Radiance is a process by means of which the thermal energy is transferred from one body to another one, having different temperature. Thermal radiation is transmitted from a warmer body to a colder one at light speed without heating the air. It is a common experience, in fact, that on sunny days our body can feel a thermal comfort even with extremely low air temperatures. On the contrary, when moving to shade, the sensation of comfort is immediately lost, even though, in both cases, the air temperature is the same. It is clear that a radiant heating system will grant the comfort conditions, keeping the air temperature lower. The temperature generating the comfort condition is not only the air temperature, but the so called operating temperature, arising from the combined effect of air temperature (ta) and average radiant temperature (tmr) of all surfaces in the room, namely ceiling, floor, vertical walls and obviously that of radiant panels.
The temperature of a body and the composition of its surface determine the value of heat transmission by radiance. In order to evaluate the thermal balance of a building, the average radiant temperature of internal surfaces must be calculated. This temperature depends on thermal conductivity of surfaces themselves, on external temperature, on internal air temperature and on the radiant thermal exchange with other surfaces, including panels. In the end, the warm surface of the radiant panel compensates the cold surface of walls and carries out the air heating. The 70% of thermal energy emitted by a good ceiling mounted radiant panel is transmitted by radiance and the remaining part by convection. When designing, the quantity of panels will be determined, which is needed to grant the required operating temperature. To obtain this result, a good knowledge of the general conditions and of all the applicable variables is necessary.