There are two completely different smells and they have different causes. A musty, damp or sour smell is biofilm — a film of detergent residue, fabric softener and bacteria that builds up when a machine only ever runs cool washes. A sewage or drain smell is coming from your plumbing, not from the machine, and no amount of cleaning the drum will fix it.
Which smell do you have?
| Smell | Where it is strongest | Cause |
|---|---|---|
| Musty, damp, sour | Inside the drum and door seal | Biofilm from cool washes and detergent residue |
| Mouldy | Door seal fold, detergent drawer | Black mould growing in trapped moisture |
| Sewage or drains | Behind the machine; worse when it drains | Plumbing — trap or standpipe, not the washer |
| Rotten eggs | Drum, standing water | Stagnant water in the pump or sump |
| Burning or hot plastic | Anywhere, during a cycle | Electrical or mechanical fault — stop using it |
If you smell burning, hot plastic or scorching during a cycle, stop the machine, unplug it and do not use it again until it has been checked. That is a motor, belt or wiring fault, not a hygiene problem.
Why biofilm builds up
Three modern habits combine to cause almost every musty washing machine:
- Cool washes. 20°C and 30°C programmes save energy but never get hot enough to kill the bacteria and break down the greasy residue that collects inside the machine.
- Too much detergent. Excess detergent does not rinse away; it coats the drum, the hoses and the sump, and that film feeds bacteria. Liquid detergents and fabric softener are worse than powders in this respect.
- A closed door. Shutting the door on a warm, damp drum after every wash creates ideal growing conditions.
The result collects in the places you cannot see: the outer tub, the sump hose, the detergent drawer housing and — most visibly — the fold of the door seal.
Clearing a musty machine
- Clean the door seal fold thoroughly
Pull the rubber open and wipe right into the fold with a cloth and hot soapy water, or a mild bleach solution. This is where the smell is most concentrated and where black mould shows first. Work all the way round, and dry it afterwards. If the seal is split or the mould has grown into the rubber rather than sitting on it, the seal needs replacing.
- Take out and wash the detergent drawer
Most drawers pull fully out, usually with a release catch. Wash it in hot water and scrub the compartments. Then scrub the roof of the empty housing with an old toothbrush — there is normally a layer of sludge up there that people never see.
- Clean the pump filter
Behind the flap at the bottom front. Trapped lint, hair and debris sitting in stagnant water is a smell source in its own right, and it is the source of the rotten-egg version.
- Run a maintenance wash at the highest temperature
Empty machine, no clothes, hottest cycle available — 90°C if your machine offers it. Use either a proprietary washing machine cleaner, or around 200 g of soda crystals, or 500 ml of white vinegar in the drum. Do not mix bleach with anything else. This is the step that actually kills the biofilm; the cleaning above just removes what you can reach.
- Run a second rinse
A plain rinse cycle afterwards flushes out what the hot wash has loosened. If the machine still smells after this, run the maintenance wash once more — a badly neglected machine sometimes needs two.
If the machine has been standing with water in it, drain it through the pump filter tube first. A hot cycle over stagnant water just circulates the problem.
If it smells of drains
A sewage smell is not biofilm and cleaning the drum will not touch it. The smell is coming back up the waste pipe into the room, and there are three usual reasons:
- No trap, or a dry trap. The standpipe the machine drains into should have a water trap, exactly like a sink. If it does not, or the water in it has evaporated because the machine has not been used for a while, sewer gas comes straight back up. Running the machine refills a dry trap.
- The drain hose is pushed in too far. If it goes past the trap it defeats it entirely. Around 10–15 cm into the standpipe is enough.
- A partial blockage. Debris in the waste pipe holds water and rots. Pour a few litres down the standpipe with the machine off — if it does not clear quickly, that is your problem, and our drainage guide covers clearing it.
Keeping it from coming back
- Run a hot maintenance wash once a month. This single habit prevents almost all of it.
- Use less detergent than you think you need — check the dose against your water hardness, and reduce it if you are getting residue.
- Cut back or stop fabric softener; it is a significant contributor to the film.
- Leave the door and the detergent drawer ajar between washes so the drum dries.
- Wipe the door seal dry after a wash, particularly the bottom of the fold where water pools.
- Take washing out promptly. Damp laundry left in a closed drum overnight will make the machine smell on its own.
