A heat pump and a radiant system are often considered a particularly effective combination. However, the reason is not simply that both are efficient technologies. The real connection lies in the temperature at which the system needs to operate to provide maximum indoor comfort and performance.
A heat pump transfers energy from an external source to the building and, to do so, must bring the system water to the temperature required by the heat emitters. The lower the required water temperature, the more favourable the heat pump’s operating conditions generally are.
This is precisely where the radiant system comes into play. Thanks to the large surface through which it exchanges energy with the room, it can distribute heat using water at lower temperatures than emitters designed for high-temperature operation.
When assessing a heat pump’s efficiency, therefore, looking at the unit alone is not enough: it is essential to understand which system it will be installed in and at what temperatures it will need to operate.
At a glance
Heat pumps and radiant systems work well together because a radiant system can heat rooms using water at a relatively low temperature. This reduces the temperature lift the heat pump must achieve and generally creates more favourable conditions for efficiency. However, the result depends on the overall design: the building’s demand, sizing, distribution, flow rates, controls and heat emission system must all be considered together.
Why does flow temperature affect a heat pump’s efficiency?
Flow temperature is one of the parameters with the greatest impact on heat pump operation: with all other conditions being equal, producing cooler water requires the unit to achieve a smaller temperature lift.
In simple terms, a heat pump extracts energy from a source, such as outdoor air, and transfers it to the system water. The greater the difference between the source temperature and the required temperature on the system side, the more work the heat pump’s refrigeration cycle must perform. Technical documentation on heat pump design shows that increasing the flow temperature tends to reduce the unit’s seasonal performance.
This also helps explain the meaning of COP, Coefficient of Performance. COP expresses the ratio between the thermal energy delivered and the electrical energy consumed under specified test conditions. In EMMETI heat pump technical data sheets, the COP is stated alongside the nominal conditions under which it was measured. For example, heating performance for Mirai SMI 4.0 and Mirai SMI Leaf is declared with outdoor air at 7 (6) °C and water at 30/35 °C. This is because COP is not a fixed value independent of context: it changes as the heat pump’s operating conditions change.
The same applies to SCOP on a seasonal basis. Comparing two heat pumps using only a nominal value, without knowing the conditions under which it was measured, can therefore provide an incomplete picture.
EU Delegated Regulation 811/2013 helps clarify the distinction: it defines a low-temperature application as one in which the heat pump delivers its declared capacity with outlet water at 35 °C, whereas the reference for a medium-temperature application is 55 °C.
This does not mean a system must always operate at 35 °C. It does mean that the temperature at which the system can meet the building’s demand is a key variable in its efficiency.
Why can a radiant system operate at lower temperatures?
A radiant system can use moderate water temperatures because it distributes energy through a very large heat exchange surface.
A traditional radiator concentrates heat exchange within a relatively small surface. Transferring a given amount of heat into the room may therefore require higher water temperatures, especially when the emitter was originally sized for high-temperature heat generators.
A radiant system works differently. The floor, wall or ceiling becomes a large active surface, distributing energy throughout the room. The required temperature difference between the radiant surface and the room can therefore be smaller.
Our EMMETI Floor systems are based on this principle and are particularly suitable for use with low-temperature heat generators, including inverter heat pumps.
Discover EMMETI underfloor heating and cooling systems
However, it is important to avoid a common oversimplification: underfloor heating alone does not “make a heat pump consume less”. The advantage comes from a correctly sized system being able to meet the building’s heating load while maintaining a lower water temperature.
It is this condition that benefits heat pump operation.
What changes when a heat pump operates with a radiant system?
As explained in the previous section, when the heat emission system can deliver the required heat using water at a lower temperature, the heat pump can operate with a smaller temperature lift and, generally, greater efficiency.
A real example: the low-energy home in Gualdo Tadino
A practical application of this principle can be found in our recent Emmeti project in Gualdo Tadino, where an air-to-water heat pump, underfloor heating, photovoltaics, solar thermal and FEBOS controls were integrated into a new detached home.
