Mould in your air conditioner: signs, causes and what it says about your home

Signs of mould in a split system: a musty start-up smell comes first

A musty smell in the first minutes after the aircon starts is usually the first sign. Daikin notes that an indoor unit can sometimes push out room odour that has been sitting inside it, so one stale puff isn't proof of mould. A damp, earthy smell that comes back every time you switch on is worth a closer look.

The second sign is visible: dark specks or patches on the louvres, around the air outlet, or on the fan behind the louvres when you shine a torch in.

The third sign is water. Drips from the indoor unit or a stain on the wall below it mean water isn't leaving the way it should. Daikin's own advice for a leaking indoor unit is to stop using it and check whether water is coming out of the drain hose outside.

Why split systems grow mould: the coil is wet by design

A split system grows mould because cooling makes water inside it, and that water has to drain away through a hose. Daikin notes that obstructions and bends in the drain hose or other parts of the drainage path can stop water draining properly. NSW Health says mould needs moisture and nutrients to grow and tends to grow indoors in wet or moist areas that lack ventilation. A closed-up unit holding a wet coil fits that description.

Filters are the other half of the story. Daikin's troubleshooting guide says a dirty filter can cause condensate to form and leak from the air outlet, which puts water where it isn't meant to be.

Sunshine Coast humidity puts more water through your aircon

Humid air means more water through an air conditioner, and the Sunshine Coast has plenty of humid air. The Bureau of Meteorology's Sunshine Coast Airport station recorded a mean 9am relative humidity of 71 per cent across the year, and between 73 and 76 per cent in every month from January to July (records from 1994 to 2010). The mean 3pm figure across the year is 66 per cent.

For comparison, the US Environmental Protection Agency's mould guide suggests keeping indoor relative humidity below 60 per cent where possible, and ideally between 30 and 50 per cent.

Those outdoor readings aren't your indoor humidity, and a running aircon pulls moisture out of the room as it cools. The point is simpler: on the coast, the coil and drain tray handle a lot of water, and a unit that sits switched off with a wet coil in warm, still air is giving mould the moisture NSW Health says it needs.

Is mould in an air conditioner harmful? Fine for most people, a trigger for some

Most people won't have any health problems from mould in an air conditioner, but it can be a real problem for people with asthma, allergies or weakened immunity.

NSW Health lists the health effects of mould exposure as a runny or blocked nose, irritated eyes and skin, and sometimes wheezing, and says inhaling mould spores may cause an asthma attack in people with asthma. It also says most people won't have any health problems from contact with mould. People with asthma, allergies or other breathing conditions may be more sensitive, and people with weakened immune systems or chronic lung disease are more at risk of mould infection.

Asthma Australia's mould fact sheet recommends making sure air conditioners are regularly cleaned and serviced.

I can't diagnose anyone, and this guide isn't medical advice. If you or your family feel unwell, NSW Health's advice is to contact your doctor.

What you can do yourself: filters, the dry setting and a dry unit

Your jobs as the owner are the filters, the settings and keeping an eye on the drain. Anything inside the unit belongs to a technician.

Clean the filters on the schedule in your owner's manual. Daikin's general recommendation is every two weeks. Daikin says to wash the filters with water or vacuum them, and after a detergent wash for heavy soiling, to let them dry in the shade.

With the unit switched off, you can wipe mould from the outside of the louvres. NSW Health's routine method for mouldy surfaces is mild detergent or vinegar diluted in water.

Use the dry setting on humid days. Asthma Australia suggests the dry setting because it reduces moisture in the air.

Skip the supermarket aircon sprays. Daikin advises against commercial cleaning agents inside the unit because they can damage resin parts of the heat exchanger and, in the worst case, cause water leaks.

Mould on the fan or coil: a job for an air conditioning technician

If mould is on the fan or coil, or the smell returns after the filters are clean, the inside of the unit needs a professional clean. Don't take the unit apart yourself: the coil, fan and drain tray sit behind covers in a powered appliance, and they are a job for an air conditioning technician. Asthma Australia's mould fact sheet makes the same point from the health side: make sure air conditioners are regularly cleaned and serviced.

Ask whoever cleans the unit to check the drain tray and hose too. The US Environmental Protection Agency's mould guide says to keep air conditioning drip pans clean and drain lines unobstructed and flowing. The EPA also advises against running a system you know or suspect is contaminated with mould, because it could spread mould through the building. If mould is clearly growing inside the unit, leave it off until it has been cleaned.

When mould on the aircon points to a damp room

If mould is on the aircon and on other surfaces in the same room, the room's humidity is the likelier cause, and cleaning the unit alone won't fix it. NSW Health puts it plainly: mould is likely to return unless you also treat the cause. The US EPA says the same thing from the other side: clean up mould without fixing the water problem and it will most likely come back.

This is where looking at the building helps. A split system is a cold, damp surface in a room. When the room itself is holding too much moisture, the unit grows mould alongside the window frames, the ceiling corners and the back of the wardrobe. The EPA notes that condensation on windows, walls or pipes can be a sign of high humidity, and that hidden mould can grow on wall surfaces behind furniture where condensation forms. The table below sorts the common clues by what they usually point to.

When a mould test or healthy home investigation makes sense

Testing makes sense when mould keeps coming back after the unit has been cleaned, when you can smell mould but can't find it, or when someone in the house has asthma or allergies and you want evidence rather than guesses.

NSW Health notes that most mould is visible, so testing generally isn't needed, but that some contamination can sit in cavities or the ceiling. It also says symptoms alone aren't a reliable indicator of mould. When I take mould samples, they go to an independent NATA-accredited laboratory, and the results come back to you as a report. Call me for a mould testing quote.

If you want a wider look at moisture and air quality across the whole home, that's a separate service: my on-site healthy home consultations start from $330 across the Sunshine Coast, Brisbane and Gympie. If you're further away, online consultations start at $88 for 30 minutes. You don't need to be buying a property: homeowners can book a healthy home investigation directly.

I don't inspect the air conditioners themselves. During a healthy home investigation I look into them instead: when each unit was last cleaned and what's going on in the room around it, and in most cases I recommend getting the units professionally cleaned.

If you rent and mould keeps growing even though the home is well ventilated, NSW Health's general advice is to raise it with the owner or real estate agent. That's general information, not legal advice.