Building Services Engineering 5th Edition Handbook

46 Energy economics
EXAMPLE 2.8
A house is continuously occupied during a 30-week heating season. The design external
air temperature is −1.0 ◦ C. Find the maximum possible number of degree days.
days = 30 weeks × 7 days
1 week
= 210 days
maximum temperature difference = [15.5 − (−1.0)] ◦ C
= 16.5 ◦ C
maximum degree days = 210 days × 16.5 ◦ C
= 3465 degree days
The load factor L is the ratio of actual to maximum degree days and is used to find the average
rate of heat loss from a building over the heating season:
L =
degree days for locality
maximum possible degree days
EXAMPLE 2.9
Find the average rate of boiler power used during the heating season when there were
2460 degree days, and steady-state heat losses were calculated to be 24.5 kW at an outside
air temperature of −1 ◦ C.
L = 2460
3465
= 0.71
seasonal average rate of heat loss = design heat loss × load factor
= 24.5 kW × 0.71
= 17.4 kW
The boiler will have an average heat output of 17.4 kW over the heating season, that is, in
addition to the hot-water service requirement and heat losses from pipework.
Degree days can be used to monitor fuel consumption and check that it is not being used
wastefully. Incorrectly serviced fuel-burning appliances would show an increasing use of energy
per degree day rather than a constant rate. Deterioration of the performance of an automatic
control system or lack of proper manual regulation of ventilation openings would also result in
a departure from expected ratios. A graph of energy consumption against degree days should