In this project, the radiant system distributes energy throughout the rooms, allowing the heat pump to operate without reaching the temperatures typical of traditional high-temperature emitters. FEBOS weather compensation also adjusts the temperature of the water leaving the heat generator according to outdoor conditions and actual demand.
The final result is a building certified in energy class A3, with an EPgl,nren of 22.880 kWh/m² per year and 70.2% renewable energy coverage across space heating, domestic hot water and summer cooling.
These figures must be interpreted correctly: they describe the performance of the entire building and its technical systems and cannot be attributed solely to the heat pump or the radiant system.
Find out more about the Gualdo Tadino project
This is the key point: unit efficiency and system efficiency are not the same thing.
Is 35 °C really the ideal temperature for a radiant system?
35 °C is a useful reference when discussing low-temperature systems, but it should not be interpreted as a universal operating temperature for every radiant system.
As mentioned above, European legislation uses 35 °C as the reference outlet temperature for classifying low-temperature heat pump applications. It is also a condition used in the nominal performance data of many units.
In practice, however, the required temperature depends on the specific characteristics of the building and the system. Factors include heat loss through the building envelope, outdoor weather conditions, available radiant surface, pipe spacing, screed type, floor covering, flow rates and required room temperature.
In a high-performance building, a lower temperature may be sufficient under certain conditions. Another building may require higher values.
The design must therefore identify the lowest temperature compatible with the building’s actual demand and the required comfort, rather than aim for a predetermined value.
Weather compensation also plays an important role here. A building’s demand does not remain constant throughout the season: as outdoor temperature rises, the energy needed to maintain indoor comfort usually decreases.
Correct controls can therefore adapt water temperature to actual operating conditions, avoiding unnecessarily high flow temperatures.
Is installing underfloor heating enough to achieve an efficient system?
The combination of a heat pump and a radiant system creates technically favourable conditions, but actual efficiency depends on the design of the entire system. An oversized or undersized heat pump, incorrectly balanced circuits, inadequate flow rates, ineffective controls or a building envelope with high heat losses can compromise performance even with a radiant system.
The EU energy labelling regulation itself points out that the actual efficiency of a system installed in a building may differ from that stated in the product documentation, because factors such as distribution losses and sizing in relation to the building’s characteristics come into play.
The correct approach is therefore not to select “a good heat pump” and “good underfloor heating” separately. It is necessary to assess how heat generation, distribution and controls will work together under real operating conditions.
The Caltana project: from class G to seven A4 nZEB apartments
A particularly significant example is the refurbishment we carried out in Caltana di Santa Maria di Sala, in the province of Venice.
The building, originally comprising five commercial units in energy class G, was converted into seven small apartments. The previous configuration, based on radiator heating and air conditioning, was replaced by seven independent systems in which heat generation, distribution, dehumidification, domestic hot water and controls were designed in a coordinated way.
Each apartment has a Mirai EH0618DC heat pump connected to the underfloor heating system, alongside an Eco Hydro Kit, ESD 25 IS EVO dehumidifiers and FEBOS Basic controls.
After the refurbishment, all seven units achieved A4 nZEB classification, with annual energy figures approximately between 5 and 20 kWh/m² per year.
Here too, it would be incorrect to attribute the result solely to the switch from radiators to underfloor heating. The transformation stems from the overall refurbishment of the building and the development of a new technical system.
This is what makes the case interesting: it shows that the right question is not “which product consumes less?”, but how to design the whole system so that every component operates under the most suitable conditions.
Heat pump with a radiant system or radiators: what changes?
Although a radiant system is particularly well suited to low-temperature operation, this does not mean a heat pump can only be used with underfloor heating.
Radiators can also be compatible, provided available output, emitter sizing, building heat losses and the required flow temperature are assessed.
We have explored this topic in our article on frequently asked questions about combining heat pumps and radiators.
Heat pumps and radiators: five frequently asked questions
The fundamental difference remains the heat emission surface. Underfloor heating naturally provides a very large surface, whereas a radiator must transfer heat through a more concentrated area.
