Every year, around the second or third week of October, my phone starts doing something predictable.
People ring up describing a smell. They’ve just put the heating on for the first time since spring, and within a day or two the house has developed a faint musty, damp, slightly earthy odour that wasn’t there in September. It’s not strong enough to be alarming. It’s just persistently, annoyingly there.
And almost every one of them has the same theory: it must be damp in the walls.
Sometimes it is. Often it isn’t. In a great many London homes, that October smell is coming from the floor, and specifically from moisture that has been sitting quietly in the carpet backing and underlay since the summer.
Here’s the mechanism, because once you understand it the smell stops being mysterious and starts being fixable.
Why the smell arrives with the heating and not before
Carpet is absorbent. That’s most of the point of it – it’s a soft, thick, fibrous layer that traps things. It traps dust, it traps allergens, and it traps moisture.
Through a London summer, moisture gets into carpet from a dozen unremarkable sources. Humid air. Damp shoes after a downpour. Condensation running down onto the skirting. Wet washing dried indoors on a rack. A spill that got blotted up on the surface but soaked through underneath. None of it individually is a problem.
That moisture sits in the carpet backing and in the underlay beneath, where there’s very little airflow. Warm, dark, damp, and full of organic material – skin cells, food particles, pet dander, general soil. Which is, unfortunately, an excellent environment for bacteria and mould spores to get on with things quietly for a few months.
Then you turn the heating on.
Warm air moving through the room accelerates evaporation from the carpet, and everything that has been developing down in the backing starts coming up with it. The smell isn’t new. It’s just newly airborne.
That’s why it appears within a day or two of the first proper heating cycle and why it seems to come from nowhere.
How to tell whether it’s the carpet or something worse
This matters, because I’d rather you spent nothing with me and fixed a real problem than paid me to clean a carpet while a leak carried on quietly behind a wall.
Signs it’s most likely the carpet
- The smell appeared within days of the heating going on, having been absent all summer
- It’s stronger at floor level – crouch down and it intensifies noticeably
- It’s worse in rooms with the thickest carpet or the least ventilation
- It fades when windows are open for a few hours and returns when they’re shut
- The walls are dry to the touch with no visible staining or bubbling paint
- The carpet is more than a year or two since its last professional clean
Signs it’s a building problem, not a cleaning problem
- Visible black or green mould on walls, ceilings, or around window reveals
- A tide mark low on the wall, often with white salt deposits – that’s rising damp
- Peeling or bubbling paint, or wallpaper lifting at the edges
- The carpet is actually wet or noticeably cold and heavy underfoot in one patch
- The smell is localised to one specific area near an external wall or under a window
- It got worse rather than better after previous cleaning attempts
If you’re seeing the second list, cleaning the carpet won’t solve it. You’d be treating the symptom while the cause carries on. Get the source of the water dealt with first, then clean. I’ll tell you this on the phone rather than turn up and take your money, because a musty carpet cleaned over an active leak smells musty again within a fortnight and neither of us wants that conversation.
Why airing the room and spraying freshener makes it worse
This is the part I find genuinely frustrating on people’s behalf, because the instinct is completely reasonable and completely counterproductive.
Carpet fresheners and deodorising powders work by masking. They add a fragrance on top of the odour without touching the source. For a day or two the room smells of synthetic linen. Then the fragrance dissipates, the underlying smell is still there, and you apply more.
The problem is that most of those powders are designed to be vacuumed up, and most people don’t get all of it back out. What’s left settles down into the pile, absorbs moisture, and becomes another layer of organic material sitting in a damp environment. You’ve fed the problem.
Plug in air fresheners do the same thing without the residue, but they also do nothing at all about the actual cause.
Ventilation genuinely helps and is worth doing regardless – opening windows for even five or ten minutes a day makes a real difference to indoor humidity, which is exactly what local authority damp guidance recommends. But ventilation removes airborne moisture. It doesn’t remove what’s already bonded into the carpet backing.
What actually clears it
Hot water extraction, properly done, with the emphasis on extraction rather than the water.
The process injects hot water and cleaning solution under pressure into the carpet, which dissolves and lifts the soil, bacteria and organic matter causing the odour. Then it immediately draws that back out. The machine I use, a Ninja 400 PSI, pulls the overwhelming majority of the moisture back with it – which is the entire point.
Two things make this work where a hired machine doesn’t.
The first is temperature. Heat breaks down the organic compounds producing the smell rather than just diluting them. The second is suction. A supermarket hire machine puts water in perfectly well but recovers a fraction of it, which means you’ve added moisture to a carpet that had a moisture problem to begin with. I get calls every November from people who did exactly that in October and made the smell twice as bad.
