Short answer: A musty or damp smell from home air conditioning usually means dirt and moisture on the indoor coil, fan or drain tray, where mould can grow. Rinse the filters first. If the smell stays, the coil and drain need a proper clean. A burning or electrical smell is different: switch the unit off and get it checked.
What the smell is telling you
| Smell | Most likely cause | What to do |
|---|---|---|
| Musty, damp, “old socks” | dirt and mould on the coil, fan or drain tray | clean filters; book a coil and drain clean |
| Damp and dripping | blocked condensate drain or pump | switch off; book a check |
| Dusty, first use of heating | dust burning off the coil after months idle | usually clears quickly; clean filters |
| Burning, electrical, plastic | wiring, fan motor or board fault | switch off and isolate; get it checked |
| Sewage or drains | condensate drain connected without a proper trap, or a dried out trap | book a check of the drain connection |
| Sweet or chemical | possible refrigerant or cleaning product residue | ventilate the room; get it checked |
Why air conditioning gets smelly
When a unit cools, the indoor coil runs below the air’s dew point, so water condenses on it, just like a cold glass on a summer day. That water runs into a drain tray and away through the condensate pipe. The coil is wet for long periods during a London summer.
The filter catches the larger dust, but fine particles get through. In a city, that includes traffic soot, pollen and cooking grease. Wet coil plus fine dirt plus warmth is ideal for mould and bacteria. The barrel shaped fan behind the louvres also collects grime and can harbour black mould. When the fan starts, that smell blows straight into the room.
Inner London flats are often affected. Islington, for example, has dense streets and many homes on busy roads such as Upper Street, Holloway Road and City Road, so windows are opened onto traffic and filters load up quickly. Our Islington area page covers the local housing.
Safe cleaning you can do
Switch the unit off at the remote and, if you can, at its isolator. Then:
- Lift the front panel. Most wall units hinge upwards.
- Slide out the filters. They are usually fine plastic mesh.
- Rinse them in lukewarm water. A little mild washing up liquid helps with grease.
- Let them dry completely in the air, not on a radiator.
- Refit them and close the panel.
Do this every few weeks when the unit is in regular use. Wipe the outside of the casing with a damp cloth. Do not spray household cleaners into the unit, push anything into the fan or take the casing off.
What a professional clean adds
An engineer’s clean goes further than the filters. It normally includes:
- applying a suitable coil cleaner to the indoor coil and rinsing it, with the unit protected and the water caught
- cleaning the barrel fan, which often needs partial dismantling
- cleaning the drain tray and flushing the drain pipe
- testing any condensate pump
- checking the outdoor coil and clearing debris
This is part of a proper service, which we explain step by step in what happens during an air conditioning service.
Smells after a period off
Many London systems sit unused from autumn until the first hot day. Water left in the drain tray and moisture in the coil can lead to a smell on the first run. Some units have a self clean or dry mode that runs the fan for a while after cooling to dry the coil. If yours has it, use it. Running the unit in fan only mode for a short time before switching off in summer also helps.
When it is not a cleaning problem
Some smells are not about dirt:
- Drain smells often mean the condensate pipe was connected to a waste or soil pipe without a proper trap or water seal, letting drain air back up the pipe. That needs a fix to the connection.
- Burning smells that persist point to an electrical problem. Leave the unit off.
- Chemical smells with weak cooling can occasionally point to a refrigerant leak. Ventilate and book a check. Only an engineer with an F-gas qualification, working for a certified company, can investigate and repair the refrigerant circuit.
How often to clean
| Task | How often | Who |
|---|---|---|
| Rinse filters | every few weeks in use | you |
| Wipe casing | as needed | you |
| Coil, fan and drain clean | yearly, as part of a service | engineer |
| Outdoor unit clean | yearly | engineer |
Busy kitchens, homes with pets and flats on main roads may need more. Our servicing page explains how to book.
Smells in new systems
A brand new system can sometimes give off a faint factory smell for the first few hours of use, from oils and coatings warming up. That should fade quickly. If a new system smells musty within its first season, the condensate drain may not be falling properly, leaving water sitting in the tray. Ask the installer to check the drain route and the level of the indoor unit.
Quick questions
Why does my air con smell musty when it starts?
Moisture condenses on the indoor coil when it cools. If the coil, fan or drain tray is dirty, that damp dirt can grow mould and smell when the fan starts. A coil and drain clean usually sorts it.
Can I clean my air conditioning coil myself?
You can rinse the filters and wipe the casing. Coil and fan cleaning involves chemicals, water near electrics and parts that bend easily, so it is best left to an engineer.
Is a burning smell from air conditioning dangerous?
It can be. A brief dusty smell when heating is first used after months off is common, but a persistent burning, electrical or plastic smell is not. Switch off, isolate the unit and get it checked.
Last updated 23 October 2026. Written for homes in London. Rules can change, so check the linked sources for the latest position.