If a radiator was designed to operate with very hot water, simply reducing the flow temperature may reduce its heat output too much. Each project must therefore be assessed in its own context.
Developments in heat pump technology also broaden the range of applications. Mirai SMI Leaf, for example, uses R290 refrigerant and can reach flow temperatures of up to 75 °C, enabling its use with radiators and in various refurbishment projects.
This broadens the unit’s range of applications, but does not change the general thermodynamic principle: when a system can meet demand at lower temperatures, operating conditions are usually more favourable for efficiency.
What if a radiant system is not the most suitable choice?
Another of our projects helps explain why no single type of heat emitter is universally appropriate.
In the refurbishment of Borgo Monchiero Heritage by Atmosphere, a former monastery converted into a hotel in the Langhe, requirements were very different from those of a home: rapid changes in occupancy, a need for extensive modulation, acoustic comfort and integration of emitters into bedroom furnishings.
The project therefore included eight 17 kW Mirai heat pumps installed in parallel, three etiKa Power 115 boilers in cascade and concealed Silence THIN ETI fan coils in the rooms.
Discover the Borgo Monchiero project
The design principle remains the same: first consider the building, its use and its demand, then choose the most appropriate technology.
How do Mirai SMI, Mirai SMI Leaf and EMMETI Floor work together?
Combining our Mirai heat pumps with EMMETI Floor systems turns the principle explained above into a practical system: producing thermal energy and distributing it throughout rooms while ensuring heat generation and emitters operate under compatible conditions.
Mirai SMI 4.0 heat pumps are inverter air-to-water units that can be connected to radiant emitters, fan coils and low-temperature radiators. Inverter technology enables delivered output to adapt to system demand, while integrated controls manage various components of the thermal system.
Find out more about Mirai SMI 4.0
The new Mirai SMI Leaf, with R290 refrigerant, further expands application flexibility, maintaining high performance in low-temperature applications while also allowing higher flow temperatures when required by the building or the emitters.
Find out more about Mirai SMI Leaf
EMMETI Floor systems provide energy distribution within the rooms. Their large radiant surface enables operation at moderate temperatures and distributes heat evenly.
The relationship between these elements is more important than the individual product: the heat pump generates energy, the radiant system transfers it to rooms at a low temperature, and the controls adapt operation to the building’s actual conditions.
It is through this relationship that technology becomes a system.
The EMMETI approach: designing efficiency as a system
For EMMETI, a system’s efficiency does not depend on a single component, but on how heat generation, distribution, emission and controls are designed to work together.
This principle is the starting point for our approach: first come the building’s characteristics, its energy demand, how its spaces are used and the required level of comfort. Only then is the most suitable combination of technologies defined.
For a heat pump combined with a radiant system, this means designing the system so that the heat generator can operate at moderate flow temperatures whenever conditions allow. It also means checking sizing, flow rates, distribution, weather compensation and the behaviour of the building as a whole.
The same logic is reflected in the Gualdo Tadino and Caltana projects: the result does not come simply from having a heat pump or underfloor heating, but from ensuring all system elements interact correctly.
This is how EMMETI designs its systems: as a coordinated whole in which every component contributes to the building’s overall operation, rather than merely a collection of products.
The ultimate goal is always the same: creating the conditions for the system to deliver the required comfort while using energy as efficiently as possible.
FAQ
Do heat pumps and underfloor heating reduce energy consumption?
The combination can improve efficiency because underfloor heating can meet heating demand using water at a relatively low temperature. However, actual consumption also depends on the climate, building envelope, sizing, controls, required temperature and how the building is used.
At what temperature does a heat pump operate with underfloor heating?
There is no single temperature suitable for every system. 35 °C is a reference used for low-temperature applications and various nominal test conditions, but the actual temperature must be determined according to the building’s demand and the characteristics of the radiant system.
Can a heat pump also work with radiators?
Yes. Compatibility must be assessed by considering required output, radiator sizing and the necessary water temperature. Radiators correctly sized for moderate temperatures can be used with a heat pump; in other cases, the emitters may need to be modified or units capable of reaching higher flow temperatures may be required.