioner, since a unit that looks fine from the outside can still be harbouring a significant problem internally.

This guide covers how mould actually establishes itself inside a unit, the warning signs worth watching for, and a safe step-by-step process for clearing it out without damaging delicate components.

How Mould Ends Up Growing Inside an Air Conditioner

Understanding why mould develops can make it much easier to prevent. Moisture and dust are key components in mould growth, and unfortunately air conditioners can easily harbour both.

Condensation is naturally produced as air conditioners cool the air in your space. The moisture can collect on evaporator coils, and if condensate lines become blocked or slow-draining, standing water can build up. Dust and organic debris carried in the air provide food for mould spores, meaning that if moisture doesn't dry between AC uses, mould can easily become established within the unit.

This is why units that run in short, frequent bursts, cycling on and off throughout the day, can sometimes be more prone to internal mould than those left running steadily. Each time the unit stops, the coils are left damp with nowhere for that moisture to go until the next cycle begins.

Signs Your AC Has Mould

Mould can contribute to respiratory irritation or allergy symptoms in some people, especially those with pre-existing allergies or asthma. Therefore, it's worth recognising the signs that there may be mould in your air conditioner:

• A persistent musty smell when the system starts working.

• Visible dark spots on filters, ventilation, and air outlets.

• Excessive condensation.

• Reduced airflow as a result of dirty inner components.

• Members of the household noticing an uptick in allergy symptoms when the AC is running.

Health and Performance Risks of Running a Mouldy Unit

Health Considerations

Airborne mould spores can aggravate symptoms for asthmatics and can trigger symptoms for allergy sufferers. Long-term exposure to mould has been linked to chronic respiratory conditions and even contributes to sinusitis 

and bronchitis. For those who are particularly vulnerable, mould can exacerbate existing conditions, so signs of persistent mould should never be ignored. The Better Health Channel has further detail on how mould exposure can affect respiratory health.

Performance Issues

You may notice the following problems with a mouldy air conditioner:

Step-by-Step Cleaning Process

Fair warning: it's messy. Here's how Green Goo tackles it.

• Always disconnect the power to your AC before any cleaning begins.

• Wash any reusable filters, following the manufacturer's instructions. Disposable filters should be replaced as necessary.

• Wipe the vents and grilles, removing all visible dust before beginning to treat mould.

• Get the bulk 5 L Anti-Mould Spray onto the coils; it's built for a job this size. Watch the mould melt away in around five minutes, then give it a brush with the Grout Remover Brush to work it loose. Avoid damaging the delicate fins.

• Remove any standing water from the drain pan and clear blockages from the condensate line so that water can drain freely.

• Finish with a Power Crystals tub on the intake. This is the step that actually stops it from coming back; most guides leave it out entirely. Our full mould remover gel range covers the right size for different room setups.

Preventing Mould From Returning in Your AC

Once you've treated the black mould in your air conditioner, keep these simple maintenance habits going to stay on top of the problem in future:

• Clean or replace the filters regularly, and keep condensate drains free of obstructions.

• Service the unit as advised by the manufacturer.

• Where possible, use the fan-only setting on occasion, as this can help to dry internal components after spells of cooling.

• Keep the surrounding vents clean and unobstructed.

• If the home is consistently damp, address the humidity promptly. Our mould killers range can help with any mould that's spread beyond the unit itself.

Window Units vs. Split Systems: Does It Make a Difference?

Both window units and split system air conditioners produce condensation and can develop mould, but the risk areas differ slightly. Window units combine everything- coils, fan, and drainage- in a single housing exposed to outdoor humidity on one side, which can make internal moisture harder to inspect and clear.

Split systems separate the indoor unit from the outdoor compressor, so mould risk is generally concentrated around the indoor coil and filters, the parts of the system this guide focuses on. Ducted systems add another layer of complexity, since mould can potentially develop inside ductwork itself, an area generally best left to a qualified technician to inspect.

Why Regular Servicing Matters More Than a Once-Off Deep Clean

A thorough clean fixes the immediate problem, but air conditioners are one of the few household appliances that create the exact conditions mould needs: moisture, warmth and trapped dust, every single time they're used. Without a regular servicing schedule, the same conditions that caused the original outbreak simply rebuild themselves over the following weeks or months.

Setting a reminder to check filters monthly during heavy-use seasons, rather than waiting for a musty smell to prompt action, is usually the difference between an occasional wipe-down and a repeated deep-clean project.

Frequently Asked Questions

Can I Run My AC While It Has Mould?

We recommend that if you suspect significant amounts of mould in your air conditioner, you should stop using it until it has been fully treated. Running a mouldy system risks spreading mould spores throughout your space.

How Often Should I Clean My Filters?

During periods of heavy AC use, you should check the filters every month. Clean or replace the filters as often as recommended by the manufacturer. Remember that homes with pets or in particularly dusty areas may need more frequent maintenance.

Is It Normal for an AC to Smell Musty When First Turned On?

A brief musty smell in the first few seconds can happen occasionally, but a persistent or strong odour that continues once the unit is running is not normal and usually points to mould or standing water somewhere in the system.