A musty smell in the first minute is the usual clue. Here is why the inside of a split grows mould, the two-week filter and the drying run the manuals ask for, what our deep clean covers (and the three things it does not), and the once-a-year rule.
Why the inside of an air conditioner grows mould
An air conditioner on cool is a dehumidifier. Room air crosses a coil cold enough to wring the moisture out of it, the water runs into a tray, and the barrel fan pushes the dried air back out. Everything on that path is damp for as long as the unit runs and for hours after it stops.
That is the condition mould needs. NSW Health's factsheet puts it plainly: mould grows in wet or moist areas that lack adequate ventilation. A closed plastic box holding a wet coil, a wet tray and a wet fan, in a room where the Bureau's Richmond station averages 47 percent humidity at 3pm in summer, is exactly that.
The manufacturers say the same thing in their own manuals. Mitsubishi Electric's MSZ-LN manual: dirty filters cause condensation in the air conditioner, which contributes to the growth of fungi such as mould. Panasonic's Z Series instructions tell you to run the unit on heat for two to three hours before a long break, to remove moisture and prevent mould.
The signs
The first one is the smell. A musty note in the first minute after start-up, before the coil cools down, is air passing over what has grown on the barrel and coil.
Mitsubishi Electric's answer when dirt or smell cannot be cleaned from the filter is to replace it, and Panasonic warns that a dirty unit can trip its H99 freeze-protection code.
The second is what you can see. Open the front panel and look at the filter, then shine a torch past the horizontal vane at the barrel fan behind the outlet.
Dark speckling on the vane or a grey fur on the fan blades is growth on the surfaces the manuals warn about, and it comes out of the unit every time the fan runs.
The third is in the household. NSW Health lists a runny or blocked nose, irritated eyes and skin and sometimes wheezing as the common reactions, and people with asthma, a weakened immune system or a chronic lung disease as the ones most affected. Most people notice nothing, which is why the smell is usually the first clue.
A unit that drips is a related sign: a blocked drain keeps the tray full, and a full tray is a mould tray. Where the water comes from covers that one.
What you can do yourself: the two-week filter and the drying run
Both manuals put the air filter on a two-week rhythm. Mitsubishi Electric says vacuum it or rinse it with water and dry it in the shade; Panasonic says wash it gently, dry it away from sun and heat, and replace any damaged filter. The MSZ-LN's black deodorising filter is a three-month job, soaked in lukewarm water for fifteen minutes.
The other habit is the drying run. Mitsubishi Electric's manuals say to operate in cool mode at the highest temperature setting, or in fan mode, for three to four hours before the unit is left unused, because moisture inside it favours mould.
Panasonic's version is heat mode for two to three hours. The point is the same: a dry coil does not grow anything.
What not to do is wash the indoor unit itself. Panasonic's safety page says do not wash the indoor unit with water, and Panasonic also says the coils and fans should be cleaned periodically by an authorised dealer. The filter and the front panel are yours; the coil, the barrel fan and the tray are the clean.
What a deep clean covers, and what it does not
- In the clean
- Filters washed
- Indoor coil and barrel: chemical anti-bacterial wash
- Barrel fan removed and washed
- Drain tray and drain line flushed
- Outdoor unit coil rinsed
- The wall covered so nothing drips
- Ducted: return air grille and filter, fan coil unit, ducts and outlets cleaned
- Ducted: leaky duct joints sealed
- Not in the clean
- Sanitising sprays or deodorisers
- Duct fogging
- A regas
- Repairs found during the clean: quoted separately
- Mould on the wall or ceiling around the unit
Our deep cleaning takes the indoor unit apart. The barrel fan comes out and is washed, the coil and barrel get a chemical anti-bacterial wash, the filters are washed, the drain tray and drain line are flushed and the outdoor coil is rinsed. The wall is covered first so nothing drips on it.
On a ducted system the return air grille and filter, the fan coil unit's coil and fan, the ducts and the outlets are cleaned, and leaky duct joints are sealed.
Three things are not part of it. There are no sanitising sprays and no duct fogging: the clean is mechanical and chemical, on the parts that get dirty.
A regas is a separate job, and so is any repair the technician finds while the unit is open, which is quoted separately. Mould on the wall or ceiling around the unit is a building problem, not an air conditioner one.
The manufacturers' own self-cleaning features sit alongside this, not instead of it. Samsung's Aero and WindFree list an Auto Clean cycle and a Freeze Wash in their specification sheets, and Mitsubishi Electric's standard air filter carries an anti-mould treatment. They keep a unit cleaner for longer; Panasonic still sends the coil and fan to the dealer.
How often
Once a year, or sooner if the unit smells musty on start-up or someone in the house has allergies. That is the rule we book to, and it sits on top of the two-week filter and the drying run, not in place of them.
A clean is not a service. A service checks and maintains the whole system, refrigerant, electrics and drain included; the clean is the layer under it, for the parts that grow things. A system that is serviced but never cleaned can still smell.
Mould on the wall around the unit
If the growth is on the plasterboard above or beside the indoor unit rather than inside it, the cause is usually water: a drain that backs up, a condensate line that sweats, or a pipe penetration that lets weather in. Fix the water first or the mould returns.
NSW Health's guidance for the surface, once the water is fixed:
- a mild detergent, or vinegar diluted four parts to one in water, for routine cleaning
- diluted bleach for stubborn patches, with gloves and the room ventilated, then let it dry thoroughly
- professional cleaning or replacement for heavily contaminated absorbent materials
- airflow while it dries: NSW Health's prevention list is exhaust fans, open windows and lower indoor humidity
If you are not sure whether the water is the unit's, a photo of the stain and the unit with a quote request is enough for us to tell you.
Sources
- Mitsubishi Electric MSZ-LN user manual (the same lines are in the MSZ-AP and MSZ-EF manuals) on our manuals and brochures page: dirty filters, condensation and fungi; filter cleaning every two weeks; the deodorising filter every three months; the three-to-four-hour drying run before a long idle period
- Panasonic Z Series operating instructions, on the same page: filters every two weeks, coils and fans cleaned by an authorised dealer, do not wash the indoor unit with water, the two-to-three-hour heat run to prevent mould, the H99 note on a dirty unit
- Mitsubishi Electric Indoor Unit Filter Guide, on the same page: the anti-mould treatment on the standard air filter
- Samsung WindFree AR9500T and Aero specification sheets, on the same page: Auto Clean, Freeze Wash and the anti-bacterial Easy Filter Plus
- NSW Health, Mould factsheet, updated 20 December 2022: where mould grows, the symptoms and who is most affected, prevention and removal
- Bureau of Meteorology climate averages, Richmond RAAF station 067105: the 3pm summer humidity
- What a Schillair clean covers and excludes, and the yearly rule: the owner, 29 August 2026, as published on our deep cleaning page