Radiators, Underfloor Heating and Emitters for a Heat Pump

A heat pump heats water to a lower temperature than a gas or oil boiler does, and that single fact drives everything on this page. An air-to-water heat pump typically sends water to radiators at 35 to 45°C, where a boiler typically runs at around 75°C¹. A radiator sized for boiler-temperature water will not deliver the same heat when fed water at 45°C, which is usual for heat pumps². The fix is more emitting surface: larger radiators, more panels, fan-assisted emitters, or underfloor heating, which turns the whole floor into one large radiator.

The good news for most homes is that this rarely means replacing every radiator in the house. Guidance on heat pump installations is consistent that some radiators may need changing, not all of them, and in many homes only the rooms with the highest heat loss need bigger emitters³. Underfloor heating is not required either: one household case study records that underfloor heating was not needed, though larger radiators and some new pipework were⁵.

Heat pump flow temperatures of 35 to 45°C compared with boiler flow of 65 to 75°C

The water a heat pump sends around the system is cooler than the water a boiler sends, and the gap is large. An air-to-water heat pump typically sends hot water to radiators at 35 to 45°C, against the roughly 75°C a fossil fuel central heating system runs at¹. Energy UK gives 55°C as an example in line with Building Regulations¹⁰, and the Scottish Government's domestic building services compliance guide states that supply water temperature to radiators on heat pump systems should be in the range 40°C to 55°C, using high-efficiency radiators with a high water volume⁶. For water underfloor heating, flow temperatures of 25 to 55°C are quoted for heat pumps and solar as the heat source, against 55 to 75°C for standard heat sources¹¹.

The reason for the lower temperature is efficiency. A heat pump extracts heat from outside air or the ground, and the cooler the water it has to produce, the less electricity it uses doing so. The Centre for Alternative Technology describes good practice as sizing the system to supply flow temperatures of only 40 to 45°C to radiators¹², and Nesta notes the marked difference between the design temperature for gas systems and the 45°C design temperature for heat pumps¹³. High temperature heat pumps exist, designed to run at usually 65 to 80°C, and they are the exception that can sometimes work with existing radiators unchanged¹⁴.

For the householder, the practical consequence is simple: the cooler the water, the bigger the emitting surface needed to deliver the same warmth. A radiator that was fine at 75°C may not be big enough at 45°C². This is a design constraint, not a fault, and it is the reason heat pump sizing and heat loss calculations come before any emitter decisions.

Why heat pumps call for larger radiators running at lower temperatures

Because the water arrives cooler, the radiator has to be bigger to give out the same heat. Energy Saving Trust explains that bigger means more surface area, which could mean a double or triple panel radiator³. The Renewable Energy Association says heat pumps benefit from the use of larger radiators or underfloor heating¹⁵, and the Heat Pump Association's consumer guidance is blunter: if you use radiators, they need to be large ones, since the heat produced will be low in temperature¹⁶.

The same point comes from the independent heat pump guide sector: heat pumps run at lower temperatures than gas or oil boilers, so they need longer run times and often larger radiators to achieve the same level of warmth¹⁷. Ground source systems follow the same physics. Which? notes that the heat produced by a ground source heat pump is at a lower temperature than other forms of heating, and that ground source heat pumps work best with underfloor heating, which requires lower flow temperatures than radiators¹⁸.

There is a second effect worth understanding. With underfloor heating, the flow temperature could be lower again compared with radiators, and this should result in the heat pump running much more efficiently¹⁹. So the emitter choice feeds back into running costs: the more surface area you give the system, the cooler the water can be, and the less electricity the heat pump draws for each unit of heat. That is the independence argument for bigger emitters in one sentence: they reduce the electricity the household must buy to stay warm.

Will you have to swap your radiators? Usually just some of them

The honest answer is that some usually do, but not necessarily all. OFTEC's guidance for off-grid homes states that when replacing a boiler with a heat pump, it is probable that the radiators will have to be changed for larger ones to produce an equivalent heat output, unless a high temperature output heat pump is installed⁴. Energy Saving Trust's heat pump fact check is more reassuring on scope: it is not mandatory to change radiators, but bigger radiators could hugely improve how well a heat pump can heat your home³.

