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Surface heating and cooling system vs. fan coil units: Lower energy consumption, less CO2 emissions 

The indoor climate in buildings is closely related to the well-being, work performance and even mood of users. A healthy climate is especially important today, where people spend 75 percent of their time indoors.

In the warm summer months, conventional air conditioning systems, such as fan coil units or split units, are the most common ways to cool rooms. These systems are able to meet the thermal needs of the premises for cooling, but they can lead to disruptive air currents, dust movements and a very low relative humidity of less than 40% —not to mention high energy consumption. Due to climate change, numerous countries have issued regulations to replace old air conditioning systems with energy-efficient models. Air conditioning with radiation offers a solution here: surface heating and cooling systems that use water as a heat carrier.

How radiation works

By radiation, surfaces – such as floors, walls or ceilings – are heated or cooled in order to exchange energy with the room. Thermal energy is exchanged in cooling mode. Objects, people, devices and light sources emit heat energy which is absorbed by the cold surface. The opposite is the case in heating mode. Energy is transferred from the heated surface by radiation and absorbed by objects and people.

Water has a 4 times higher specific heat capacity than air and, due to the much higher density, requires less transport energy. Water is therefore considered a better heat transfer medium than air. Due to the very large active area of the surface heating and cooling system, an even radiation exchange between the room and the room enclosing surfaces is generated, whereby the temperatures during cooling are perceived lower and during heating higher than the air temperature indicates (-> operative temperature). In contrast to conventional air conditioning systems surface heating and cooling systems work in summer at higher temperatures and in winter at lower temperatures. This leads to a reduction in both energy consumption and CO2 emissions. And, in a radiant system, the same circuits can be used for heating or cooling.

Combination with a ventilation system

The room climate is determined by the following factors: temperature, humidity and air speed. If a surface heating and cooling system is responsible for covering the sensible loads of the premises, a ventilation system is often essential to compensate for the latent exposure to moisture in the room and from people, and to ensure adequate air quality.

There is still scepticism in the area of radiant cooling. However, even in humid climates a well-designed system will handle the load without difficulty. Constant control of the humidity in a surface cooling system ensures that the surface temperature is always above the dew point, which prevents condensation on walls, ceilings and floors. The combination of a surface heating and cooling system with a controlled ventilation system significantly reduces the required air volume flow that circulates in the air-conditioned room, leading to a reduction in air movement. Unpleasant drafts, noise and dust are decreased and the comfort for occupants is increased. The lower air volume flow allows for a smaller dimensioning of the air ducts. The height of the suspended ceilings can be reduced. This has positive effects on the planning of the building body.

Ideally suited for renewable energies

One of the great advantages of a radiant heating and cooling system is its energy efficiency and its ease of use with renewable energies. Energy consumption and CO2 emissions decrease thanks to the reduced flow temperature when heating (35° C) and the high temperature when cooling (> 15° C). This enables the use of a multi-purpose heat pump which can generate heat and cold at the same time. The installation of two different machines (boiler and cooling system) can be avoided and thus costs during installation saved.


For our comparison, two water-bearing systems are used: fan coil units and a surface heating and cooling system. Small changes in operating temperatures of the surface heating and cooling system can save up to 31% of annual energy use and 30.2% of CO2 emissions. The building in this example must be heated and cooled. It has a heating requirement of 210 kWt and a cooling requirement of 230 kWt (sensible heating load in both cases).

The operating temperatures for both systems are:

Before each calculation, the operating parameters of the heat pump must be selected (heat pump model WSHN-XSC3 from Clivet). The heat pump must be able to cover the worst case, in this case in cooling mode 230 kWt.

The most important data are summarized in the following table:


As can be seen from the tables, changing the operating temperature not only increases the performance of the heat pump, but the installation can also be dimensioned with a smaller model (model 70.4 instead of model 75.4).


The annual power consumption of the heat pump is:

Total energy (kWh) = electrical energy of the compressor x operating hours

Fan coil units heating = 60.27 kWe x 4,000 h = 241,080 kWh

Fan coil units cooling = 45.21 kWe x 2,000 h = 90,420 kWh

Surface heating and cooling system heating = 39.7 kWe x 4,000 h = 158,800 kWh

Surface heating and cooling system cooling = 36.16 kWe x 2,000 h = 72,320 kWh

Total fan coil energy = 241,080 + 90,420 = 331,500 kWh / year

Total energy surface heating and cooling system = 158,800 + 72,320 = 231,120 kWh / year


Annual energy savings = 30,2%



With these results we can calculate the CO2 reduction:

CO2 emissions (fan coil units) = 331,500 kWh* 0.75 = 248,625 kg CO2

CO2 emissions (surface heating and cooling system) = 231,120 kWh * 0.75 = 173,340 kg CO2


Annual CO2 reduction = 30.2%




A surface heating and cooling system not only leads to a reduction in the CO2 balance, but also provides economic benefits. For example, a heat pump of smaller size can be installed. In addition, a surface heating and cooling system offers many other advantages:

  • more comfort as unpleasant drafts, noise and dust are avoided
  • energy saving through more efficient operating temperatures
  • reduction of CO2 emissions due to reduced energy use
  • lower operating costs
  • freedom of design of the interior, since the installation takes place under the surface
  • higher durability
  • nearly maintenance free



In this example, the surface heating and cooling system aquatherm black system was compared with a modern fan coil system. If other air conditioning systems are used or if the building is being renovated, the energy saving of the surface heating and cooling system increases considerably since the conventional air conditioning units work in heating mode at higher flow temperatures and in cooling mode at lower temperatures.

Contact:  aquatherm GmbH | Biggen 5 |  57439 Attendorn | Germany | Tel: +49 2722 950 0 |