I’ve written about that in more detail in our post on why stains reappear after carpet cleaning – the mechanism is the same. Water goes down, doesn’t come back up, and the problem migrates rather than leaves.
Mould in carpet: when it’s beyond cleaning
Worth being straight about the limits here.
Mould needs moisture to grow, and the threshold where growth accelerates is around sixty percent relative humidity. Below that, spores sit dormant. Above it sustained, they get to work. London homes through autumn and winter frequently sit above that line, particularly in ground floor rooms, basement flats and anywhere with poor ventilation.
Surface mould in carpet fibres responds well to professional cleaning. The high temperature of hot water extraction kills spores, and the extraction removes them along with the organic matter feeding them.
Mould that has established in the underlay is a different matter. The National Carpet Cleaners Association is candid about this: mould on absorbent materials like carpet can be very difficult to remove, and in severe cases replacement is the realistic option. If I lift a corner and the underlay beneath is black, I’ll tell you that the underlay needs cutting out and replacing before the carpet is worth cleaning.
That’s not an upsell – it’s the opposite. It’s me telling you the job is bigger than the one you rang about.
If you rent, this matters legally as well as practically. Awaab’s Law places specific obligations on social landlords to investigate and act on reports of damp and mould within set timeframes. Private tenants have protections too under the Homes (Fitness for Human Habitation) Act. If the damp is structural rather than incidental, that’s your landlord’s responsibility to fix, not yours to clean around.
Stopping it coming back next October
The smell is a symptom of a carpet that’s been holding moisture and soil for a long time. Deal with both and it doesn’t return.
| What to do | Why it works |
| Professional clean before the heating season | Removes the organic matter that produces odour when it warms |
| Open windows briefly each day, even in winter | Cuts indoor humidity below the level where mould accelerates |
| Dry washing outdoors or in a vented room | Indoor drying is one of the biggest single moisture sources |
| Use extractor fans and keep using them | Kitchen and bathroom moisture migrates to colder rooms |
| Move furniture off carpet edges occasionally | Airflow under and around furniture prevents damp pockets |
| Deal with wet shoes at the door | Repeated damp in the same hallway spot is a common cause |
The timing matters. A clean in late September or early October, before the heating goes on properly, gets the organic load out while there’s still enough warmth and daylight for fast drying. A clean in January works but takes longer to dry. I’d rather do yours in autumn.
Frequently asked questions
Why does my carpet smell musty only when the heating is on?
Warm air accelerates evaporation from the carpet backing and underlay, carrying up odours from bacteria and organic matter that developed there over the summer. The smell isn’t new, it’s newly airborne. It appears within a day or two of the first proper heating cycle and is usually stronger at floor level.
Can professional carpet cleaning remove a damp or musty smell?
Yes, in most cases. Hot water extraction removes the bacteria, mould spores and organic matter producing the odour rather than masking it. The high temperature breaks down the compounds causing the smell. It won’t work if there’s an ongoing source of water such as a leak or rising damp – that needs fixing first.
Is a musty carpet smell dangerous?
The smell itself isn’t, but what causes it can matter. Mould spores and dust mites in damp carpet are common triggers for asthma, eczema and allergic rhinitis. If anyone in the household has breathing difficulties or unexplained symptoms that worsen in autumn, damp carpet is worth ruling out.
Will carpet freshener or baking soda get rid of the smell?
Only temporarily. Both mask odour rather than removing its source, and powder residue left in the pile absorbs moisture and adds to the organic load. The smell returns once the fragrance fades, usually within days, and repeated applications make the underlying problem worse.
How do I know if it’s the carpet or damp in the walls?
Crouch to floor level – if the smell intensifies, it’s likely the carpet. Check walls for visible mould, tide marks, salt deposits or bubbling paint, which point to a building problem. Carpet odour also fades with ventilation and returns when windows shut. Structural damp doesn’t behave that way.
When is the best time of year to have carpets cleaned?
Late September or early October, before the heating season begins properly. There’s still enough warmth and daylight for carpets to dry within three to five hours, and you remove the organic matter before warm air starts driving odour up out of it. Spring is the other good window.
Sort it before the clocks go back
This is one of those problems where the fix is genuinely straightforward and people put it off for months because they assume it’s something structural and expensive.
Most of the time it isn’t. It’s a carpet that’s been quietly holding on to a summer’s worth of moisture and soil, and three or four hours of proper extraction clears it out completely.
I’ve been doing this across London for seventeen years, and October is when I have this conversation most. Ring me and describe the smell. If it sounds like a builder’s job I’ll tell you that instead, and you’ll have lost nothing but a phone call.
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