Something’s off before you even look up. A split system air conditioning unit in decent condition runs without any smell at all, so when one starts producing an odour, you’re already getting useful information — the type of smell narrows it down considerably.
Musty or Damp Smells: Mould in the Indoor Unit
Moisture sits inside the unit after every cooling cycle. Split system air conditioning handles this through a drain line, but if that line is partially blocked or the unit has been sitting idle for a few months, moisture hangs around long enough for mould to establish itself in the filter, on the coil, or in the drain pan. The smell that comes out is exactly what you’d expect.
Cleaning the filter is the first step, and it should be happening every six to eight weeks during heavy use anyway. What a lot of people don’t know is that running fan-only mode for about 20 minutes after you stop cooling dries the internals out significantly — it’s a small habit that makes a real difference. If the smell is still there after a proper filter clean, the mould has gone deeper, into the coil or housing, and that’s not something a can of spray fixes.
Burning or Electrical Smells
A faint burning smell in the first few minutes after a unit starts up following a long break is usually just dust on the heating elements — it clears fast. If it doesn’t clear, or if what you’re smelling is sharper, more chemical, something closer to hot plastic, stop running the unit. Overheating components, a failing capacitor, wiring that’s starting to break down — these aren’t things that improve with time. Turn it off and get someone out.
People wait on this one more than they should.
Rotten Egg or Sulphur Odours
A cooling system pulling air through roof cavities and wall spaces will occasionally pull through whatever else is in there. A dead rodent or bird is the most common source of a sulphur smell, and while that’s unpleasant to deal with, it’s a straightforward problem once the animal is found and removed.
The other possibility — a gas leak somewhere in the house — is not straightforward. If there’s any real uncertainty about the source, leave and call your gas provider. The air conditioner can wait.
Sweet or Chemical Smells: Refrigerant Leaks
It’s subtle. Refrigerant leaks tend to produce a faint sweetness, sometimes described as nail polish remover or a mild solvent, and because it’s not aggressive or obviously alarming, people often file it under “weird smell, probably nothing.” Months later the system is struggling to cool on a 38-degree day and the leak has been quietly running the whole time.
Modern split system air conditioning refrigerants aren’t acutely dangerous in the concentrations you’d encounter at home, but a leaking system costs more to run, cools worse, and the leak itself requires a licensed technician to fix — it’s not a DIY repair. If the smell is there, get it checked. The longer it goes, the more refrigerant is lost and the more the compressor works to compensate.
Dirty Sock Syndrome
The name is doing a lot of work here, and it’s accurate. Bacterial growth on the evaporator coil — typically in units that switch between heating and cooling frequently, where the coil never fully dries — produces a smell that is genuinely difficult to describe as anything other than a gym bag left in a hot car. The unit works fine. It just smells terrible.
A coil clean from a technician resolves it. Some will apply an antimicrobial coating afterward to slow regrowth, which is worth asking about.
Smells That Need a Technician, Not a Filter Change
Poor maintenance causes most odour issues in a cooling system, and a decent annual service catches most of them before they get bad. But some faults sit behind the smell:
- Refrigerant leaks require specialist equipment and a licence to handle — not a DIY job under any circumstances
- Electrical burning smells mean the unit shouldn’t run again until it’s been inspected
- Mould that keeps returning after cleaning usually points to a drainage fault keeping the unit persistently damp
An annual service — before summer starts, ideally — is the most cost-effective way to stay ahead of all of this. A well-maintained unit also runs more efficiently, which across a full Melbourne summer is a noticeable difference on the electricity bill.