A real installation bears this out. In one household case study, underfloor heating was not needed with the heat pump, but larger radiators and some new pipework were⁵. The pattern installers report is that the rooms with the highest heat loss, usually the largest room or the one with the most external wall and glazing, are the ones whose radiators fall short, while well-insulated small rooms often keep their existing radiators. This is why an emitter survey, room by room, forms part of a proper heat pump installation.

Where a home cannot be insulated further, larger radiators are one of the recognised compensations. Energy Saving Trust notes that if you cannot install insulation, you could increase your radiator size²⁰. The cost of any radiator changes sits alongside the heat pump itself; the Boiler Upgrade Scheme grant for an air-to-water heat pump in England and Wales is £7,500, with vouchers valid for 3 months²¹, and 116,832 such vouchers had been issued between May 2022 and July 2026²². Emitter work is part of what the household budgets for around the grant, not something the grant separately covers.

Types of emitter: double and triple panels, fan assisted and reflector options

The most common upgrade is a deeper radiator, not a longer one. Double and triple panel radiators do not take up extra wall space but add surface area, which is exactly what a lower flow temperature needs². Where wall space is genuinely tight, fan-assisted radiators are the other route: Renewables First reports that models such as the Jaga Strada or Dimplex SmartRad can provide around 3 times as much heat output as a standard radiator of the same size, which allows a low flow temperature, though these need a power supply at the radiator⁷.

Two cheaper measures sit alongside replacement. Radiator reflector panels help get warmth from radiators into your room and may make your heating more efficient²³, and they are most effective when fitted to radiators on uninsulated external walls²⁴. Neither is a substitute for adequate emitter size, but both reduce the heat lost into the wall behind a radiator, which matters more when every watt of output counts.

It is also worth knowing what avoids radiators altogether. An air-to-air heat pump, which heats with warm air rather than water, does not need underfloor heating or radiators²⁵. And underfloor heating removes radiators from the room entirely, which is one of its selling points for designers: with no need for wall mounted radiators, architects and designers have greater flexibility in room layouts, glazing design and window placement²⁶.

The case for underfloor heating with heat pumps, and how it operates

Underfloor heating essentially warms your home from the ground up by turning your floor into a large radiator²⁷. Because that radiator is the entire floor area, the water flowing through it can be cooler still than the water feeding even oversized wall radiators, and a cooler flow temperature means the heat pump runs more efficiently¹⁹.

The fit between the two technologies is repeatedly noted across independent and industry guidance. The Consumer Protection Association states that underfloor heating systems work particularly well with heat pumps because they provide consistent warmth at lower temperatures²⁸. NICEIC notes underfloor heating is often installed in combination with a heat pump, and that it works with all types of heating systems²⁷. OFTEC calls heat pumps ideal for underfloor heating systems⁴. The Centre for Sustainable Energy adds the operational reason: air or ground source heat pumps are particularly effective with wet underfloor heating because heat pumps are designed to be left on for longer periods and operate at lower temperatures²⁹.

That last point is the one to take seriously before choosing. Underfloor heating rewards a house that is heated steadily, and it responds slowly (see the final section). It suits a household that runs its heating for long stretches, which is also how a heat pump runs best. It suits less well a household that wants a room hot in half an hour. The RECC consumer guidance also flags cost: heat pumps work best with underfloor heating, so you need to factor this into the cost¹⁶.

Choosing between wet and electric underfloor systems for different jobs

There are two kinds of underfloor heating, and they suit different jobs. Water (wet) underfloor heating circulates warm water through pipes in the floor, and it is the kind that pairs with a heat pump. Electric underfloor heating uses heating cables or mats under the floor finish and runs on electricity directly.

