Radiant cooling

Great alternative to air conditioning

Radiant systems have experienced rapid development thanks to their ability to provide high levels of heating comfort. Increasingly the market wants their same heating system to operate in summer as a cooler. The reasons are manifold. First of all, for reasons of comfort, since radiant transfer with a cold surface not too far off ambient temperature produces a sensation of personal comfort. Also, in comparison with air conditioning systems, there is minimal discomfort caused by mass movement of air or noise from fans. Finally, for energy considerations. The cold floor or cold ceiling uses water at a higher temperature than a traditional air system which means the device cooling the air (which can be an air-water heat pump) will be able to work more efficiently. A ceiling heating system is generally more efficient than a floor system due to the higher coefficient of convective heat transfer (see the section on Efficient Design). EN 1264 does not recommend adopting floor temperatures below 19°C, while this limit does not apply to ceilings. However, all radiant systems must be designed and adjusted based on the relative room humidity. The flow temperature of the water is therefore selected on the basis of the room‘s dew point, as described by the regulations. Since in summer it is common to have a high room humidity due to external conditions or people‘s activity, it is highly recommended to install an air dehumidification system if fitting a radiant cooling system, for reasons that will be explained later.

Performance cooling

A radiant cooling system works differently according to the surface on which it is located. A cold floor transfers heat primarily by radiation with other surfaces, because the capacity for convective transfer with the air is limited. Conversely, with a radiant ceiling system, the capacity for heat exchange with the air is much greater thanks to the higher coefficient of convective transfer of a cold surface which exchanges downwards. Add to this the fact that the presence of people, computers or electrical appliances in the room creates an upwards movement of hot air and the overall effect is an increase in output exchanged by the ceiling.

A radiant wall system works in a similar way both when heating and cooling due to the the convective coefficient being the same for both conditions. In general, a radiant floor system is sized in heating mode, as described by UNI EN 1264 and later on. According to that reported in Part 5 of UNI EN 1264, it is possible to evaluate power in cooling mode using a formula. Once familiar with the winter performance of a floor-installed radiant system, it is possible to estimate the performance of the same system if it were installed on a wall or ceiling (when heating and cooling) thanks again to Part 5 of UNI EN 1264. Alternatively, a radiant system‘s performance can be evaluated using numerical simulations, as set out in UNI EN 15377-1:2008. Since a radiant ceiling system‘s performance is also affected by the presence of people, computers and electrical equipment, it is advisable to evaluate the performance of a radiant ceiling in a laboratory simulating a room environment. The regulations UNI EN 14240:2005 require the performance of a radiant ceiling system to be checked once installed in a room where heat loss to the outside can be controlled.

The ceiling must be able to remove the heat flow from various load simulators (simulating people or electrical appliances) positioned on the floor. When a condition of equilibrium is reached, the flow rate, water temperature, air temperature and temperature of the globe probe are recorded. The result is expressed by the characteristic curve of the ceiling, through which the cooling power can be estimated for each temperature difference between the water and the room. Evaluating the performance in accordance with EN 14240 in a laboratory ensures accurate results, as laboratories necessarily have to limit measurement uncertainties in order to be able to issue the certificate.

Just a few laboratories, located in Germany, are able to perform this test. The WSPLab in Stuttgart, where Eurotherm has tested its products, is one such laboratory.

Characteristic curve of the Leonardo ceiling system tested at the WSPLab

The final component of the radiant system: dehumidification

The final component of the radiant system: dehumidification. To maximise the air-conditioning potential of a radiant system it‘s essential to install an air dehumidification system. Humidity inside the home can be significant during the summer months. This is in part due to unfavourable weather conditions and the presence of people whose indoor activity contributes to an increase in water vapour. Conditions such as these can lead to condensation forming on the cold surface, but this should be avoided so as not to damage the structure. Another aspect is the perceived comfort of occupants. In accordance with that advised by UNI EN 7730, to ensure a sensation of comfort and keep indoor air healthy, relative humidity should not exceed 60-65%. Several studies have shown that fitting a radiant system in a Mediterranean climate without installing an air dehumidification system will likely result in the relative humidity exceeding permitted limits causing personal discomfort. In conditions such as these it is preferable to close the passage of cold water to the cold floor or ceiling in order to prevent condensation. A room can be dehumidified using a special machine called an isothermal dehumidifier, which is capable of lowering the relative humidity of the air removed from the room. A refrigerant (or compressor) dehumidifier works by dehumidifying the air using an internal chiller and a cold water flow. Alternatively, in a non-compressor unit, the air flows over a system of air-to-air heat exchangers and a cold water coil and exits dehumidified and slightly cooled. The water can be further cooled using an additional water coil inside the machine described above. In this case it is called a dehumidifying air conditioner. A room can also be dehumidified using a fan convector. However, a fan convector‘s capacity for dehumidification is significantly less than a dehumidifier at the same flow of treated air. This means that the number of fan convectors in a home must be appropriate to the actual dehumidification requirements of the environment. In addition, it should be remembered that the main function of a fan convector is to heat. Therefore, the control system for any radiant system combined with such a machine must be carefully evaluated at the design stage. Finally, there is the option of installing a mechanical ventilation (MV) system with dehumidification, which injects clean, dehumidified air, drawn from the outside, into each room while at the same time removing pollutant-rich air, after heat recovery if it is energetically efficient to do so.