Building Services Engineering 5th Edition Handbook

Heat loss calculations 73
if this is done, the quality of the materials remains largely unknown and assumptions about the
water content and the integrity of each layer would have to be made. The constructional detail
is unknown. There may be air spaces, vapour barriers and layers of thermal insulation in place,
but these are hidden from view.
Figure 3.1 represents a cross-section through the unknown structure. It could be an external
wall, internal wall, roof, floor, glazing or door. All that can be realistically assessed are the
temperatures on either side, at nodes 1 and 4, and on the surfaces at nodes 2 and 3. A shielded
surface-contact thermocouple probe can be used to measure each surface temperature. An
exposed thermocouple junction or a sling psychrometer can be used to find the air temperatures.
The values for the inside and outside surface film resistances, R si and R so m 2 K/W, are assumed
to be their normal, tabulated values for the appropriate applications. The heat transfer equations
(Chapter 10) that describe the heat flow through the structure, Q W, are as follows.
For the whole structure:
Q = U
For the interior film:
Q = 1
R si
W
m 2 K × A m2 (t 1 − t 4 ) K
W
m 2 K × A m2 (t 1 − t 2 ) K
For the unknown structure:
Q = 1 R
W
m 2 K × A m2 (t 2 − t 3 ) K
Unknown structure
t 1
1
t 2
2
Detail of gradient
between nodes 2 and
3 is unknown
R so
t °C
R si
t
3
3
t ae °C
t 4
4
Any thickness
l m
3.1 Temperature gradient through a structure.