The cost positions are clear from independent testing. Electric underfloor heating is generally slightly cheaper to install than water underfloor heating⁹, and water underfloor heating is around 25% more expensive to install per m2 than electric³⁰. But the running costs run the other way: electric underfloor heating is generally more expensive than radiators, because gas is cheaper than electric⁸, and it is more expensive than radiators both in installation and running costs, with the exception of heating a single room where radiators cannot be zoned⁹. Water underfloor heating is more expensive than radiators to install, but cheaper to run⁹.

For a heat pump household the choice is largely settled by the physics: a heat pump heats water, so it feeds a wet system. Electric underfloor heating is not connected to the heat pump at all, and running it is running direct electric heating, which carries the highest running cost per unit of heat of any option on this page. Its sensible use is a single small room, typically a bathroom, where the cost of running it matters less than the cost and disruption of extending a wet system.

What underfloor heating requires: floor coverings, insulation and coverage

The floor covering decides how well the heat gets through. Tiles and stone conduct heat efficiently, which is why they are the most commonly recommended flooring for underfloor heating⁸. Underfloor heating is also used with most types of thin carpet, engineered wood, vinyl and laminate⁸. It is not used with thick carpet, because it traps too much heat, or with solid wood, because it becomes warped by the temperature changes⁸. Carpet can be installed over underfloor heating, but it takes longer to heat the room as it reduces heat transfer¹⁹.

If underfloor heating is to be the only heat source in a room, two conditions apply. The system needs to cover between 80% and 90% of the room's total floor area, wall to wall, and insulation boards must be installed beneath the underfloor heating to reduce the amount of heat being lost downwards⁸. Both conditions are about the same thing: enough of the floor emitting, and the emitted heat going up into the room rather than down into the ground.

Installation standards also apply. The Welsh Government's Approved Document L consultation states that all installed equipment in underfloor heating systems should be commissioned in accordance with BS EN 1264-4³¹, the standard covering underfloor heating systems. For a householder this is a check to make at handover: ask which standard the system was commissioned to and to see the commissioning record.

Underfloor heating versus radiators: costs, speed of response and control

The running cost picture favours water underfloor heating over radiators. It will generally always be cheaper to heat a room with water underfloor heating than radiators⁸. For a 25m2 room run for three hours a day for roughly 6 months, a water underfloor heating system costs around £140 in total⁹. Electric underfloor heating is the outlier: more expensive than radiators to install and to run⁹.

Response time is the trade-off. Underfloor heating takes between an hour and 90 minutes to heat a room, with electric slightly quicker than water⁸. Wet systems generally take much longer to heat up and cool down than traditional radiators, partly due to the screed or concrete in which most systems are embedded²⁹. Radiators respond faster, which is why mixed systems, underfloor heating downstairs and radiators upstairs, are common: the slow, steady emitter suits rooms occupied all day, and the fast emitter suits bedrooms. A mixed system is entirely workable with a heat pump, provided each circuit is designed for the flow temperature it will receive.

Installation itself brings a pause. A heat pump takes a few days to fit, and the heating system is turned off for most of that time¹. For underfloor heating in screed, the switch-on schedule after installation is set by the installer, following the commissioning standard, rather than by the household's eagerness to feel the floor.

Day to day, radiators still need basic care on a heat pump system, exactly as on a boiler system. Bleeding means releasing trapped air so hot water can circulate properly and heat the radiator evenly, and it usually takes only a few minutes per radiator³². The signs a radiator needs bleeding are a radiator that is cold at the top, slow to warm up, or making gurgling noises³², or a top that is cool or much colder than the bottom while the heating is on³³. Bleed at least once a year, preferably every few months, ideally before winter³⁴, and always switch the central heating off and let it cool completely first, to avoid scalding³⁴. Bleeding more than once a month, or bleeding that has no effect at all, is the point to seek professional help³⁴.

For faults under a finished floor, the news is better than most householders fear. Specialist engineers can pinpoint faults under a finished floor to within a few millimetres, and for water underfloor heating, faults can be identified using a thermal camera or a moisture tester⁸. A leak does not mean lifting the whole floor.