A musty or mildew-like smell when a split system air conditioner first switches on, especially after it’s been idle for a while, is one of the most common mould-related calls we field that has nothing to do with the building itself. The cause almost always sits inside the head unit: a cold evaporator coil that stays damp between cycles, combined with dust and skin cells that build up on the coil fins and filter, gives mould exactly what it needs to establish inside a space most homeowners never look into. This guide covers what actually happens inside a split system to cause this, what you can reasonably clean yourself, and when the problem needs a professional rather than a DIY spray.
Why Split Systems Grow Mould in the First Place
A split system’s indoor head unit works by passing warm room air over a cold evaporator coil, which cools the air and, in the process, condenses moisture out of it, exactly the mechanism that makes the unit useful for cooling and also for dehumidifying a room. That condensation runs off into a drain tray and out through a drain line, but the coil and the surrounding fins stay damp for a period after the unit switches off, particularly in the Illawarra’s already-humid coastal air where ambient moisture is higher than average to begin with.
Dust, pollen, skin cells and general household particulate settle on the coil and filter constantly during normal operation, and in a unit that runs on cooling mode through summer, that combination of a damp surface and an organic food source is close to ideal for mould growth. Units that sit unused for weeks over winter, then get switched straight to cooling mode on the first hot day, are especially prone to producing a strong musty smell in the first few minutes of operation, since the mould that’s established over the idle period gets blown directly into the room.
Fan or “dry” mode, ironically, can make the problem worse rather than better if a unit’s self-cleaning function isn’t used, since dry mode still passes damp air over the coil without the intense cooling cycle that would otherwise flush more condensate through. Units installed in bathrooms, laundries or bedrooms with generally higher ambient humidity, common in older brick-veneer Illawarra homes with limited general ventilation, see this problem more often than units in well-ventilated living areas.
Is That Black Mould in Your Air Conditioner?
Most people find this problem by looking, not smelling: you open the front cover, or shine a torch up past the louvres, and there is black speckling across the vanes, the barrel fan or the coil fins. Whether you search for it as black mould in an air conditioner, black mold in an air conditioner, black mould in an aircon or black mould in the air con, it is the same thing, and it is one of the most common places mould turns up in an Illawarra home that has no building moisture problem at all.
Two honest points about it. First, the colour tells you almost nothing. Dark growth inside a head unit is usually a mix of species growing on a biofilm of dust, condensate and skin cells, and no one can identify a species by looking at it, including the person quoting you for the clean. “Black mould” is a colour, not a diagnosis, and our black mould facts and myths guide covers why that distinction matters when someone uses the phrase to justify a price. Second, what looks like mould on the vanes is sometimes just dust and grime bound together by condensate. That still needs cleaning, but it is a different urgency.
What matters more than the colour is where it is. Growth on the front louvres and vanes is the visible edge of what is usually a much larger colonised area on the evaporator coil and inside the blower wheel behind it, surfaces you cannot see or reach without partially dismantling the unit. Wiping the vanes clean makes the problem invisible without making it smaller, and the smell returns within days of the unit running again. That is the single most useful thing to know before deciding whether to clean it yourself or book a service.
What You Can Safely Clean Yourself
Some maintenance is genuinely within reach for most homeowners and worth doing regularly regardless of whether you’re already smelling mould:
- Removing and washing the air filters. Nearly every split system has slide-out plastic mesh filters behind the front cover, designed to be removed without tools. Rinsing them under running water and letting them dry fully before reinstalling is standard manufacturer-recommended maintenance, ideally every one to three months during heavy use.
- Wiping the front panel and visible louvres with a damp microfibre cloth and a mild detergent, which removes surface dust but doesn’t reach the coil itself.
- Running the unit’s self-clean or dry-out function, a feature on many modern split systems that runs a drying cycle after the unit is switched off, specifically to reduce residual moisture on the coil between uses. Using this consistently is one of the more effective preventive habits available.
- Checking the drain line isn’t visibly blocked where it’s accessible outside, since a slow or blocked condensate drain can cause water to back up inside the unit, though clearing an internal blockage isn’t a DIY job.
What you generally can’t safely reach yourself is the evaporator coil, the fan (blower) wheel, and the internal housing behind the filters, all of which sit behind the unit’s casing and require partial disassembly to access properly.
