Air conditioning smells bad: what to do

The smell tells you when to stop, but it cannot diagnose the fault.

The answer in 2 lines. For a mild musty smell, stop the appliance and check its user-accessible filters according to the instructions. Persistent odours may involve moisture, internal dirt, drainage or another source in the room, so ask an engineer to investigate. Do not spray deodoriser into the unit. A burning smell means stop using it. If you suspect a gas escape, avoid electrical switches, leave and call the gas emergency service from outside.

The first steps

Notice the circumstances: does the smell occur only when cooling, whenever the fan runs, or even with the appliance off? Record this without repeatedly restarting it.
For a mild musty smell only: stop and isolate the system as instructed before checking user-accessible filters. Follow the specified cleaning or replacement method.
Check the surroundings: look for visible damp, a nearby drain smell or another household source. Ventilate when safe, but do not use the air conditioner to disperse a suspected hazardous release.

Stop: danger

Burning or hot-plastic smell, crackling or repeated tripping: stop using the appliance. Isolate it only if safe and arrange professional inspection.
Smoke or fire: leave, stay out and call 999. Do not investigate inside the appliance or tackle an electrical fire with water.
A sulphurous smell that might be gas: do not assume it comes from the air conditioner. Avoid switches and flames, open doors and windows if safe, leave and call 0800 111 999 from outside.

A smell is a clue, not a diagnosis

A musty odour can prompt checks for dampness or contamination, but it does not establish what is growing inside the appliance. A drain-like smell could involve the condensate arrangement or come from elsewhere. A smell noticed at the first use after storage should still be assessed rather than automatically dismissed as harmless dust.

Describe the smell in ordinary language and say when it starts. If other people notice it too, record that. Do not deliberately sniff close to vents, open covers to investigate or keep running the equipment to see whether an unpleasant smell becomes stronger.

Clean only what the instructions allow

Find the filter-care section for the exact model. Some filters are washable; others need replacement or different treatment. Before removing anything, follow the isolation instructions. Avoid touching internal fins, wiring or fan components.

Refit cleaned filters only as the instructions specify. If the unit has a drying or self-cleaning function, check what it actually does and when it may be used. Do not assume it disinfects the whole appliance or clears a blocked drain. There is no universal fan-running time suitable for every system.

If access is difficult, the filter is damaged or the instructions are unclear, ask for servicing instead of improvising.

What not to do

Spraying perfume, bleach, disinfectant or cleaning foam into the air inlet.
Mixing cleaning products or using a product that the equipment instructions do not permit.
Leaving a persistent smell untreated because the appliance still cools.
Treating an unusual smell as proof of either a refrigerant leak or a particular health hazard.

When to arrange a service visit

Arrange assessment if the smell returns after permitted filter care, if you see internal contamination that is not accessible for user cleaning, or if there is accompanying water leakage or poor cooling. Ask the engineer to investigate the source, including the condensate collection and drainage arrangement where relevant, and to explain the proposed cleaning method.

If work involves F gas, check the applicable qualification and certification requirements through GOV.UK's F-gas guidance. Use it to establish what evidence to request. Ordinary filter care does not justify opening the refrigerant circuit.

Reporting the problem in England

Renters should report recurring smells, damp and equipment faults in writing. Include photographs and explain whether the system also provides heating. Check the tenancy agreement before booking work. Read GOV.UK's repairs guidance for the general duties and escalation steps; responsibility for a cooling-only appliance depends on the circumstances.

For a suspected gas escape, follow the official gas emergency instructions and call the emergency number rather than arranging routine servicing. This housing advice concerns England; Wales, Scotland and Northern Ireland have different systems.

Good to know

Do not use smell to decide whether the refrigerant circuit is safe or needs topping up.
Keep filter-care and service records so the engineer can see what has already been done.
The fire service's emergency guidance explains what to tell the operator: give your location, describe the smoke or fire and say whether anyone is trapped.

Illustrative example: in Bristol, Sam notices a musty smell whenever the bedroom unit runs. Cleaning the permitted filters does not resolve it. Sam stops using the unit and records the pattern for the engineer, who investigates the internal surfaces and drainage before recommending treatment. Sam avoids spraying fragrance into the vents.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on 5 November 2026, for England. Confidence: high for electrical, fire and suspected gas emergency precautions; medium for maintenance advice, which must follow the model's instructions. Confidence: limited for assigning a specific cause from an odour alone: no official domestic diagnostic table was confirmed. No fixed drying time or universal servicing interval is asserted.