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Heat pumpsUnderfloor heating and heat pumps: the natural pairing
Underfloor heating and heat pumps want the same thing: low water temperatures across a large surface. That makes them the most efficient combination available, and it also creates one design problem worth understanding before you sign anything.
Updated 4 August 2026 · written by the Clearline guidance team
Why they suit each other
A heat pump is most efficient at low flow temperatures, typically 35 to 45C, against 70 to 80C for a conventional boiler system. Underfloor heating runs at 35 to 45C natively and spreads its heat over an entire floor rather than a metre of wall. The surface area does the work that temperature would otherwise have to do.
The practical consequence is that underfloor heating needs no output correction when you move from a boiler to a heat pump. A radiator rated at Delta T 50 loses roughly 58 percent of its output at 45C flow. An underfloor circuit designed for 40C is already designed for 40C.
Low temperature is the whole point
Samsung quotes its EHS Mono R290 5 kW at SCOP up to 4.84 at 35C flow, and its high temperature range at SCOP 2.82 to 3.03 at 65C flow. Underfloor heating is the emitter that lets you live at the top of that range instead of the bottom, and that difference is roughly 40 percent of your heating electricity.
What underfloor heating gives you
- The lowest realistic design flow temperature, which is the single biggest lever on running cost.
- No emitter replacement work in the rooms it covers, because the circuit is already low temperature by design.
- Even heat distribution, which suits the steady continuous operation a heat pump prefers over short hot blasts.
- No wall space taken by radiators, which matters in kitchens, bathrooms and open plan spaces.
- A large thermal mass in a screed floor, which smooths out the system's response and can help with load shifting onto cheaper tariff windows.
The upstairs problem
Most retrofits do not end up with underfloor heating throughout. The common British outcome is underfloor downstairs, either from a previous project or as part of a new floor, and radiators upstairs. That is a mixed emitter system, and mixed emitter systems have one design decision at the centre of them.
One heat pump produces one flow temperature at a time. If the underfloor circuits are happy at 38C and the upstairs bedroom radiators need 50C to hold temperature, the system cannot serve both at once without help. In the room by room MCS design that has to be resolved, and there are only two honest ways to do it.
Design to the demanding circuit
Set the system flow temperature to whatever the worst performing emitter circuit needs, then throttle the underfloor circuits back with their own controls. Simple and cheap, but the whole system pays the efficiency penalty of the least capable room.
Upgrade the demanding emitters
Increase the upstairs radiators so they perform at the lower temperature the underfloor circuits want. More capital cost, better efficiency for the life of the system, and usually the better long term buy.
Split the temperatures with mixing
Use mixing controls or a blending valve so the underfloor circuits run cooler than the radiator circuits from the same source. More kit and more commissioning, but it lets each circuit sit where it belongs.
Ask which option your quote assumed
The three approaches produce very different bills and very different install prices, and a quote rarely spells out which one it chose. Ask for the design flow temperature and the reason for it. If the answer is a high number because of two upstairs bedrooms, that is a conversation worth having before the order goes in, not after.
Retrofitting underfloor heating is a bigger job than it sounds
Adding underfloor heating to an existing house is a floor project rather than a heating project. Traditional screed systems need floor build up height, which affects door heights, skirtings and thresholds. Low profile overlay systems reduce that but add cost per square metre. Neither is trivial in an occupied house.
Which is why the practical advice for most retrofits is straightforward. If you already have underfloor heating, it is an asset and the design should be built around exploiting it. If you do not, adding it purely to improve heat pump efficiency rarely pays back on its own, and the same money spent on radiator upgrades and insulation usually goes further.
Mixed system, part underfloor and part radiators? The free remote design call will tell you which of the three approaches your house actually needs.
Get your free quote →Controls matter more with underfloor
A screed floor is slow. It takes hours to warm and hours to cool, which is a feature rather than a fault, but it does mean the old habit of setting a timer to fire the heating for two hours in the evening does not translate. Underfloor and heat pumps both prefer steady low output over long periods, with setback rather than full off.
That slowness is also useful if you move onto a time of use electricity tariff. A floor charged with heat during a cheap window holds it, in a way that a radiator system simply cannot. The tariff structures that make this worth doing are set out in our tariffs guide and in heat pump running costs versus gas.
What to check before you commit
- The room by room heat loss calculation, which the MCS design standard requires to BS EN 12831 before any certified installation.
- The design flow temperature, stated as a single number for the system.
- How the underfloor and radiator circuits are being separated, if they are being separated at all.
- Which upstairs radiators are changing, with their output quoted at the design flow temperature.
- The controls strategy, because a heat pump on underfloor heating run like a boiler on a timer will disappoint.
The wider sizing picture is in heat pump sizing and what a proper survey covers, and the emitter arithmetic is in do you need new radiators. Every county page on this site carries a local calculator, so use our coverage map for the position where you live, alongside the main heat pumps guide.
Clearline gives independent guidance and does not install. Installations are carried out by an MCS-certified installation team for your area.
Underfloor heating questions, answered
Is underfloor heating necessary for a heat pump?
No. Radiators work perfectly well provided they are sized for the design flow temperature, and most retrofits use radiators. Underfloor heating is simply the emitter that allows the lowest flow temperature, and therefore the best efficiency, which is why it is described as the ideal pairing.
Can I have underfloor heating downstairs and radiators upstairs?
Yes, and it is the most common arrangement in British retrofits. The mixed system is handled in the room by room design, and the practical choice is between running everything at the temperature the radiators need, upgrading those radiators, or splitting the circuits with mixing controls.
What flow temperature does underfloor heating need?
Underfloor circuits typically run at 35 to 45C, which is the range where a heat pump performs best. That is the reason no output correction is needed when moving an underfloor system from a boiler to a heat pump.
Should I retrofit underfloor heating to improve efficiency?
Rarely as a standalone decision. Retrofitting underfloor heating is a floor construction project with implications for build up height, doors and thresholds. If a floor is coming up anyway it is worth considering, but the same budget spent on radiator upgrades and insulation usually does more for running cost.
Does underfloor heating take a long time to warm up?
Yes, particularly in screed. That suits a heat pump, which prefers steady continuous operation over short bursts, but it means controls should use setback rather than switching the system fully off. It also makes the floor a useful place to store heat bought during a cheap tariff window.
Will underfloor heating alone heat my house?
It depends entirely on the calculated heat loss of each room against the floor area available in it. Rooms with high heat loss and little floor area, such as a hallway with a large glazed door, sometimes need a radiator as well. That is precisely what the room by room calculation exists to find.
See the numbers next: Tesla Powerwall 3, installed, or what a battery costs.
Figures and specifications in this guide are sourced below and were checked on the date shown. Rates and product specifications change; we confirm the current picture on your free design call.
Sources
- APMI, radiator sizing and low temperature systems: underfloor heating at 35 to 45C flow, no output correction required, typical heat pump versus boiler flow temperatures apmi.uk
- Staffordshire Training, radiator sizing for heat pumps: handling of mixed emitter systems in the room by room design staffstraining.co.uk
- Squote, radiator sizing for heat pumps: approximately 42 percent of Delta T 50 rated output at 45C flow squote.app
- Samsung Climate Solutions, EHS Mono R290 and EHS Mono HT Quiet product pages: SCOP at 35C and 65C flow samsung-climatesolutions.com
- MCS, MIS 3005-D Heat Pump Design Standard 2025 V1.0: BS EN 12831 room by room heat loss calculation, design flow temperature and emitter sizing mcscertified.com
- Reonic, heat loss survey guide: survey outputs covering capacity, design flow temperature and emitter sizing per room reonic.com
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