How the two differ in kind
Every wet emitter obeys the same relationship: what it gives out depends on how much hotter the water inside it is than the air around it, and the output falls steeply as that difference narrows. A radiator is rated at a large difference — water at seventy or eighty degrees against a room at twenty — and it is compact because of it. An underfloor loop runs at thirty-five to forty-five, a difference a third the size, and it compensates by being enormous: the emitter is the entire floor.
That is why the heat source decides. A condensing boiler is happy producing seventy-degree water and merely happier producing fifty. A heat pump is the opposite: its efficiency depends more on the temperature it has to reach than on almost anything else, and pushing it to radiator temperatures costs a large share of the reason it was installed. Run the same radiators at forty degrees instead of seventy and their output falls to somewhere around half — the room that was warm is not, and nothing about the heat pump is faulty.
The third fact sets a ceiling nobody can design around. A floor's output is limited by its SURFACE TEMPERATURE, capped for comfort and for the covering at around twenty-nine degrees in occupied areas — a little more at a perimeter or in a bathroom. Below that cap, pipe spacing sets the output density; above it there is nothing to be done, however hot the water is. A room whose heat loss per square metre exceeds what a floor can deliver cannot be heated by its floor alone, and that is a fabric question answered before any of this is chosen.
The factors that actually differ
Show
| Radiators and baseboard convectors | Underfloor heating loops | |
|---|---|---|
| What flow temperature it wants | Sized at a large temperature difference. Compact at seventy degrees, marginal at fifty, and roughly half its rated output at heat-pump temperatures. | Designed for thirty-five to forty-five. It is the emitter that lets a heat pump run where its efficiency actually lives. |
| Output ceiling | Raise the water temperature or fit a bigger unit. There is no hard cap short of the room running out of wall. | Capped by floor surface temperature at roughly twenty-nine degrees occupied, so there is a maximum watts per square metre no water temperature exceeds. |
| Response time | Minutes. A room can be brought up for an evening and let go afterwards, which suits intermittent occupancy. | Hours in a screed, far less in a low-profile board. Run continuously with weather compensation rather than on a timer — a different control strategy, not a slower version of the same one. |
| What the floor covering does | Nothing. The emitter is in the room, not under the finish. | Everything. A carpet and underlay sit between the pipe and the room as thermal resistance, and a thick one can cost a large fraction of the output — a decision usually made by somebody who was not in the heating conversation. |
| Usable area | Consumes wall. Under a window is the traditional position and often the only one that works with the furniture. | Consumes none — but floor under fitted units, a bath or a wardrobe emits into a cupboard. The effective area is the exposed floor, not the room. |
| Fitting it to an existing house | A pipe run and a bracket. Disruptive for a day, reversible, and the floor never comes up. | A build-up in floor height that has to be found somewhere: door bottoms, stair risers, thresholds. Low-profile systems reduce it and reduce the output with it. |
| Zoning | Per room by thermostatic valve, but every radiator shares one flow temperature, so the coldest room sets it for the house. | Per loop from a manifold, each with its own actuator — and each room's floor can be laid at a different pipe spacing for its own load. |
| Joints and repairs | Every unit is a set of joints you can see, and a leaking one is swapped in an hour. | Continuous loops with no joints buried in the screed — every connection is at the manifold, where it can be reached. That is the whole reason for the design, and a damaged loop is a floor-up repair rather than a fitting. |
| Shape of the cost | Cheap to install, and the running cost depends entirely on what is producing the water. On a heat pump it is the expensive option because it forces a higher flow temperature. | Front-loaded — pipe, manifolds, insulation, screed and a longer programme — and cheaper to run wherever the source rewards low temperatures. On a boiler in a well-insulated house, the running difference is modest. |
Which one, and when
Choose radiators and baseboard convectors when…
- The heat source is a boiler and is staying one, and the house is already piped for radiators.
- Rooms are used intermittently and you want them warm quickly rather than kept warm continuously.
- The floor cannot rise: existing thresholds, stair geometry, or a finished floor that is not coming up.