When the Problem Needs a Professional Service
A few signs point to needing more than a filter wash:
- A persistent musty smell that continues after filters are cleaned, which usually means the mould is on the coil or fan wheel itself, surfaces a household clean can’t reach.
- Visible black or grey buildup inside the vents or on the fan blades when you look up into the unit with the front cover open, a strong indicator of established growth rather than just dust.
- A smell that’s gotten stronger over successive seasons, suggesting the mould has had time to build up rather than being a one-off.
- Anyone in the household with asthma or mould sensitivity noticing symptoms that seem to track with when the air conditioner is running.
Professional split system mould cleaning, sometimes marketed as a “deep clean” or “chemical clean” by air conditioning technicians, typically involves partially disassembling the head unit, covering surrounding surfaces, and pressure-washing or chemically treating the coil and fan wheel in place, flushing the residue out through the drain line. This is a different service to our building-focused mould remediation work; where it becomes relevant to us is when a unit’s mould smell is actually a symptom of a bigger moisture problem in the surrounding room or wall cavity, such as a unit installed in a persistently damp bedroom in an older brick-veneer home, or where condensate has been leaking from a poorly maintained unit into a wall or ceiling cavity over time. Our guide to subfloor ventilation for Illawarra homes and our guide to mould in 1970s-80s brick veneer homes cover the building-side moisture problems that can sit behind a room’s general humidity and make a split system’s job harder than it should be.
Split System Mould vs a Building Moisture Problem
It’s worth distinguishing the two clearly, since they need completely different fixes:
| Symptom | Likely cause | Who to call |
|---|---|---|
| Musty smell only when the aircon first starts | Mould on the coil or fan wheel inside the unit | Air conditioning technician (deep clean) |
| Smell persists in the room even with the unit off | Building moisture: wall cavity, subfloor, or general ventilation issue | Mould inspection and remediation |
| Visible mould around the unit’s wall bracket or ceiling line | Condensate leak or a building moisture source behind the wall | Mould inspection first, to establish the cause |
| Water staining on the ceiling or wall near the unit | Condensate drain fault or roof leak, not just the unit itself | Air conditioning technician and, if staining persists, a moisture inspection |
If you’re not sure which category your situation falls into, particularly where there’s visible staining around the unit rather than just a smell, an inspection is the more efficient path than guessing.
FAQs
Can I use a spray-on mould cleaner inside my split system myself?
Generic surface mould sprays aren’t designed for the enclosed, electrically live environment inside a split system’s coil housing, and using them without proper disassembly and drainage can push contaminated water into places it shouldn’t go, or onto electrical components. Filter washing and the unit’s own self-clean cycle are the appropriate DIY-level steps; anything beyond that is a job for an air conditioning technician with the right equipment.
Is a musty aircon smell actually a health risk?
Airborne mould spores from an aircon coil are generally the same category of concern as any other indoor mould exposure, most people won’t react noticeably, but people with asthma, allergies or mould sensitivity can experience worsened symptoms. If anyone in the household is affected, addressing the source promptly is worthwhile regardless of how mild the smell seems.
Does running the unit in fan-only mode help dry it out?
It can help somewhat by moving air across the coil, but it’s less effective than a proper self-clean or dry-out cycle specifically designed for this purpose, if your unit has one. Fan-only mode alone won’t remove existing mould from the coil or fan wheel.
Why does the smell only happen at the start of summer?
Units that sit largely idle over the cooler months build up dust and residual moisture undisturbed, and mould that’s established during that quiet period gets released into the room as soon as the unit is switched back to cooling mode on the first hot day. Regular use and periodic self-clean cycles through the year reduce this seasonal spike.
Should I get my home checked if my aircon smell keeps coming back despite cleaning?
If a professional aircon clean doesn’t resolve the smell, or if you notice dampness or staining around the unit’s wall bracket, it’s worth having the surrounding wall or ceiling cavity checked, since a recurring smell despite a clean unit sometimes points to a building moisture source rather than the aircon itself.
Get Advice on a Persistent Mould Smell
If cleaning your split system hasn’t fixed a musty smell, or you’re seeing staining around the unit rather than just noticing an odour, get a free quote with a few photos and we’ll advise honestly on whether this is a building moisture issue worth inspecting.