Musty, mouldy or damp — the most common one
If your aircon smells like a damp cupboard when it starts, it's almost always mould and bacteria on the evaporator coil and the barrel fan. The coil is cold and wet every time the system runs, room dust sticks to it, and within a season or two there's a slimy film growing on the fins and on every blade of the fan behind them.
Washing the filter won't fix it, because the filter sits in front of the problem. Neither will a deodorising spray — it covers the smell for a few days while the mould keeps growing. The fix is a clean that takes the cover off, bags the unit and washes the coil and fan until the water runs clear. That's our split system clean.
A "dirty sock" smell when the heating starts
Common in winter with reverse cycle units. The same build-up on the coil gets warmed up in heating mode and the smell comes out stronger. Same cause, same fix: a proper clean of the coil and the fan.
Swampy or sour — evaporative coolers
An evap cooler that smells like a pond usually has stagnant water in the tank and old, scaled pads. It's worst on the first hot day after winter. The fix is to drain and descale the tank, flush or replace the pads, and clean the pump and float — see evaporative cooler cleaning.
A bad smell from some ducted outlets only
If the whole house is fine but one or two rooms smell, the cause is often in the roof space or a duct run to those rooms — dust and mould, or occasionally something that has got into the roof. We check the fan coil and the return air, photograph what we find, and tell you if it's a job for a pest controller rather than a cleaner.
Burning, electrical or chemical smells — switch it off
If you smell burning plastic, hot wiring or a sharp chemical smell, turn the system off at the isolator and call a licensed electrician or air conditioning technician. That's not a cleaning problem and it shouldn't be left running.
A faint dusty smell for a few minutes the first time you use the heating each year can just be dust burning off. If it doesn't clear quickly, treat it as above.
How to stop it coming back
- Get the system properly cleaned once a year — twice with pets, allergies or a beachside home.
- Wash split filters every month or two while it's in use.
- Run the fan only for 10 to 15 minutes after cooling, if your unit has a dry or fan mode, to help dry the coil.
- Keep the drain clear: a slow-release tablet in the tray helps stop biofilm building up.
Smell won't go away? Send us a photo of the unit and we'll tell you what we think it is and what it costs to fix.