Building Services Engineering 5th Edition Handbook

Energy economics 45
occupants, lighting, cooking and hot-water usage at an external air temperature of 15.5 ◦ C;
this is known as the base temperature t b . The value taken for the base temperature is
an estimate of the conditions under which there will be no net heat loss from a traditionally
constructed residence; thus no fuel will be consumed at this and higher outside
temperatures.
Calculation of the actual base temperature for a particular building may reveal another value;
consequently, care is needed in the application of degree day data, and correction factors
may be included for other than traditionally constructed dwellings: for example, highly insulated
or commercial structures and where internal heat gains from electrical equipment are high
(CIBSE, 1986).
The standard method of use is to assess the daily difference between the base temperature
and the mean value of the external air temperature during each 24-h period. A modified
calculation is made when the base temperature is below the external mean temperature, as
this would indicate a net heat gain to the building. Degree day data are not used for airconditioning
cooling-load calculations as they are not appropriate. Figure 2.1 shows a typical
fluctuation in external air temperature relative to base temperature. As the maximum and minimum
air temperatures are 10 ◦ C and 6 ◦ C respectively, the 24-h mean is 8 ◦ C. Therefore, as there
is a difference of 7.5 ◦ C per day, 7.5 degree days are added to the accumulated total for that
month.
The maximum possible number of degree days for a particular location and period of heating
system operation can be found as shown in the following example.
15.5
15 Base temperature t b
14
13
Outside air temperature (°C)
12
11
10
9
8
7
6
5
7.5°C
Outdoor air temperature
24 h mean
4
3
2
1
0
0 1200
Time (h)
2400
2.1 Method of calculating degree days during a 24-h period.