Key Takeaways
- Underfloor heating runs at 35°C to 45°C flow; traditional radiators need 60°C to 75°C.
- Drop a radiator to a 55°C flow and it gives about half its rated output, which is why heat pump retrofits usually mean upsizing every radiator.
- Wet underfloor heating costs £70 to £100 per m² in a new slab and £100 to £190 per m² retrofitted; electric £50 to £85 per m²; a radiator installed £100 to £300 (The Floor Heating Warehouse, Checkatrade and MyJobQuote, 2026).
- Wet underfloor heating tops out at roughly 40 to 100 W per m², so it cannot heat a poorly insulated room on its own.
- A low-profile retrofit still adds 15mm to 20mm of floor height, and that means trimming doors and altering thresholds.
Underfloor heating is not simply a more luxurious radiator. It is a low-temperature system with a hard output ceiling, and that single fact decides whether it suits your house, your heat source and your floor build-up. Here is the comparison for London homes in 2026.
Underfloor heating vs radiators
The defining difference is flow temperature. Underfloor heating spreads gentle heat over a whole floor; a radiator concentrates it into a small hot surface and needs much hotter water to do it.
| Factor | Wet underfloor heating | Radiators |
|---|---|---|
| Flow temperature | 35°C to 45°C | 60°C to 75°C traditional |
| Output ceiling | about 40 to 100 W per m² of floor | Sized to the room, no practical ceiling |
| Time to warm up | 2 to 3 hours through screed | About 15 minutes |
| Retrofit cost | £100 to £190 per m² | £100 to £300 per radiator |
| Works with a heat pump | Yes, as designed | Usually only after upsizing |
| Floor height added | 15mm overlay to about 100mm full build-up | None |
What each actually costs
| System | New build or new floor | Retrofit into an existing floor |
|---|---|---|
| Wet underfloor heating | £70 to £100 per m² | £100 to £140 per m² as a low-profile overlay, £120 to £190 per m² with the screed replaced |
| Electric underfloor heating | £50 to £85 per m² | £50 to £85 per m² |
| Radiators | £100 to £300 per radiator installed | £100 to £300 per radiator installed |
| Whole three-bed, 80 to 120 m², wet system | £6,000 to £13,000 | |
| Whole three-bed, 6 to 8 radiators | £900 to £1,600 | |
One quirk worth knowing: the manifold, pump and controls cost much the same whatever the floor area, so small rooms never reach the headline rate. A 2m² cloakroom at £340 to £380 works out at nearly £190 per m², against £100 or less for a 20m² room. If you are doing one small room, electric is usually the sensible answer.
Underfloor heating
Wet underfloor heating gives even warmth with no cold spots, frees every wall, and runs at a flow temperature that suits a condensing boiler or a heat pump. The constraint people miss is output. A wet system delivers roughly 40 to 100 W per m² of floor, capped by the maximum safe floor surface temperature of about 27°C over timber, laminate or carpet. If a draughty London room loses more heat than the floor can supply, underfloor heating will never hold temperature, however long you run it. Insulate first. See our underfloor heating cost guide and flooring comparison.
Thermal mass cuts both ways. A screeded floor takes 2 to 3 hours to come up to temperature, so it is not an on-demand system, but once charged it holds heat and cycles less. Electric mats warm in 30 to 60 minutes.
Radiators
Radiators are cheap, fast and easy to retrofit, and their output is far more flexible. The number that matters is how it is quoted. Radiator outputs are published at ΔT50 under EN 442, meaning 80°C flow and 60°C return into a 20°C room. Run the same radiator at a modern 55°C flow and 45°C return and the correction factor is about 0.5: a radiator rated 1,000 W delivers around 500 W.
That is not academic. Approved Document L now asks for heating systems newly installed or fully replaced in existing dwellings to be sized for a maximum flow temperature of 55°C. Meeting it properly means roughly doubling emitter size, so budget for larger radiators rather than assuming the existing ones carry over. Replacing all the radiators in a house to run at low temperature typically costs £900 to £1,600 for a three-bed, plus around £400 for a power flush and £70 to £140 per thermostatic valve. See combi vs system vs regular boilers.
Source: Zehnder correction factors, EN 442, MyJobQuote and Approved Document L Volume 1, 2021 to 2026.Why underfloor heating suits a heat pump
Because a heat pump's efficiency falls as flow temperature rises, and underfloor heating is designed for exactly the temperature a heat pump likes. Air source heat pumps are at their best at 35°C to 40°C flow, which is the underfloor design point and roughly half what an old radiator system was built around.
The industry has formalised this. Since March 2025, MCS 031 estimates performance using generic seasonal figures keyed to the design flow temperature rather than a manufacturer's headline figure, so a system designed around 50°C is now assessed at a seasonal performance factor of about 3.1 where the product directory average had been nearer 3.6. Designing low is no longer optional if you want the numbers to work. Note that the Boiler Upgrade Scheme's £7,500 pays for the heat pump, not for the underfloor heating or the upsized radiators it needs. See our air source heat pump cost guide.
Source: MCS 031 update, 2025, and Ofgem Boiler Upgrade Scheme, 2026.Floor height, thresholds and running costs
In a retrofit, height is the deciding practicality. A full traditional build-up of insulation, pipe and screed adds around 100mm, with sand and cement screed at up to 65mm and liquid screed from 40mm. Low-profile overlay systems using 12mm pipe add as little as 15mm, with 12mm to 20mm the usual band. Even that 15mm means trimming internal doors, reworking thresholds and rethinking the kitchen plinth, and it changes the relationship with the stairs.
On running cost, the fuel decides it. On the Ofgem cap for July to September 2026 (the cap is reset every quarter, so check the current rates), gas is 7.33p per kWh and electricity 26.11p. Allowing for boiler efficiency, heat from gas costs around 8p per kWh delivered, so electric underfloor heating costs roughly three times as much to run as a wet system on gas. That is why electric belongs in bathrooms and small rooms as comfort heating, not as a whole-house solution.
Source: Ofgem price cap, 1 July to 30 September 2026, and UK Underfloor Heating, 2026. Delivered-heat costs are calculated and indicative.The best of both
Fit wet underfloor heating wherever the floor is already coming up: a new extension, a new screed, a ground floor being relaid, or a whole-house renovation. It is the right call with a heat pump and it needs no upsizing later. Keep radiators upstairs and anywhere the floor is staying put, and if you are replacing a heating system, size them for 55°C flow now rather than paying twice. Use electric underfloor heating only in bathrooms and cloakrooms, as comfort heating under tile, never as the main heat source.
The usual London answer is underfloor heating downstairs and upsized radiators upstairs, and it is usual because it is right. We plan both into renovations across London, with the heating engineer we appoint designing and installing the system, including home renovation in London. For a free quote, contact us or call 07472 424 226.