You can smell it from the doorway before you get your shoes off. Sour, slightly sweet, something between a damp tea towel and a wet dog, and it lifts when the radiators come on in the evening. The carpet looks fine. It felt dry by the Tuesday. The machine went back to the supermarket a fortnight ago with a clean tank and a good conscience.

This is the most common call I get in January and again in late August, and the householder has almost always done the work themselves with a hired machine and done it thoroughly. Thoroughness is the problem.

Why does the smell arrive ten days after the carpet feels dry?

Because two different things dry at two different speeds, and only one of them is under your hand when you check.

Pile is designed to shed moisture. A cut-pile carpet in a warm room with the windows open will feel dry to a palm within eighteen hours, and that is the test everyone uses. Underneath it sits the primary backing, then the secondary backing, then underlay, then whatever the floor is. Each layer down holds water longer and moves air worse. The underlay in a normal London lounge can still be carrying free moisture nine or ten days after a weekend of extraction, particularly when the carpet above it has sealed over and slowed evaporation to a crawl.

Smell is on a timer of its own. Bacteria in the underlay need water, warmth, food and a few days to get going, and old soil plus hire-shop detergent gives them all four. What you notice at day ten is a colony reaching a size where its waste products register in the air. Before that there was nothing to smell, which is why nobody connects the cause with the effect and why I turn up to be told the carpet was fine after the clean.

What “dry to the touch” is measuring

The top four millimetres. Nothing else. I carry a moisture meter with pin probes and I push them through to the backing in five or six places, and a carpet that feels bone dry at the surface regularly reads wet at the base of the yarn, in a flat in Tooting as readily as in a house in Chislehurst. If you do nothing else after a DIY clean, borrow a cheap meter before you decide the job finished.

How much water does a hire machine leave in the floor?

More than most people would believe, and the arithmetic is not complicated.

A domestic hire unit puts down solution through a spray bar and recovers what it can through a single vacuum motor, and that motor is doing the same work as the one in your upright. Professional portables run two or three vacuum motors in series or a truck-mounted blower, and the difference in lift at the wand is not a matter of degree. A hire machine can typically recover somewhere in the region of half to two thirds of what it lays down in a single pass over a medium-soiled carpet. Everything else goes downward.

Then there is operator behaviour. The tank empties, the user sees filthy recovery water, reads that as progress and does the area again. And again. A professional would make one wet pass and two or three dry passes with no solution flowing, because dry passes are where the water comes back out. I have watched people do four wet passes in a row on a hallway and I have never once seen a hire customer do a dry pass on purpose.

The bit of the instructions everyone skips

The hire shampoo is high foam by design, because foam looks like cleaning. Foam also kills vacuum recovery inside the machine and leaves a sticky surfactant film in the pile and in the backing. That film holds moisture, attracts soil and feeds bacteria, and it is the reason a DIY-cleaned carpet resoils in six weeks. Half the recommended dose would serve most households better than the full one.

What in the floor build-up is doing the smelling?

Open a corner and you will usually find the answer within a minute.

PU foam crumb underlay, the speckled multicoloured stuff under most carpets fitted since the nineties, behaves like a sponge with a skin. It takes water in readily through its open cells and gives it back slowly, and it carries the entire history of the room inside it. Every drink knocked over, every puppy accident from two owners ago, every dropped soup bowl went through the carpet and stopped there. Nothing in a hire machine’s wand reaches that layer, but the water you push down does, and it rehydrates old residue and lets it go back to work. A great many of my “the cleaning caused a smell” jobs are old deposits reactivated rather than anything new.

Felt underlay is worse again. It is organic, it holds several times its own weight in water, and it sours fast. Rubber waffle underlay in an older house is the mildest of the three, because the water tends to run off the ribs and sit on the floor instead, and you can smell it from the doorway there too, only you are now smelling floorboards.

Chipboard is the one that costs money

Where a chipboard subfloor sits under the underlay, a weekend of over-wetting can lift the board at the joints, and swollen chipboard does not go back down. In a maisonette in Beckenham I lifted a corner to find the board crumbling at the edge under the window, and that job left my hands entirely and went to a joiner. Find out which floor you have before you hire anything.

Can it be dried out without lifting the carpet?

Sometimes, if you get to it early and you accept how long it takes.

The method is air movement plus dehumidification, running together. A single air mover angled low so it slides air across the pile rather than at it, a domestic dehumidifier with the doors and windows shut, and forty-eight to seventy-two hours of running. Open windows help the surface and do nothing for the layer that matters, because ambient London air in winter carries plenty of moisture of its own. Warmth without extraction moves water from the underlay into the room and back down again after dark.

Where to point the air, and for how long

The mover goes at the far end of the room facing along the length of the floor, not into a corner, so the air sheets across the whole surface and returns. Move it a quarter turn every twelve hours. The dehumidifier sits in the middle with a clear metre around it, and you empty it morning and evening until it stops filling, which is the only honest signal you have that the floor has given up its water. A household that runs both for three days gets further than one that runs a fan heater for a fortnight.

Floating the carpet is the stronger version. Pull one edge off the gripper, feed air underneath from a mover on its lowest setting, and let the whole cavity breathe. This is a proper technician’s job and it risks the carpet going back badly if the gripper pins are tired or the fitting was tight to start with. On a wool carpet I would rather lift it than leave it wet, and on a cheap polypropylene one in a rental I would think twice about touching the fitting at all.

When is booking another clean the wrong answer?

Most of the time, and I will keep saying it even though a second clean is a paid visit for me.

If the room smells at day ten, more cleaning adds water to a floor that already has too much. You would be paying me to make the cause worse and to buy about a week of masking from whatever deodoriser sits in my van. I turn these jobs down on the phone several times a month, and the conversation goes the same way every time: the customer wants the smell gone this week, and the floor is working to a timetable that has nothing to do with anyone’s plans. You can smell it from the doorway for as long as the underlay stays wet, and no chemistry alters that. The honest sequence is drying first, then assessment, then cleaning if anything still needs it, and often nothing does.

The second thing I say, and it costs me work, is that hiring a machine is a perfectly sensible decision for a small flat if you use it sparingly. Half the detergent or none at all, one wet pass, three dry passes, and stop while the carpet still feels barely damp rather than pleasantly soaked. Done that way, a hire unit on a bedroom in Crouch End will get you seventy per cent of what I would achieve for a tenth of the price, and there is no sense pretending otherwise.

Where the money belongs, when the smell is already ten days old and the underlay is sodden, is a fitter lifting the carpet and putting down new underlay. Twenty-odd pounds a square metre supplied and fitted in most of London, no return of the smell, and no contribution from me beyond telling you to spend it there.

What I charge to be wrong about this

I will come and meter the floor and give an opinion for the price of a call-out, and about a third of those visits end with me taking the fee and recommending nothing. Some of my trade thinks that is bad business. I think a customer who has already spent money on a hire machine, a weekend and a bottle of shampoo has earned a straight answer.

The corner I lifted in Beckenham is still in the photos on my phone, with the meter reading against the skirting and the dark line across the underlay where the water had spread as far as the sofa.