Twenty-seven degrees is the number that matters, and almost nobody in a wet flat knows it.
That’s the surface temperature a domestic underfloor system is designed to run a carpeted floor at. It’s a comfort figure rather than a drying figure, and the gap between those two ideas is where most of the trouble starts. A customer with a damp carpet looks at a thermostat, thinks of it as a hairdryer built into the building, and turns it to maximum.
The floor is not a dryer. It’s a low-grade heat source with several hours of lag and no means of removing a single gram of water from the room.
Why Does Turning The Heating Up Make A Carpet Dry Slower?
It doesn’t slow the evaporation. It slows everything after that, which in a modern flat is the part that counts.
Heat drives moisture out of the pile and into the air, and in a draughty Edwardian semi that air goes up the chimney and out through the sash windows. In a 2019 flat in Nine Elms, SW8, with triple glazing, no trickle vents open and a front door that seals like a fridge, the water comes out of the carpet and stays in the room. Relative humidity climbs to eighty per cent and evaporation stalls, because air that’s already carrying its fill of moisture won’t accept any more.
A flat in Nine Elms with the trickle vents shut has perhaps a hundred and twenty cubic metres of air in it, and a freshly extracted living-room carpet can put a litre and a half of water into that air over an afternoon. The air has nowhere to put it.
Then it condenses on the coldest surface available – the glazing, the window reveals, the outside wall behind the sofa – and some of it drips back down into the same carpet.
I’ve been called back to jobs where the carpet was still damp forty hours after cleaning and the customer had the floor running flat out the whole time, convinced they were helping. In one case the reveals were beaded with water and there was a faint bloom of mould on a cold corner of plaster, which had nothing to do with my machine and everything to do with a sealed box full of steam.
What the windows are telling you at midnight
Walk into the room four hours after a clean and look at the glass. Clear glass means the moisture is leaving the building. Fogged glass means it’s circulating, and you need ventilation or a dehumidifier rather than another degree on the thermostat.
What’s Happening Underneath The Carpet That You Can’t See?
Thermal mass, mostly, and the reason the system behaves nothing like a radiator.
A wet system runs pipes through a screed that might be sixty or seventy millimetres deep. That screed takes three or four hours to respond to a change at the manifold and holds heat for hours after the call stops, so anything you adjust on the day of a clean arrives late. Electric mats under a thin levelling compound respond faster and run hotter in patches, which brings its own problem – I’ve felt fifteen-degree variations across a single hallway in a conversion in Colindale, NW9, where the mats stopped short of the walls.
The carpet sitting on top is a blanket, and a measured one. Combined carpet and underlay is meant to come in under about two and a half tog for the system to work properly, a figure the underfloor heating trade and the Carpet Foundation settled on after testing, replacing an older and lower limit of one and a half.
Wet that carpet and its behaviour changes completely. Water conducts heat far better than air does, so a soaked pile transmits floor heat straight up into itself, and the base of the tuft gets warmer than it ever does in normal use.
Why the backing is the thing I worry about
Heat plus moisture at the base of the pile is where damage happens. Latex secondary backings soften, and on a stuck-down carpet the adhesive can plasticise enough to let the carpet move. Jute-backed wool held warm and wet for two days is a maintenance problem I’d rather not create, and any rug sitting on that floor gets the same treatment from below with no airflow underneath it at all.
The floor is not a dryer. It’s a heated plate under a wet blanket.
How Do I Set A Room Up Before The Wand Goes On?
Three things, and they’re all conversations rather than equipment.
First, the flow temperature. I ask for the system to be left at its normal set point, and I ask for any boost or party mode to stay off. Steady and moderate beats hot and enthusiastic every time. If the customer has a heat pump feeding the manifold, the flow is low anyway and there’s nothing to argue about.
Second, ventilation. Trickle vents open, one window on the latch if the weather allows, internal doors open so the moist air can spread into a bigger volume of building. In a flat with mechanical ventilation and heat recovery, the boost setting on the MVHR does more good than the thermostat ever will.
Third, air movement at floor level. Two air movers on low, pointed across the pile rather than at it, moving the saturated layer of air that sits immediately above the carpet. That layer is the bottleneck, and a moving fan beats a hot floor by a distance.
I also clean differently. More vacuum passes, a slower wand speed with the trigger released on the return stroke, and one extra dry pass per section – the aim is to leave less water in the carpet rather than to remove it faster afterwards.
The conversation I have with the concierge
In managed blocks around Canary Wharf, E14, I now ring the building manager before the job rather than the resident. Half of those flats have the heating controlled centrally with the resident only able to nudge it a couple of degrees, and the difference between a four-hour dry and a two-day dry is a phone call I could have made on the Monday.
What Goes Wrong When A Damp Carpet Sits On A Warm Floor Too Long?
Smell, first and fastest.
Warmth and moisture at the base of a pile are ideal conditions for bacterial growth, and a carpet that stays damp at twenty-six or twenty-seven degrees for two days will develop a sour, slightly sweet odour that the customer describes as a wet dog smell. It comes from the microbial activity rather than from anything in my tank, and it needs the carpet dried properly and often treated again.
Then the underlay. Water that has reached the underlay in a heated floor dries from the top down, so the carpet feels dry to the hand while a sponge layer underneath is still holding moisture at body temperature. That’s the state where you get odour returning a week later and everybody blames the cleaning.
Delamination is slower and rarer, and it’s the one that ends in a replacement rather than a re-clean. Repeated cycles of warm and wet at the base of the pile work at a latex secondary backing over years, and the symptom is a carpet that starts to feel loose and slightly crisp underfoot.
Rugs on heated floors are the third problem, and the one I see most in new-builds.
Do not put the rug back down on the same day
A wool rug laid back onto a warm floor while the foundation is still damp has no ventilation on its underside at all. The heat holds the moisture in, the cotton foundation stays wet for days, and I’ve unrolled rugs in Richmond, TW9, that smelled of a cellar four days after a competent clean. Rugs come off the floor, go over a banister or a couple of chairs, and stay there until the foundation is dry to the back of the hand. That’s usually overnight in summer and two days in February.
Should Some Of These Rooms Be Cleaned Another Way Entirely?
Yes, and it costs me money to say so.
A sealed one-bedroom flat with underfloor heating, no openable windows worth the name, a mid-January booking and a low-pile synthetic carpet is a room where hot water extraction is the wrong tool. A low-moisture method puts a fraction of the water down, dries in an hour or two, and the floor heat becomes a modest help rather than a liability. I charge less for it.
Where extraction still wins is deep soiling, anything with contamination in it, and any carpet where the soil has got past the surface. No dry method reaches that, and a compromise clean in a filthy hallway is money wasted on a better-smelling version of the same problem.
What I ask before I quote a flat with a heated floor
Four questions on the phone. Wet system or electric mats. Whether the windows open. Whether the block runs the heating centrally. And whether there’s a tumble dryer or a bathroom extractor in the flat, because in a small one-bed I’ve been known to ask people to run the shower extractor for an afternoon with the bathroom door open, which is a crude dehumidifier and free. If the answers add up badly I’ll bring a dehumidifier and add it to the price, and most people would rather pay forty pounds for that than spend two days walking on a damp floor in socks.
The honest position is somewhere in the middle. Underfloor heating helps drying when it’s paired with air movement and ventilation, and it works against you when it’s the only thing you’ve got. Add a fan and it earns its keep. Leave it alone in a sealed room and the floor is not a dryer, it’s a warm lid on a wet sponge.
The moisture meter I use for carpet has a setting for screed, which I’ve never once needed, and I keep meaning to read that page of the manual.