The short answer
A musty smell usually comes from damp dirt on the coil, drain tray or fan, and a dusty smell when heating first starts usually fades. A burning or electrical smell means switching the system off straight away.
On this page
Most wall-mounted units recirculate the air already in the room rather than bringing in fresh air from outside. So a smell from the unit has either been picked up indoors or built up inside the unit itself. Most smells are a cleaning matter and a few are a safety matter, and the table below helps tell them apart.
Common smells and what they mean
| Smell | Likely source | What to do |
|---|---|---|
| Musty, damp or stale | Damp dirt on coil, tray or fan | Clean filters, dry the unit, book a service |
| Warm dust on first heating | Dust on the coil | Wait; it should fade |
| Drains or sewage | Dry trap or drain connection | Engineer or plumber |
| Cooking, smoke or pets | Room air being recirculated | Ventilate, clean filters |
| Burning or electrical | Overheating wiring or part | Switch off, call an engineer |
| Mains gas | Not the air conditioner | Gas emergency number |
Musty or damp smells
In cooling and dry modes the coil is cold and wet, and water sits in the drain tray below it. Dust that gets past the filters settles on these damp surfaces and on the fan, and mould and other growth can build up there. The result is a stale, damp smell, sometimes compared to old socks, that is often strongest in the first few minutes after the unit starts.
What you can do:
- Clean the filters, following how to clean the filters.
- After using cooling, run fan-only mode for a while, or use the unit's drying or self-clean function if it has one. These are designed to dry the inside so it doesn't sit wet.
- Air the room.
If the smell stays once the filters are clean, the coil, fan and drain tray need cleaning, which is part of a service. Don't spray disinfectants, air fresheners or household cleaners into the unit. They can damage parts, and the smell comes back while the dirt is still there.
Other everyday smells
When heating starts. A warm, dusty smell for the first few minutes of heating after months off is usually dust warming on the coil, and it should fade quickly. A stale smell as heating starts usually has the same cause as a musty one, and the same fixes. A smell that carries on, or smells of hot plastic or wiring, should be treated as a burning smell.
Drains. A smell like drains or sewage usually means the condensate pipe joins a waste pipe indoors and drain air is getting back up it. That connection should have a trap or valve to stop this, and a trap that relies on water can dry out during months without cooling. If the smell keeps coming back, an engineer or plumber can check the connection. Where the water goes explains how drains are usually run.
The room. Cooking, smoke, pets, paint and new furniture all leave smells in the air, and the unit moves that air around. Filters and a damp coil can hold on to smells for a while afterwards. Clean the filters and ventilate the room.
Burning, electrical, gas and refrigerant
- Burning, hot plastic or electrical smells, especially with buzzing, crackling or smoke: switch the system off with the remote, and at its isolator switch or circuit breaker if you can reach it safely. Don't switch it back on, and call an engineer. If there's smoke or fire, get everyone out and call 999.
- A smell like mains gas isn't coming from the air conditioner, which doesn't use mains gas. Treat it as a gas leak and call the gas emergency number, which is 0800 111 999 in Great Britain. Northern Ireland has its own number.
- Refrigerant leaks can't be relied on to give off a smell you'd notice, so don't depend on your nose. Hissing from the pipes or the unit, oily marks at pipe joints or ice where it shouldn't be are better clues. If you suspect a leak, switch the system off, open the windows and call an engineer. R32, the refrigerant in many home systems, is mildly flammable, although a home system holds only a small amount. Keep naked flames away from the unit until it has been checked.
Related guides
- How to clean the filtersSwitch the unit off, lift the front panel and take out the mesh filters, then vacuum or rinse them in lukewarm water and let them dry fully before refitting. In regular use, this usually needs doing every few weeks.
- What a service involvesA service is a clean and check of the whole system, indoors and out, plus tests of how it runs and a look for leaks and wear. Manufacturers typically recommend one regularly, often once a year.
- Where the water goes: condensate drainageWater that condenses in the indoor unit as it cools runs through a drain hose to a suitable outlet, with a small pump where it can't fall naturally. In heating mode the outdoor unit produces water too.
- When to call an engineerCall an engineer when simple checks don't fix a problem, and switch off straight away for burning smells, smoke, a circuit breaker that keeps tripping or signs of a refrigerant leak. Refrigerant, electrics and anything inside the casing are work for a qualified engineer.