Why does my aircon smell like vinegar?

A sour, vinegary smell is usually bacteria on the wet coil and in the drain tray rather than anything chemical. The exception is the first day or two after a service, when it is more likely to be cleaning residue drying off — that clears by itself.

Will a general service remove the smell?

Often, yes, if the smell is recent and mild. A service washes the filter, cover, reachable surfaces and drain line, which is where a new smell usually starts. If it has been there for months, or has come back after a clean before, a service buys a week or two and then it returns.

Is a smelly aircon harmful?

What is known is that it is mould and bacteria growing in a wet, dark box, and the fan blows air across it into the room. People with asthma or allergies often notice it. We will not claim more than that — the practical point is that cleaning removes it.

How often should I do a chemical overhaul?

About once a year for a unit running most days. In between, a general service every three months is the routine we recommend, and it is what stops the yearly overhaul turning into a rescue job.

The smell came back a month after servicing. Was the service done badly?

Not necessarily. A general service cleans the unit with it still on the wall, and on most models there are surfaces it cannot reach. Once growth is established there, the smell returns no matter how carefully the service was done. That is when the strip-down is worth the money.

Aircon smells bad

A musty or sour smell is mould and bacteria growing on the wet parts inside the indoor unit — the coil, the drainage tray and the blower wheel. Nothing is broken. Cleaning removes it. A light smell often clears with a normal service; one that returns within weeks needs the unit taken apart.

Written by Wilson Lee, owner and lead technician at LM Brothers Aircond · 10 years in the trade · Reviewed 21 August 2026

One blower wheel in three stages: fins packed solid with black grime, then mid-wash with the loosened dirt scattered across the floor, then clean with every fin clear

Why it happens

An aircon cools by pulling warm room air across a cold coil. Water condenses on that coil, the same way a cold drink sweats, and runs into a tray and out through a drain pipe. So the inside of every working aircon is wet the whole time it runs.

It is also dark, it turns warm the moment you switch off, and dust blows through it all day. In Singapore that sits there all year — there is no cold dry season to dry the unit out. Growth in there is the default outcome, not bad luck and not something you did wrong.

The smell is the growth itself. When the fan starts, air is pushed across those surfaces and into the room — which is why it is strongest in the first minute.

What is normal, and what is not

A faint smell for the first thirty seconds after switching on, which then clears, is common and not worth spending money on. That is stale air being pushed out.

A smell that fills the room, or is still noticeable an hour in, is not normal. That is growth on the surfaces, and it will not clear on its own.

A sharp chemical or vinegar-like smell in the first day or two after a service is usually cleaning residue drying off. It passes. If it is still there after a week, say so to whoever did the job.

A burning, hot-plastic or electrical smell is a different problem. Switch the unit off at the wall and leave it off until someone has looked at it.

What causes it, and what fixes each one

Mould on the coil and in the drainage tray
The most common cause by a wide margin. The coil is a block of thin aluminium fins that stays wet whenever the unit runs, and the tray underneath holds standing water. Dust settles into both and growth follows. On a unit that has gone a year or more without a proper clean, the dirt has hardened onto the fins and only a strip-down shifts it.
The fix: A general service washes the filter, cover and reachable surfaces. If the smell has been there a while, or comes straight back, the unit has to come apart so the coil and tray can be washed properly. Aircon chemical overhaul in Singapore
A blocked or slimy drain line
The pipe carrying condensation out of the unit builds up a soft black sludge. Once it partly blocks, water backs up and sits in the tray instead of draining, and standing water smells within days. Usually comes with drips or water marks below the unit.
The fix: The drain line is vacuumed and flushed clear. That is part of both a general service and an overhaul. Aircon general servicing in Singapore
A filthy blower wheel
The barrel-shaped fan behind the coil is the dirtiest part of most units, and the one most often skipped because reaching it takes time. Once the gaps between its blades pack up with damp grime, all the air entering the room passes over it.
The fix: Some models allow it to be cleaned in place during a service. Many do not, and on those it only comes properly clean once the unit is dismantled. Aircon chemical overhaul in Singapore
Something dead inside the casing
Less common, but it happens — a lizard or cockroaches inside the housing. The giveaway is that the smell is foul rather than musty, and it appeared suddenly instead of creeping up.
The fix: The casing is opened and cleared out, normally as part of a service visit. Aircon general servicing in Singapore
Sewer smell coming back up the drain
The rarest of the group. If the condensation pipe was run into a floor trap or waste pipe without a trap of its own, drain smell can travel back up the line and out through the indoor unit. The clue is that it smells like a drain rather than a damp cloth.
The fix: A pipework problem, not a cleaning one. Cleaning the unit will not fix it. Aircon repair in Singapore

The same aircon drainage tray and blower wheel before and after cleaning: mould and grime on the left, clean parts on the right

What you can check yourself

Take the filter out and rinse it: Open the front flap, slide the filters out, rinse them under a tap and dry them fully before putting them back. Free, ten minutes, and on a mildly smelly unit it makes a real difference. If the smell goes, you did not need us.
Run it on fan-only for half an hour: Switch the mode to fan with the cooling off and leave it running. The airflow dries out the coil and tray. Ten minutes of this before you switch off at night is the best habit for keeping the smell away — some units have a self-dry setting that does it for you.
Check the smell is actually the aircon: Stand near the unit while it is off, then switch it on. If the smell only arrives with the airflow, it is the unit. If the room already smelt that way, look elsewhere — a dry floor trap or a damp corner smells very similar and gets blamed on the aircon.
Look at where the water goes: Find the drain outlet outside the window or on the ledge. After twenty minutes of running there should be a steady drip. Nothing, or a trickle where it used to run freely, points at a blocked line. Worth mentioning when you book.

When to call someone

The smell came back within a few weeks of a clean. That is the clearest sign the growth is on surfaces a surface clean cannot reach.
You can see black or green patches on the fins, the flap or the tray with a torch.
There are water stains, drips or damp patches on the wall below the unit.
The unit has never had a full clean and is more than a year old.
Someone in the house has asthma or an allergy that got worse since the smell started.

What it costs

A general service is $50 for one unit and drops the more units you do in one visit — $20 a unit at 6. No GST, and every job carries a 90-day workmanship warranty.

That is the right answer when the smell is mild, when it turned up after the unit sat unused for weeks, or when it was cleaned within the last few months. The smell is then on the filter and the surfaces we can already reach.

A chemical overhaul is $130 for the first unit and $88 for each one after. The unit comes off the wall and comes apart, so the casing, coil, blower wheel and drainage tray are each washed separately instead of wiped in place. That is the right answer when the smell returned within weeks of a clean, or the unit has gone a year or more without one. At that point paying $50 for a clean that will not shift the dirt is the more expensive choice.

Worth saying plainly, because this is the symptom most used to sell overhauls people do not need: if you are told you need one without the unit being opened and shown to you, ask why. The smell does not prove it — the inside of the unit does.

If you would rather we look first and tell you which it is, that call-out is $50 ($120 for VRV and VRF), free if we go on to do the work.

See the full price list