Mould inside a split system is a condensation problem: humid air meets the cold indoor coil, water collects on the coil and drain tray, and a dusty filter or coil that stays damp between cycles grows mould. That’s the musty blast you get on start-up. A dusty filter is a DIY clean; a smell that returns after cleaning, or mould on the coil or fan, needs an air conditioning technician.
Why does the aircon smell musty when it starts?
Air passing over a damp, mould-affected filter or coil picks up the odour and pushes it into the room. It’s usually strongest in the first minute or two and fades as the airflow settles. That timing is the giveaway: if the smell tracks the unit switching on, the unit is the source.
The reverse check matters too. If the unit runs odourless but the room still smells musty with it off, look elsewhere: a wardrobe on a cold wall, the subfloor or the roof void. The musty smell guide walks through those patterns.
What causes mould inside a split system?
- Residual moisture. A coil and drain tray that stay damp between cycles in a shut-up room never fully dry.
- An overdue filter. Dust and household grime on the filter give mould something to feed on.
- A slow drain. Dust, slime and mineral build-up in the drain line leave water sitting in the tray.
- Short cycling. A unit that cools quickly then stops repeats the cold, wet, then still cycle more often.
- Humid rooms. Coastal humidity and closed-up winter rooms give the coil more moisture to deal with.
None of this is specific to one suburb; it’s what happens when refrigerated cooling runs in a humid climate. The same conditions are why bathroom and bedroom ceilings in the same homes grow mould.
What can I clean myself?
DIY is reasonable when:
- The mould or dust is on the filter, which is designed to be removed, washed and dried
- It’s the first time, not a repeat after cleaning
- You can reach it without opening the casing
- There’s no staining or smell on the wall or ceiling around the unit
Most split system filters slide or clip out. Follow the manual: wash in mild detergent, rinse, and let it dry completely before refitting.
Call an air conditioning technician when:
- The smell or speckling returns within weeks of a filter clean
- Mould is visible on the coil, fan barrel or inside the housing
- The unit drips or the drain isn’t working
- It’s a ducted system, where the coil and ducting aren’t accessible without equipment
Don’t open up the unit to clean the coil yourself. The Australian Refrigeration Council notes that under the Australian Government’s Ozone Protection and Synthetic Greenhouse Gas Management Regulations 1995 it is an offence to handle refrigerant without a Refrigerant Handling Licence, and an amateur coil clean can damage the unit or void a warranty.
Technician or mould remediation?
| What you’re seeing | What’s usually going on | Who to call |
|---|---|---|
| Musty smell on start-up, dusty filter, first time | Dust and light mould on the filter | Nobody: wash the filter |
| Smell or speckling back within weeks of a filter clean | Mould on the coil, fan or drain tray | Air conditioning technician |
| Musty smell with the aircon off, or you can’t tell if it’s the unit | Probably a building moisture source | Mould inspection and moisture investigation |
| Mould, staining or bubbling paint on the wall or ceiling around the unit | Condensation or a drain fault has reached the building | Mould treatment of the affected surface |
Servicing the appliance is air conditioning work. Illawarra Mould Removal doesn’t arrange it. Where it crosses over is the building around the unit: if an overflowing drain or dripping unit has left mould on the ceiling or wall, that’s a building-surface job, handled much like bathroom and ceiling mould removal. It’s also worth an inspection if the unit has been serviced, its smell is gone, and the room still smells musty, because that points to a separate moisture source.
Stopping it coming back
- Clean the filter on the manufacturer’s schedule, more often in a bedroom unit used every night.
- Run fan-only or the unit’s dry or clean mode after cooling, if it has one, so the coil dries rather than sitting wet overnight.
- Keep the room’s humidity down by airing it on dry days and running exhaust fans when showering or cooking, as NSW Health recommends for preventing mould generally.
- Have the drain line checked at each service; a slow drain is invisible until it overflows.
- Glance at the wall below the unit each season for staining or bubbling paint.
If the smell still comes back, describe the pattern (on start-up, constant, worse after rain) and your suburb through the quote form. The enquiry goes to an independent local contractor, who can tell you whether it’s a servicing issue or a building one.
Is aircon mould a health risk?
This page doesn’t give medical advice. NSW Health says mould spores can cause symptoms such as a runny or blocked nose, eye and skin irritation and sometimes wheezing in people who are sensitive or allergic, and can trigger asthma, while most people have no symptoms. If anyone at home has symptoms, see a GP. The mould smell and symptoms guide has more on what the evidence says.
Frequently asked questions
Can I just wash the filter myself?
Yes, for a first-time dusty or lightly mouldy filter: remove it as the manual describes, wash it in mild detergent, rinse and dry it completely before refitting. If the smell returns within weeks, the problem has likely reached the coil or drain tray, which is a technician’s job.
Is it safe to clean the coil myself?
Not recommended. The coil sits on a sealed refrigerant circuit, handling refrigerant without a Refrigerant Handling Licence is an offence, and coil cleaning done wrong can damage the unit or void the warranty. The filter is the DIY boundary.
Does mould in my aircon mean the whole house has a mould problem?
Not necessarily. Aircon mould is often contained in the appliance and clears with a filter clean or a service. Look wider if the smell persists with the unit off, or there’s mould on the wall or ceiling around it.
Who do I call: an aircon technician or a mould service?
A technician for anything inside the unit: filter, coil, fan, drain line or refrigerant. A mould service when the mould or smell has spread to the surrounding ceiling or wall, or you can’t tell whether the aircon is the real source.
Will a dehumidifier fix mould in my air conditioner?
Not directly. It can lower room humidity, but it won’t remove mould already on a filter or coil; see do dehumidifiers help with mould.