Efficient design

Exchange coefficient

Inside the room, overall heat transfer is given by combining the heat transfer by convection and the heat transfer by radiation which affects all the surfaces and anyone in the room.

Convective exchange affects the air inside a room and can be determined by a difference between the temperature of a surface and the air. In a room that has an active radiant surface, the main mechanism of convective exchange is by that surface, although other convective phenomena can exist next to a cold window or above people or a computer usually at a temperature higher than the air. If the room has a primary air inlet (natural ventilation), the coefficients of convective exchange could be locally high if the inlet speed of the air is higher. However, when considering a radiant system‘s efficiency at heating or cooling, only the convective exchange between surfaces with average temperature θsm and an ambient reference temperature is considered.

Surface

Warm
floor

Cold floor

Warm ceiling

Cold ceiling

Wall

hc [W/(m2K)]

5.3

1.0 1.0 5.3 2.5


Heat transfer by radiation affects all surfaces of a room and is determined by the thermal radiation emitted by a surface. The magnitude of this heat flow is influenced by several factors: the emissivity of the surfaces (a property of the material), the temperature and area of the surfaces, and the view factor between the two surfaces. The view factor is a purely geometrical factor which depends solely on the mutual position of the two surfaces. When considering a radiant system‘s efficiency at heating or cooling, only the thermal exchange between the active surface and the average temperature of the other surfaces is considered, ignoring the presence of other bodies (such as people) in the room.

Adding up the contributions of transfer by radiation and convection gives a single coefficient, called the threshold coefficient of exchange (hl). According to UNI EN 1264-5 this is:

Surface

Warm
floor

Cold floor

Warm ceiling

Cold ceiling

Wall

hl [W/(m2K)]

10.8

6.5 6.5 10.8 8