Updated
Musty Smell When The Air Conditioning Starts
A damp, sour or gym-bag smell fills the room for the first few minutes after the unit comes on.
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What's happening?
The indoor coil spends every cooling hour wet. That constant damp, plus the dust the filters do not catch, is an ideal home for mould and bacteria, which colonise the fins, the drain tray and the fan barrel. When the fan restarts it drives air across that growth and pushes the smell into the room, which is why the odour is worst in the first minutes and fades afterwards. Engineers call the growth biofilm. It is the same problem behind most blocked condensate drains, so a musty unit and a dripping unit are usually the same fault at different stages. A sour or sewage-like smell rather than a musty one points somewhere else — at a condensate line joined into a waste without a trap, letting drain gases back through the pipe.
What usually causes it?
- Mould and bacterial biofilm on the indoor coil, tray and fan barrel
- Filters left unwashed, so dust settles onto a permanently wet coil
- System always switched straight off after cooling, so the coil never dries out
- Condensate tray holding standing water because the drain is partly blocked
- Condensate drain connected into a waste pipe without a trap, letting drain smells back
- Damp or a leak in the wall or ceiling void the pipework runs through
- Unit oversized for the room, so it short-cycles and leaves the coil wet without ever drying it
Is it dangerous?
The smell itself is a nuisance, but mould in the home is not something to leave running. Government guidance for housing providers is clear that damp and mould can cause or worsen respiratory problems, and that it should be treated as a health risk rather than a lifestyle issue — that logic applies to mould growing inside the machine that blows air at you. Anyone with asthma, an allergy or a weakened immune system has a stronger reason to get it cleaned rather than masked with a spray.
Can I fix it myself?
Wash the filters properly — warm water, dry fully, refit — and keep doing it through the cooling season. Then change how you shut the unit down: most systems have a fan-only or dry mode, and running that for ten to fifteen minutes before switching off dries the coil so there is nothing for mould to grow on. Some units do this automatically and the setting is worth finding. Do not spray aerosol cleaners or bleach through the vents; they do not reach the fan barrel and they damage the coil coating.
When should you call an air conditioning engineer?
Call an air conditioning engineer if the smell survives clean filters and a drying-out routine, if you can see black speckling on the coil fins or the fan when you look up into the head, or if the smell is sour or drain-like rather than musty. Deep cleaning means stripping the head down, and that is not a domestic job.
What will the air conditioning engineer actually do?
- Strip the indoor head down to expose the coil, tray and fan barrel
- Deep-clean the coil and blower with a purpose-made coil cleaner, catching the run-off
- Clear and sanitise the condensate tray and drain line, which is where the growth reseeds from
- Check whether the drain is trapped correctly if the smell is drain-like rather than musty
- Check the unit's run time and setpoints for short-cycling that leaves the coil permanently wet
- Set up or enable an automatic dry cycle so the coil dries after every cooling run
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