Underfloor heating lets a heat pump run at about 35 °C instead of 45–55 °C, which is worth roughly 20–40% on running cost. Radiators work perfectly well too when sized for 40–50 °C — which usually means upgrading only the rooms whose existing panels fall short. The real decision is driven by floor build-up, ceiling height and retrofit disruption, not by whether the heat pump functions.
Does underfloor heating make a heat pump more efficient?
Yes, and the mechanism is simple: a large emitting surface can deliver the same kilowatts at a much lower water temperature. A screed floor emitting across an entire room needs 30–35 °C; a steel panel on one wall needs 45–55 °C to do the same job. Since heat pump efficiency changes by roughly 2–2.5% per degree of flow temperature, that 15–20 °C gap is worth 20–40% of the heating bill. Put the other way round, a heat pump on unmodified boiler-era radiators consumes about a quarter more electricity than the same unit on properly designed underfloor heating.
What actually changes between the two?
| Underfloor heating | Heat pump radiators | |
|---|---|---|
| Design flow temperature | 30 – 35 °C | 40 – 50 °C |
| Relative running cost | Baseline | +20 to +40% |
| Response time | 2 – 6 hours (thermal mass) | 20 – 40 minutes |
| Retrofit disruption | Floor up, or 15–20 mm overlay | Swap panels only |
| Best control strategy | Continuous, weather-compensated | Continuous, weather-compensated |
| System water volume | Screed circuits add volume | Low volume — usually needs a buffer |
| Summer cooling | Possible with dew-point control | Not possible |
| Furniture and floor coverings | Thick carpet or rugs cut output | No constraint |
Two lines in that table are decisive in practice. Response time means underfloor heating should never be run intermittently — the mass that makes it efficient also makes it slow. System volume means a radiator system frequently needs a buffer tank to reach the manufacturer's minimum volume, while a screed floor often already satisfies it. Whether yours does is set out in do I need a buffer tank with a heat pump?
Can I mix underfloor downstairs and radiators upstairs?
Yes, and it is the most common European retrofit. The trap is temperature. A single heat pump produces one flow temperature, so if the upstairs radiators need 50 °C the underfloor circuits get 50 °C too — and then a blending valve throttles them back down to 35 °C, wasting the efficiency you paid for. There are two correct answers: size the upstairs radiators generously so the whole house runs at 40 °C, or accept a mixing group and treat the underfloor purely as a comfort feature. Sizing the radiators is almost always cheaper over ten years. The uplift factors are in do I need bigger radiators for a heat pump?
Is underfloor heating always worth the retrofit cost?
No. Lifting a ground floor to install underfloor heating in an existing house is expensive and displaces the occupants, and the same money spent on fabric insulation plus larger radiators often reduces the bill by more. Underfloor heating wins clearly in three situations: new build and extensions where the screed is going down anyway, ground floors being replaced for other reasons, and rooms where wall space for a radiator genuinely does not exist. Low-profile overlay systems at 15–20 mm exist for retrofit, but they carry less water and less mass, so they behave more like a fast emitter than a classic screed floor.
Whichever emitter you choose, the domestic hot water side is unaffected: the cylinder charges at 48–52 °C through a large coil regardless of what heats the rooms, and a weekly pasteurisation handles legionella. What changes is only the space heating flow temperature — which is where nearly all the energy goes, and why the decision matters at all.
Frequently asked questions
Is underfloor heating essential for a heat pump?
No. Correctly sized radiators running at 40–50 °C work well and are far less disruptive to retrofit. Underfloor heating is the efficiency optimum, not the entry requirement, and the difference between them is roughly 20–40% of the heating bill.
What flow temperature does underfloor heating need with a heat pump?
Typically 30–35 °C in a screed floor sized for a heat pump, and up to 40 °C in a low-profile overlay or a poorly insulated ground floor. Above 40 °C the floor surface itself becomes uncomfortable, which is a practical ceiling as well as an efficiency one.
Should underfloor heating be switched off at night?
No. A screed floor takes hours to respond, so setback simply forces a higher flow temperature at the coldest part of the morning. Run it continuously on a weather-compensated curve and let the mass smooth the load.
Can I put underfloor heating downstairs and radiators upstairs?
Yes, and it is the standard retrofit. Size the upstairs radiators for the same flow temperature as the floor circuits, around 40 °C, so the heat pump never has to make hot water for one circuit and blend it down for the other.