More specifically, the number of heating degrees in a day is defined as the difference between a reference value of 65°F (18°C) and the average outside temperature for that day. The value of 65°F is taken as a reference point because experience shows that if the outside temperature is this value then no heating or cooling is normally required. Occupants and equipment within a building usually add enough heat to bring the temperature up to a more comfortable level.
Suppose, for example, that the average temperature for a given day is 55°F. Since this value is ten degrees lower than the reference point of 65°F then one would say this is a ten degree-day. Obviously, the outside temperature is not always constant, so one needs a method to determine the average temperature. A simple way to do this is to compute the arithmetic mean of the high and low temperatures for the day. While not always correct, this is sufficiently accurate for most purposes and is done for practicality because these temperatures are always recorded by the weather bureau. Thus, in the previous example, if the high temperature were, say, 65°F and the low 45°F, then the average would still be 55°F for a ten degree-day.
65°F is known as the "base temperature" of the degree days. Degree days are commonly found with a base temperature of 65°F or 18ºC, and 15.5ºC is common in many countries such as the UK. However, heating and cooling degree days can actually be calculated using any base temperature - the most appropriate base temperature to use depends on the application (e.g. the temperature that the building is heated to and the amount of heat supplied by people and equipment).
Heating and cooling degree days can be added over periods of time to provide a rough estimate of seasonal heating and cooling requirements. In the course of a year, for example, the number of heating degree-days for New York City is around 5,000 whereas that for Barrow, Alaska is over 20,000. Thus, one can say that, for a given home of similar structure and insulation, four times the energy would be required to heat that home in Barrow than in New York.
The degree-day system has several problems. Heat requirements are not linear with temperature, and heavily insulated buildings have a lower "balance point". The amount of heating and cooling required depends on several factors besides outdoor temperature: How well insulated a particular building is, the amount of solar radiation reaching the interior of a house, the number of electrical appliances running (e.g. computers raise their surrounding temperature) the amount of wind outside, and individuals' opinions about what constitutes a comfortable indoor temperature. Another important factor is the amount of relative humidity indoors; this is important in determining how comfortable an individual will be.
To convert ºC HDD to ºF HDD: ºF HDD = (9/5) x (ºC HDD)
Note that, because degree days are relative to a base temperature (as opposed to being relative to zero), it is incorrect to add or subtract 32 when converting degree days from Celsius to Fahrenheit or vice versa.
Sources of free degree-day data:
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