Why Does My Split System Smell Musty?

If your split system in Melton starts up and pushes out a musty, stale, or damp earthy smell, you’re not imagining it — and it’s not something to ignore. That odour is almost always the result of mould, mildew, or bacteria growing inside the indoor unit, and in some cases within the ductwork. Beyond being thoroughly unpleasant to live with, a contaminated split system is actively circulating air through biological growth before delivering it into your living space.

For households in West Melbourne with children, elderly residents, or anyone managing asthma, allergies, or respiratory conditions, this is a genuine indoor air quality concern, not a cosmetic issue. This guide explains exactly where the smell is coming from, why it develops, what you can do yourself, and when heating and cooling specialist in Melton is needed to fix it properly.

Where the Smell Is Coming From

A musty smell from a split system can originate from several different components inside the indoor unit. Identifying which part is affected helps explain why a simple filter clean doesn’t always resolve the problem — and why a proper deep clean often requires disassembly of the unit.

Mould on the Evaporator Coil

The evaporator coil is the core component of the indoor unit, and it operates in permanently wet conditions by design. During cooling operation, the coil surface drops below the dew point of the incoming air, which causes condensation to form continuously on the coil and its surrounding fins.

Mould and mildew establish on the wet coil surface over time, particularly in the narrow gaps between the fins where moisture sits and airflow is limited.
When the fan forces air across the coil, the air picks up spores and the characteristic musty odour before being delivered through the vents into the room.
Coil mould is significantly more common in systems that run heavily through Melbourne’s humid summer periods, and in systems where a clogged filter has forced debris past the filter and deeper into the unit.
A contaminated coil often produces a smell that’s strongest at startup and reduces slightly once the system has been running for a few minutes — though it rarely disappears entirely.

Mould in the Drip Tray and Drain Line

The condensate drip tray sits directly beneath the evaporator coil and collects the water that drips off the coil surface during operation. In a healthy system, this water drains promptly and continuously through the condensate drain line. When the drain is partially blocked or slow, problems develop quickly.

Standing water in the drip tray — even for short periods — creates an ideal environment for mould, bacteria, and algae to establish in the warm, enclosed interior of the unit.
A contaminated drip tray typically produces a damp, earthy smell that’s particularly noticeable at startup when the fan first begins circulating air through the unit.
In Melton and Bacchus Marsh homes where the split system runs heavily through summer and then sits unused through winter, water that wasn’t fully drained before shutdown can sit in the tray for months, allowing significant biological growth to develop before the system is next used.
A slow or blocked drain may also eventually cause the tray to overflow, leading to water damage in the wall or ceiling around the indoor unit.

Mould on the Blower Wheel

The blower wheel is the cylindrical fan that draws air from the room across the evaporator coil and pushes the conditioned air through the vents. Over time, a combination of fine dust particles and condensation moisture accumulates on the wheel’s fins and hub.

This combination of organic dust and moisture creates excellent conditions for mould to establish on the blower wheel surface.
Unlike coil mould, which tends to produce a smell mainly at startup, blower wheel contamination typically produces a musty smell that persists consistently throughout operation — because the wheel is always in motion and always pushing air through the contaminated surface.
The blower wheel is one of the most difficult components to access for cleaning without disassembling the indoor unit, which is why consumer spray products rarely resolve the problem when the blower is the source.

Dirty or Mould-Affected Filters

The filter is the most accessible component of the indoor unit and the easiest to check. Occasionally, the filter itself is the source of a musty smell — but more often, a contaminated filter is a symptom of the wider problem rather than the sole cause.

Filters that were put back wet after cleaning, or that have accumulated enough organic material to support biological growth, can produce a musty odour independently.
Visible mould on the filter surface — often appearing as dark spots on the mesh — confirms the filter is contributing to the smell.
Clean or replace the filter and ensure it is completely dry before reinstalling. If the smell persists after the filter is clean and dry, the source is further inside the unit.
Filters should be cleaned every six to eight weeks during active use in Melton and Bacchus Marsh homes, and always allowed to dry in a shaded area rather than reinstalled damp.

Mould in the Ductwork

For ducted split systems — where the indoor unit distributes air through flexible ductwork in the roof cavity to multiple rooms — mould can establish itself within the duct runs, not just inside the unit itself.

Older flexible ductwork retains moisture in its insulation layer and inner lining, providing both moisture and organic material for mould to grow on.
In Melton’s older homes where ductwork hasn’t been replaced in 15 or more years, duct contamination is a common source of musty odours that persist even after the indoor unit has been professionally cleaned.
Duct mould is more difficult and expensive to address than indoor unit mould, and in severe cases may require partial or full duct replacement rather than cleaning alone.

