Air conditioning works by passing room air over a cold coil. Moisture condenses on that coil and runs away through a drain. Dust and skin flakes travel with the air. Put moisture, dust and a dark space together and you have the conditions for bacteria and mould. That is why the most common air conditioning smell is musty, and why it often appears when you switch the system on after a few weeks off.
This guide goes through the smells people describe, what usually causes each one, what you can do yourself and what needs an engineer. Our diagnosis is the same as any good doctor’s: work out the cause before treating the symptom.
Musty, damp or “old socks”
This is the classic smell, and it usually comes from one or more of these:
- Dirty filters. Filters clogged with dust hold moisture and odours, and they cut airflow, which keeps the coil wetter for longer.
- A dirty evaporator coil. The fins behind the filters collect a film that bacteria feed on.
- The fan barrel. The long cylindrical fan in a wall-mounted unit picks up a grey or black coating that you can sometimes see through the outlet.
- The drain tray and pipe. If condensate sits rather than drains, it stagnates. A slow drain can also cause drips; see our guide to air conditioning leaking water.
Visible black spots on the louvres or fan are a sign of mould growth and a reason for a proper clean, not a spray.
Burning, electrical or hot dust
A faint dusty smell for a few minutes when you first use heating mode after the summer can be dust warming on the coil. It should fade quickly.
A sharp burning, hot plastic or electrical smell is different. Switch the system off at the isolator and call an engineer. It can point to an overheating fan motor, a failing component or a wiring problem. Don’t keep running it to see if it clears.
Sweet, chemical or solvent-like
Refrigerant leaks are often described as having little or no smell, so don’t rely on your nose to find one. A chemical smell can come from cleaning products used on the unit, from new materials in the room, or from something drawn in from outside. If you also notice the system is cooling less well, or you see oily marks on pipe joints, ask an F-Gas certified engineer to check it. Leak finding and repair is regulated work under GOV.UK’s F gas qualification rules.
Want it done properly? AirCon Doctor installs, repairs and services air conditioning and heat pumps across London and the South East, from our base in East Grinstead. We’re an F-Gas certified company and NICEIC approved. Get a fixed-price quote.
Sewage or drain smells
Where a condensate drain connects into a waste pipe, a missing or dry trap can let drain smells back up into the unit. This is more common where systems have been off for a long time or where the drain was connected without a proper trap or waterless valve. It needs an engineer to check the drain run, not air freshener.
Smoke, cooking or tobacco
Air conditioning recirculates the air in the room. Filters and coils can absorb smells from cooking, smoking or pets and release them later. A thorough clean helps. Opening windows and ventilating the room properly matters too, because a split system doesn’t bring in fresh outdoor air. For whole-home fresh air, see our ventilation and MVHR page.
What you can clean yourself
- Switch the unit off at the remote and at the isolator.
- Open the front panel and slide out the filters, following the manufacturer’s manual.
- Vacuum gently, then rinse in lukewarm water if the manual allows. Let them dry fully before refitting.
- Wipe the casing and louvres with a damp cloth.
- Run the unit in fan mode for a while after cooling, or use the dry or clean function if your model has one, to help the coil dry out.
Our step-by-step guide to cleaning air conditioning filters covers this in more detail. Avoid pushing anything into the fins, spraying household chemicals into the unit or getting water near the electrics.
What needs an engineer
Cleaning the coil properly, cleaning the fan barrel, clearing and treating the drain tray and pipe, and checking electrical connections all need the unit partly dismantled. That is part of a proper service. Our annual service costs £95–£150 + VAT, and a regular service is the best way to stop smells coming back. See what an annual service includes.
Quick answers
Is a musty air conditioning smell harmful?
It is a sign of microbial growth on the filters, coil, fan or drain. Clean the filters and book a proper service rather than living with it.
Why does my air con smell only when I first turn it on?
Moisture left on the coil after the last use allows odours to build up. Running fan or dry mode after cooling helps the coil dry.
Can I use an antibacterial spray on my air conditioning?
Only products the manufacturer approves, and only on parts you can reach safely. A spray won’t clean a blocked drain or a coated fan barrel.
My air conditioning smells of burning. What should I do?
Switch it off at the isolator and call an engineer. Don’t keep using it.
Related services
More advice
- How to Clean Air Conditioning Filters: Step by Step Guide
- Air Conditioning Annual Service: What’s Included and Why It Matters
- Why Is My Air Conditioning Leaking Water? Causes and Fixes
- Can Air Conditioning Improve Air Quality?
- All advice
This guide is general information, checked against the sources linked on 30 October 2026. It isn’t legal, tax or financial advice. Rules, grants and product ranges change, so check the current position before you rely on it.