A musty smell from an air conditioner usually comes from mould or bacteria growing in the damp parts of the indoor unit: the coil, the drip tray, the drain and the fan. In cooling mode the unit collects water, and dust plus moisture is all mould needs. Dirty filters, a blocked drain and switching off straight after cooling make it worse. Cleaning the filters helps, but a smell that persists usually needs a professional clean.
This guide covers why the smell happens, what Daikin and other manufacturers advise, whether it is a health concern, and what a proper clean involves.
At a glance
| Smell | Likely cause | What to do |
|---|---|---|
| Musty, damp or mildew | Mould or bacteria on the coil, drip tray, drain or fan | Clean the filters, run a drying mode, then book a professional clean |
| Musty smell only at start-up | Moisture left in the unit after cooling | Use the unit's drying or mould-proof mode after cooling |
| Smells like the room, cooking or smoke | Odours absorbed by the filters and coil | Clean the filters and run fan mode with windows open |
| Burning smell | Electrical fault | Switch off at the breaker and call an engineer |
| Musty smell with water leaking | Blocked drain or tray | Switch off and have the drain cleared |
Why air conditioners smell musty
In cooling mode, moisture from the air condenses on the indoor coil and drips into a tray, then drains away. That leaves the inside of the unit damp. Add the dust and skin flakes that get past a dirty filter, and the coil, tray and fan become a good place for mould and bacteria to grow. When the fan starts, it blows the smell into the room.
A few things make it worse:
- Dirty filters. More dust reaches the coil and fan. Daikin recommends cleaning the filters every 2 weeks.
- A slow or blocked drain. Standing water in the tray stays damp and stagnant. Our guide to an air conditioner leaking water covers drain problems.
- Switching off straight after cooling. The coil stays wet with no airflow to dry it.
- Long periods unused. A damp unit left sealed up for weeks often smells when it starts again.
Smells that are not mould
Daikin's troubleshooting guide explains that the indoor unit can discharge room odours that have collected inside it. Meaco, a UK air conditioner maker, makes the same point in its help centre: air conditioners contain no odour-producing components, so smells come from what the unit has absorbed from its surroundings, such as cooking, smoke or pets. For that kind of smell, Meaco suggests running the unit in fan mode with the windows open to let fresh air pass through.
Daikin also notes that units with its Streamer air purifier generate a very small amount of ozone, which can cause a slight smell at the outlet, and says the amount is too small to be a health hazard. A burning smell is different: Daikin's advice is to stop the unit, switch off the power and contact the installer or service provider.
Health and musty smells
A faint smell for a minute at start-up is usually more of a nuisance than a risk, but it is a sign that mould or bacteria are growing, and it is worth dealing with. The UK Health Security Agency's guidance on damp and mould says they produce allergens, irritants and mould spores that mainly affect the airways and lungs, and that while they can cause ill health in anyone, people with underlying health conditions and weakened immune systems are at greater risk. If anyone in the home or workplace has asthma or allergies, or the smell is strong and constant, have the unit cleaned rather than waiting.
Our guide to air conditioning and indoor air quality covers the wider picture.
What you can do yourself
- Clean the filters. Daikin suggests washing them with water or vacuuming them, using a neutral detergent in tepid water for heavy soiling, and drying them in the shade.
- Dry the unit after cooling. Many units have a drying, self-clean or mould-proof mode that runs the fan after cooling to dry the coil. Daikin notes its mould-proof setting runs automatically after cooling or dehumidifying stops, so the unit keeps running for a while, which is normal.
- Air out absorbed smells. Run fan mode with the windows open, as Meaco suggests.
- Before a long break, Daikin recommends running the mould-proof operation on a sunny day to dry the inside thoroughly, then switching off at the breaker.
Do not spray household cleaners into the unit. Daikin warns against using commercial cleaning agents inside an air conditioner, because they can damage the resin parts of the heat exchanger and other components, and in the worst case cause water leaks.
What a professional clean involves
If the smell comes back after the filters are clean, the mould is on parts you cannot safely reach. A proper clean typically includes:
- Cleaning the indoor coil with a cleaner suited to it, and rinsing.
- Cleaning the fan barrel or blower wheel, which collects a lot of dirt.
- Cleaning the drip tray and flushing the drain, checking the condensate pump if there is one.
- Checking the unit is draining properly so the smell does not return.
Planned air conditioning maintenance covers coil, tray and drain cleaning, which prevents most smells. For offices, shops and restaurants, our air conditioning repair team can deep clean units that already smell.
When to call an engineer
- A musty smell that returns within days of cleaning the filters.
- Visible black spots on the louvres or inside the outlet.
- A smell with water leaking from the indoor unit.
- Any burning or electrical smell.
Questions people ask about musty air conditioner smells
Why does my air conditioner smell musty?
Because mould or bacteria are growing in the damp parts of the indoor unit, usually the coil, drip tray, drain or fan. Cooling leaves those parts wet, and dust from dirty filters feeds the growth.
Is a musty smell from my AC dangerous?
It is a sign of mould or bacteria. The UK Health Security Agency says damp and mould mainly affect the airways and lungs, and that people with health conditions or weakened immune systems are at greater risk. A persistent smell is worth having cleaned.
How do I get rid of a musty smell in my air conditioner?
Clean the filters, use the unit's drying or mould-proof mode after cooling, and air out absorbed room smells with fan mode and the windows open. If it returns, the coil, tray, drain and fan need a professional clean.
Can I spray cleaner into my air conditioner?
Daikin advises against using commercial cleaning agents inside an air conditioner, because they can damage the heat exchanger's resin parts and other components, and can lead to water leaks.
Why does my AC only smell musty when I first turn it on?
Moisture left inside after the last cooling run lets mould grow while the unit is off, and the fan blows that smell out at start-up. Running a drying mode after cooling usually helps.
How often should an air conditioner be cleaned to prevent smells?
Clean the filters about every 2 weeks when the unit is in regular use, as Daikin recommends, and have the coil, tray and drain cleaned during an annual service.
The short version
A musty smell means the inside of the unit is staying damp and dirty. Clean the filters, dry the unit after cooling, and avoid spraying cleaners into it. If the smell keeps coming back, a professional clean of the coil, tray, drain and fan sorts it.
Last updated: 4 November 2026
Written by
Ali Elm
Ali is the Head of Operations at Be Cool Refrigeration with over a decade of hands-on experience in HVAC and commercial refrigeration. He oversees every installation, repair, and maintenance project, making sure the work meets the highest standards. Ali holds full F-Gas certification and has worked across residential, commercial, and industrial refrigeration systems throughout London and the South East. When he is not on site, he writes these guides to help business owners and homeowners understand their cooling systems better.