Why Mould Develops More Often Than You’d Expect

The interior of a split system’s indoor unit creates conditions that are genuinely favourable for biological growth. Understanding why helps explain the pattern of how and when the problem develops in Melton and Bacchus Marsh homes.

The Conditions Inside a Split System

Darkness: the interior of the unit receives no UV exposure that would otherwise inhibit biological growth.
Periodic moisture: the coil and drip tray are wet during cooling operation, providing regular moisture.
Moderate temperatures: the interior of the unit is never too hot or too cold to support mould growth — conditions stay within the range where biological activity is sustained.
Dust supply: every split system draws room air through the unit, and a proportion of fine dust always passes through even a clean filter, providing an ongoing nutrient source for mould.

The Summer-Winter Pattern

The most common pattern that leads to a severely contaminated split system is one that’s particular to Melton and Bacchus Marsh homes: heavy summer use for cooling, followed by months of complete inactivity through the cooler months when ducted gas heating is used instead.

At the end of summer, the coil and drip tray contain residual moisture from the final cooling sessions.
If the system isn’t run in fan-only mode to dry out before being shut down for winter, that moisture sits in a warm, closed, dark environment for three to six months.
By the time the system is switched on again in the following summer, mould has had an entire off-season to establish and spread through the unit.
This pattern explains why the worst mould problems are often discovered at the start of the cooling season, and why a system that smelled fine at the end of last summer now smells significantly worse at the beginning of the new one.

Can You Fix a Musty Split System Yourself?

Partially. There are things you can do at home that address the most accessible causes, and preventive habits that significantly reduce the likelihood of mould developing in the first place. But for a unit that’s already developed a significant mould problem, DIY measures have real limitations.

What You Can Do at Home

Clean and dry the filter thoroughly. Remove it from the unit, rinse it under running water from the back face outward to push trapped debris out rather than forcing it deeper, and allow it to dry completely in the shade before reinstalling.
Run the system on fan-only mode for 30 to 60 minutes after each cooling session. This dries out the residual moisture on the coil and in the drip tray before the system sits idle, and is one of the most effective preventive habits you can establish. Many manufacturers recommend this in their operating manuals.
Run fan-only mode for an hour before shutting the system down for the season. This is the seasonal version of the same habit — giving the unit a chance to dry out thoroughly before months of inactivity.
Use a consumer foaming coil cleaner spray if the smell is mild. These biodegradable sprays are applied to the coil surface through the front of the unit without disassembly, and the foam carries contaminants into the drain. They can reduce mild coil mould and are better than nothing, but they don’t reach the blower wheel and don’t always address the drain adequately.

Where DIY Falls Short

Accessing the blower wheel properly requires removing the front panel and often other components of the indoor unit. Consumer spray products simply don’t reach the wheel in most indoor unit designs.
Clearing a partially blocked drain and treating it to prevent algae regrowth requires flushing the drain line and often applying a drain treatment tablet or solution — steps that aren’t practical without partial disassembly.
If the smell is already strong and persistent, a consumer coil spray will typically reduce it temporarily rather than resolve it. The mould is often established on multiple surfaces simultaneously, and surface treatment of one component doesn’t address the others.

What a Professional Deep Clean Involves

A professional split system deep clean is substantially different from a routine filter service. Here’s what a thorough service by a qualified technician actually involves.

The Process Step by Step

Removal of the front panel, louvre blades, and internal covers to fully expose the evaporator coil, drip tray, and blower wheel.
Application of a biodegradable, low-toxicity coil cleaner that penetrates the fin gaps on the evaporator coil, killing mould and bacteria at the surface level and in the areas a spray alone can’t reach.
High-pressure rinsing of the coil using a containment bag that fits over the indoor unit to capture all wastewater, which is contaminated with mould, bacteria, and cleaning solution and should not be allowed to run down walls or into ceiling cavities.
Cleaning of the blower wheel using specialist tools — typically a stiff brush or purpose-designed wheel cleaner — to dislodge mould and accumulated dust from each fin of the wheel.
Flushing and treatment of the condensate drain line to clear any partial blockages and treat the drain to inhibit future algae and mould growth.
Cleaning of the drip tray, including removing any standing water, biological growth, and debris.
Full reassembly, followed by a test run through a complete cooling cycle to confirm the smell is resolved, the unit is draining correctly, and all components are operating normally.

When to Book a Deep Clean Rather Than a Routine Service

If you’re booking a service specifically for a musty smell, make this clear when you book. A standard annual service may not include blower wheel cleaning unless the technician knows it’s needed.
If the smell has been present for more than one season, a deep clean is almost certainly required rather than a surface treatment.
If other household members have noticed worsening allergy or asthma symptoms during periods when the split system is running, a deep clean should be prioritised.

