Why does my air conditioner smell musty or like dirty socks?

A musty or dirty-sock smell from an air conditioner usually means moisture, dust and mould or bacteria have built up on the cold parts inside the indoor unit or its drain. It is common in humid Brisbane summers. Stop using the unit if the smell is strong and have it inspected and serviced by a licensed technician.

Why do air conditioners get smelly?

When an air conditioner cools the air, moisture collects on the indoor coil and is meant to drain away. If the coil stays damp and dust builds up, mould and bacteria can grow, and the fan then blows that smell into the room. Trade websites often call this “dirty sock syndrome” because of the sour smell. It is more common in damp climates and when a unit is used often for short periods.

Are all smells the same?

No. Different smells point to different causes:

• Musty or sour, like damp socks: mould or bacteria on the coil or in the drain tray.
• Rotten egg or sewer-like: can come from a drain problem and is worth reporting to a technician.
• Burning or hot plastic: an electrical or motor problem. This is a safety issue, not a cleaning issue.
• Chemical or oily: could involve a refrigerant leak or something else, and needs a licensed technician.

Is a musty smell harmful?

Mould and bacteria can irritate some people, particularly those with asthma, allergies or sensitive airways. A mild smell is usually an annoyance rather than an emergency, but it is worth dealing with, especially in bedrooms or if anyone in the home is unwell. If someone has breathing problems, speak to a doctor as well.

Why does the smell come back?

If the underlying moisture or dust problem is not dealt with, the smell returns. Common reasons are a drain that is not working properly, a unit that is rarely serviced, or a system that is too big for the room so that it keeps starting and stopping and never dries out properly. A technician can look at the whole system rather than only the visible parts.

Can sprays or home remedies fix it?

Sprays and household products can mask a smell without fixing the cause, and putting products into a unit may damage it or create a safety risk. The manufacturer’s maintenance guidance and owner’s manual set out what is suitable for your model. For anything beyond that, rely on a licensed technician.

How can I reduce the chance of smells?

Following the manufacturer’s maintenance guidance and having the unit serviced regularly by a licensed technician helps keep the unit in good condition. Using it normally, rather than leaving it off for months and then switching it on, also helps. Your manual will explain what the maker recommends.

What does a servicing visit involve?

A licensed technician will inspect the indoor unit, the drain and the condition of the parts that handle air and moisture, and carry out the cleaning that the manufacturer’s guidance allows. They will tell you if anything is worn or faulty and whether the system suits the room.