Degree days has been the standard measure for estimating gas heating consumption since the 1930's. A degree day is defined as the deficiency of the daily mean temperature below 65°F. This definition does not take into consideration the distribution of hourly temperatures during the twenty four hour period. The 65°F base was chosen since it was considered that heating was required when the outside temperature cooled to this level. When the degree day concept was first developed, inside house temperatures were maintained in the low-to-mid seventy range with little or no thermostat set-back at night. Also, few homes contained attic insulation or storm windows and doors. Rapid increases in the cost of home heating fuels has caused home owners to dramatically change their heating requirements and conserve energy. Their hourly, daily and seasonal load profiles have changed accordingly. The degree hour concept was developed because it accurately tracks the differential between inside and outside temperatures which in turn is directly proportional to building heat loss. A degree hour is defined as the deficiency of the hourly temperature below any specific base temperature. Today, I would like to explain the methodology used to develop degree hour tables, explore the practical application of degree hours to determine home heating profiles and give the results of a detailed test on my home heating system last winter. In order to explain the development of temperature data for degree hour tables, let us consider the following mix of hourly temperatures: 64°F, 63°F, 63°F, 62°F and 60°F. These five hourly temperatures represent 1, 2, 2, 3 and 5 degree hours respectively and total 13 degree hours at a 65°F base. The data is tabulated on the next page in a cumulative-distributive manner or what I call the duration method. The occurance of each temperature is tabulated by diminishing value of temperature. Then the occurances are added cumulatively starting at the coldest temperature in order to determine the number of hours colder than various temperatures. They are then readded cumulatively to determine the degree hours colder than various temperatures. This tabulation shows that there were 13 degree hours when the temperature was 64°F or colder and they occured on 5 hours. This checks with the original data and also gives similar information for temperatures colder than 64°F. The base temperature is always one degree warmer than the tabulated temperature. For example, if the degree hour base temperature is 63°F, then there are 4 degree hours occuring on 2 hours. Please turn to the back of this paper where the results of a ten year study of Boston, Massachusetts degree hours are tabulated. A temperature duration curve is shown on the blue cover page. The triangular shaped area (DH) above the hourly temperature (T) on the vertical axis represents the average degree hours colder than each temperature and (H) on the horizontal axis represents the average number of hours per year on which they will occur.
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Tenney, Charles M. "A Logical Extension Of Degree Days." Paper presented at the PSIG Annual Meeting, Savannah, Georgia, October 1980.
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