Preventing a Musty Smell From Returning

Once the unit has been deep cleaned, the right maintenance habits will significantly extend the time before the problem returns — and in many cases, prevent it from returning at all at the same severity.

Ongoing Maintenance Habits

Clean the filter every six to eight weeks during active use, and always allow it to dry completely before reinstalling.
Run fan-only mode for 30 to 60 minutes after each cooling session during summer. Set a reminder if needed — it’s the single most effective preventive habit for reducing coil and drip tray contamination.
Run a fan-only session for at least an hour at the end of the cooling season before switching the system off for winter.
Book a professional service at least once per year. In Melton and Bacchus Marsh, where systems run hard through long, hot summers, an annual service before the cooling season starts is the recommended minimum.
Ask specifically about blower wheel condition during your next service — a technician can advise whether the wheel is clean or beginning to accumulate growth.

Conclusion

A musty split system isn’t a minor inconvenience that you run through summer hoping it improves on its own. The smell won’t go away by itself. The mould causing it doesn’t reduce through normal operation — if anything, running a contaminated unit continuously during summer gives the mould more moisture and more time to spread further through the unit and into the ductwork.

Left unaddressed for multiple seasons, a split system mould problem becomes progressively more difficult and expensive to resolve, and the air quality impact on your household becomes more significant over that same period.

New Image Heating and Cooling in Bacchus Marsh and Melton provides professional split system deep cleaning services. If your unit smells musty at startup, if household members have noticed allergy or breathing symptoms when the system is running, or if it’s simply been more than a year since your last professional service, call us now. We’ll assess the unit, give you an honest report on what’s needed, and carry out a thorough clean that actually resolves the problem.

Call 0468 125 098 or submit our online form to book a split system service. Don’t spend another summer running a contaminated system through your home.

Frequently Asked Questions

Is the musty smell from my split system harmful to breathe?

Mould spores and bacteria circulated through a contaminated split system can aggravate asthma, allergies, and other respiratory conditions. For generally healthy adults, short-term exposure is unlikely to cause serious harm. However, ongoing exposure in a home where the system runs for multiple hours daily is a meaningful air quality concern, particularly for children, elderly residents, and anyone with existing respiratory sensitivities. The prudent approach is to have the system cleaned rather than continue running it contaminated.

My split system only smells musty for the first few minutes and then the smell goes. Is that normal?

A brief dusty or stale smell at first startup after a long idle period — particularly at the start of the cooling season — can be normal as accumulated fine dust is displaced. A musty or mouldy smell that dissipates after a few minutes most likely indicates mould is present but still at an early stage. It is unlikely to stay at that level: left untreated, it will typically worsen over successive uses as mould spreads further through the unit. A professional clean before it develops further is the right response.

I cleaned the filters but the musty smell is still there. What next?

If a thorough filter clean and dry didn’t resolve the smell, the source is further inside the unit — most likely the evaporator coil, blower wheel, or drip tray, all of which require partial disassembly to access and clean properly. Book a professional service and specifically request a coil and blower wheel clean, not just a routine filter service. A technician who knows the issue in advance will come prepared with the right equipment and cleaning products.

How do I prevent my split system from developing a musty smell again after it’s been cleaned?

The most effective prevention is running the system on fan-only mode for 30 to 60 minutes after each cooling session, which dries out the coil and drip tray before the unit sits idle. Clean the filter every six to eight weeks during active use and always dry it fully before reinstalling. Have the unit professionally serviced at least once a year. At the end of the cooling season, run fan-only mode for at least an hour before shutting the system down for winter. These habits, consistently applied, significantly reduce the likelihood of mould re-establishing at the same rate.

Could the musty smell be coming from somewhere other than the split system?

If the smell is only present when the split system is running and stops when it’s switched off, the system is the source. If the smell is present regardless of whether the system is running, the source may be elsewhere in the home — damp roof cavity insulation, mould in wall cavities, damp carpet or underlay, or similar. A technician can confirm during an inspection whether the unit is the source. In homes where the smell persists after a professional clean of the unit, duct contamination in older flexible ductwork should also be investigated.

How much does a professional split system deep clean cost?

The cost varies depending on the unit size, how accessible the indoor unit is, and the extent of the contamination. A standard split system deep clean — including coil, blower wheel, drip tray, and drain flush — is typically priced differently from a routine filter service. New Image Heating & Cooling provides upfront pricing before any work begins. Call 0468 125 098 for a specific quote based on your unit and its current condition.

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