- The room's heat loss per square metre is above what a floor can deliver under its surface temperature cap.
Choose underfloor heating loops when…
- The heat source is a heat pump, or will be — this is the pairing that lets it run at the temperatures it is efficient at.
- The floor is coming up anyway, or the slab is being poured, so the build-up costs nothing extra.
- The rooms are large and open, where wall space for emitters is scarce and the floor area is generous.
- You want the emitter invisible and the wall free, and you are content to run the system continuously rather than in bursts.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Is underfloor heating actually cheaper to run?
- Only where the heat source is rewarded for producing cooler water, and the size of the reward is the whole answer. A heat pump's efficiency rises steeply as its flow temperature falls, so moving from radiator temperatures to floor temperatures is a large and repeatable gain — this is the case where the claim is straightforwardly true. A condensing boiler gains much less: it condenses better on a lower return, which is a real improvement, but it is a few per cent rather than a transformation. And a floor run continuously in a leaky house can use more energy than radiators run in bursts, because the losses run continuously too. The honest form of the question is not which emitter is cheaper but which one lets your particular heat source work where it is efficient.
- Can I keep my existing radiators if I fit a heat pump?
- Sometimes, and the check is arithmetic rather than opinion. Take each radiator's rated output, which is quoted at a stated temperature difference, and recompute it at the flow temperature the heat pump will actually run — the output falls faster than linearly, and around forty-five degrees a typical radiator delivers roughly half its catalogue figure. Compare that against the room's heat loss. Where it falls short, the fix is a bigger emitter rather than hotter water, because raising the flow temperature is exactly what destroys the efficiency you installed the heat pump for. In practice this means some rooms keep their radiators, some get larger ones, and one or two get a fan-assisted unit or a floor loop. Replacing every radiator is rarely necessary and assuming none need replacing is the commonest way a retrofit disappoints.
- Can underfloor heating go under carpet or timber?
- Yes, within a limit on the total thermal resistance of everything above the pipe — and it is that total, not the product category, that matters. Carpet plus a thick underlay is usually where installations get into trouble, because the underlay is chosen separately and often after the heating was designed. Engineered timber is generally fine and solid timber has its own movement constraints at floor temperature. The practical approach is to decide the covering before the heating is designed rather than after: a floor designed against a bare screed and then carpeted is a floor that will not reach its output, and the remedy at that point is a higher water temperature, which is the thing the design existed to avoid.
- How long does a floor take to warm up, and is that a problem?
- A screeded floor responds over hours because the screed is a large thermal mass that has to be warmed before the surface rises. That is a characteristic to design around rather than a fault: such systems are run continuously with weather compensation, so the floor never goes cold and never needs to catch up. A low-profile system on boards over joists has far less mass and responds much faster, at the cost of lower output. The mistake is running a screeded floor on a conventional timer like a radiator system — it spends the morning warming up and the evening giving back heat nobody is there for, which is the version of underfloor heating that gets a bad reputation.
- What actually decides the pipe spacing?
- The output the room needs per square metre, bounded above by the floor surface temperature limit. Closer spacing delivers more watts per square metre and also a more even surface, because the temperature between pipes dips; wider spacing shows up as warm and cool stripes underfoot before it shows up as insufficient heat. Perimeter zones are commonly laid tighter than the middle of a room, because that is where the loss is and because the surface limit is relaxed slightly in a strip along an external wall. And the total length of a loop is bounded by pressure drop, which is why a large room is several loops rather than one long one — the calculator returns the pipe length for a given spacing, and the split into loops is the next decision after it.
- Can a floor heat a poorly insulated room at all?
- Not beyond its cap, and that is the one constraint on this page that no amount of plant overcomes. Work out the room's heat loss and divide by the exposed floor area — the floor under fitted units and baths does not count. If that figure exceeds what a floor can deliver under its surface temperature limit, the room needs either less heat loss or another emitter, and adding a supplementary radiator or a fan unit is a normal and unembarrassing answer. This is also why underfloor heating and a well-insulated fabric tend to arrive together: the emitter with the lowest output ceiling on the site is the one that most needs the loss reduced first.