Degree Day Method

Frequently, in energy calculations, simpler methods are required. One such simple method, which can give comparatively accurate results, is the degree day method. This method is used to predict the seasonal energy consumption. Each degree that the average outdoor air temperature falls below a balance temperature, Tb, of 18.3°C (65°F) represents a degree day. The number of degree days in a day is obtained approximately by the difference of Tb and the average outdoor air temperature, Tav, defined as (Tmax + Tmin)/2. Therefore, if the average outdoor air temperature of a day is 15.3°C, the number of heating degree days (DD), for the day is 3. The number of heating degree days over a month is obtained by the sum of the daily values (only positive values are considered) from

Similarly, cooling degree days are obtained from

Degree days for both heating (DD), and cooling (DD)c are published by the meteorological services of many countries. Appendix 7 lists the values of both heating and cooling degree days for a number of countries. Using the degree days concept, the following equation can be used to determine the monthly or seasonal heating load or demand (Dh):

where UA represents the heat loss characteristic of the building, given by


Qh = design rate or sensible heat loss (kW).

Ti - To = design indoor-outdoor temperature difference (°C).

Substituting Eq. (6.25) into (6.24) and multiplying by 3600 X 24 = 86,400 to convert days into seconds, the following equation can be obtained for the monthly or seasonal heating load or demand in kJ:

For cooling, the balance temperature is usually 24.6°C. Similar to the preceding, the monthly or seasonal cooling load or demand in kJ is given by

Example 6.1

A building has a peak heating load equal to 15.6 kW and a peak cooling load of 18.3 kW. Estimate the seasonal heating and cooling requirements if the heating degree days are 1020°C-days, the cooling degree days are 870°C-days, the winter indoor temperature is 21°C, and the summer indoor temperature is 26°C. The design outdoor temperature for winter is 7°C and for summer is 36°C.


Using Eq. (6.26), the heating requirement is

Similarly, for the cooling requirement, Eq. (6.27) is used:

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