Key takeaways
A musty or sour smell means damp and mould inside the unit, not just a dirty filter.
A burning smell is the only one that means switch off straight away.
If the smell comes back soon after a clean, the hidden wet parts still need cleaning.
Running fan mode for 10 to 15 minutes after cooling helps the inside dry out.
Table of contents
What causes a musty aircon smell?
What each smell usually means
How to fix a musty aircon smell today
Why it can still smell after a clean
If it smells like burning, switch off first
1. What causes a musty aircon smell?
Your aircon cools the room by pulling warm air in through a filter and passing it over cold metal fins behind it. When warm, humid air hits something cold, water forms on it, the same way a glass of iced water gets wet on the outside. That water drips into a tray under the fins and runs outside through a small pipe.
So the inside of every aircon is dark, damp and slightly dusty, which is exactly what mould likes. Midea explains that the smell comes from mould and bacteria on the fins, and that if the pipe is blocked, the water left in the tray starts to smell within a few days. You notice it most when you switch the aircon on, because the fan blows it all into the room at once.
Your room adds to it. LG notes that fish tanks, pet water bowls and laundry dried indoors all raise the humidity the aircon has to deal with.
2. What each smell usually means
What it smells like | Likely cause | What to do
|
|---|---|---|
Musty, like damp laundry | Mould on the fins or in the tray | Wash the filter; if it stays, the fins and tray need a professional clean |
Sour, like food gone off | Aircons in or close to the kitchen pick up cooking grease from the air. It sticks to the damp parts inside and turns sour (Haier). | Professional clean to wash off the grease |
Like urine or droppings | Geckos, cockroaches or ants nesting inside the unit, which is common in Thailand. Their droppings and remains cause the smell (Carrier Thailand) | Professional clean so the unit can be opened and cleared out |
Burning or hot plastic | Electrical fault or an overheating part | Switch off now and call a technician |
3. How to fix a musty aircon smell today
These help straight away and slow the smell's return:
Open the windows for a while to let fresh air through.
After cooling, run fan mode or dry mode for 10 to 15 minutes before switching off. Haier says this dries the inside, and that switching off straight after cooling makes the smell worse.
Wash the filter and let it dry completely. Our article on cleaning your AC at home shows how.
Skip the room air freshener. Midea warns that spraying it into the unit leaves residue and only covers the smell for a while.
4. Why it can still smell after a clean
A clean that only reaches the front of the unit leaves the real source behind. Midea lists the usual reasons: the tray or its pipe wasn't fully cleaned, mould on the fins wasn't reached, or the filter went back in while still damp. Cleaning sprays have the same problem: they only reach the front of the metal fins, not the tray or pipe behind them where the mould grows. Our article on aircon cleaning sprays and foam explains why.
A thorough clean covers all of it. The technician will hang a waterproof bag under the unit, take off the casing, wash the fins, the fan and the tray with a high-pressure pump, and flush the drain pipe so water runs out freely. If that pipe was blocked, you may also have noticed water dripping from the aircon.
If a smell keeps coming back even then, LG suggests the unit may be too powerful for the room. It cools so fast that it switches off before it has removed enough moisture.
The smell is worth fixing. In PPTV's report drawing on Thailand's Department of Health, aircon units are listed among the places mould builds up at home, and mould is linked to asthma, allergies and skin rashes.
5. If it smells like burning, switch off first
A burning or hot plastic smell is not a cleaning problem. Midea says it can come from an overheating motor, damaged wiring or a fault on the circuit board, so switch off at the breaker and don't try to fix it yourself. An aircon technician can find the fault and make it safe.
Get rid of the smell for good
A technician takes the unit apart and washes the hidden wet parts where the smell starts, which a filter wash and a can of spray never reach. To book one, post an aircon cleaning task on gigit with a budget, or leave it open, and pick the offer that suits you from local gigit heroes. Your room goes back to cool air that smells of nothing.