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degree-day,a unit of measure used to estimate the fuel and power requirements in heating and cooling a building; it is equal to a difference of 1 degree between the outdoor daily average temperature (the mean of the maximum and minimum daily dry-bulb temperatures) and a reference temperature. Degree-days are an indicator of how far the average temperature departs from a human comfort level called the base. In the United States the base is generally 65°F; (18°C;), although in very warm or cold locations an alternative may be used, while in Great Britain the base is 15.5°C; (60°F;).
Each degree of outside average temperature below the base is one heating degree-day (HDD), and each degree above the base is one cooling degree-day (CDD). To calculate the number of heating degree-days in a month, for example, the outdoor average temperature for each day is subtracted from the base, and the results for each day are added (with negative remainders being treated as 0).
Heating degree-days are a measure of the severity and duration of cold weather; the colder the weather over a given period the higher the cumulative heating degree-day value. Similarly, the warmer the weather over a given period, the higher the cumulative cooling degree-day value. The ability to compare one week, month, or other period with another using degree-days permits the analysis of seasonal patterns of energy consumption, enables the setting and tracking fuel and power budgets, and can be used to verify that projected economies are achieved by energy-saving measures.
The growing degree-day (GDD), an extension of the degree-day concept, is defined as a day on which the mean daily temperature is one degree above the minimum temperature required for the growth of a particular crop. The GDD is used as a guide to planting times and for determining the approximate dates when a crop will be ready for harvesting.
A unit used in estimating energy requirements for building heating and, to a lesser extent, for building cooling. It is applied to all fuels, district heating, and electric heating. Origin of the degree-day was based on studies of residential gas heating systems. These studies indicated that there existed a straight-line relation between gas used and the extent to which the daily mean outside temperature fell below 65°F (18°C).
The number of degree-days to be recorded on any given day is obtained by averaging the daily maximum and minimum out-side temperatures to obtain the daily mean temperature. The daily mean so obtained is subtracted from 65°F and tabulated. Monthly and seasonal totals of degree-days obtained in this way are available from local weather bureaus.
A frequent use of degree-days for a specific building is to determine before fuel storage tanks run dry when fuel oil deliveries should be made. Number of Btu which the heating plant must furnish to a building in a given period of time is where “Btu required” is the heat supplied by the heating system to maintain the desired inside temperature. “Heat rate of building” is the hourly building heat loss divided by the difference between inside and outside design temperatures. When the estimating procedure is applied to buildings with high levels of internal heat gains, as in a well-lighted office building, then degree-day data on other than a 65°F basis are required. See Air conditioning