Heating degree day (HDD) is a measurement designed to reflect the demand for energy needed to heat a building. It is derived from measurements of outside air temperature. The heating requirements for a given structure at a specific location are considered to be directly proportional to the number of HDD at that location. A similar measurement, cooling degree day (CDD), reflects the amount of energy used to cool a home or business.
Heating degree days are defined relative to a base temperature—the outside temperature above which a building needs no heating. The most appropriate base temperature for any particular building depends on the temperature that the building is heated to, and the nature of the building (including the heat-generating occupants and equipment within it). The base temperature is usually an indoor temperature which is adequate for human comfort (internal gains increase this temperature by about 1 to 2°C).
For calculations relating to any particular building, HDD should be selected with the most appropriate base temperature for that building. However, for historical reasons HDD are often made available with base temperatures of 60 °F (16 °C) or 65 °F (18 °C)—base temperatures that are approximately appropriate for a good proportion of buildings. Also, base temperature of 17 °C (63 °F) is used at least in Finland, but also 19 °C (66 °F) is used is some locations. Because the base temperature can be chosen and it can therefore vary, the base temperature used in calculating the heating degree days has to be presented.
There are a number of ways in which HDD can be calculated: the more detailed a record of temperature data, the more accurate the HDD that can be calculated. HDD are often calculated using simple approximation methods that use daily temperature readings instead of more detailed temperature records such as half-hourly readings. One popular approximation method is to take the average temperature on any given day, and subtract it from the base temperature. If the value is less than or equal to zero, that day has zero HDD. But if the value is positive, that number represents the number of HDD on that day. This method works satisfactorily if the outside air temperature does not exceed the base temperature. In climates where this is likely to occur from time to time, there are refinements to the simple calculation which allow some 'credit' for the period of the day when the air is warm enough for heating to be unnecessary. This more accurate algorithm enables results to be computed in temperate climates (maritime as well as continental) throughout the year (not just during a defined heating season) and on a weekly as well as monthly basis.
HDD can be added over periods of time to provide a rough estimate of seasonal heating requirements. In the course of a heating season, for example, the number of HDD for New York City is 5,050 whereas that for Barrow, Alaska is 19,990. Thus, one can say that, for a given home of similar structure and insulation, around four times the energy would be required to heat the home in Barrow than in New York. Likewise, a similar home in Los Angeles, California, whose heating degree days for the heating season is 2,020, would require around two fifths the energy required to heat the house in New York City.
However, this is a theoretical approach as the level of insulation of a building affects the demand for heating. For example temperatures often drop below the base temperature during night (daily low temperature in diurnal variation), but because of insulation heating is unnecessary. In the end of spring and in the beginning of fall or in the winter depending on the climate, sufficient insulation keeps the indoor temperature higher than the outdoor temperature with little or no heating. For example in southern California, during winter heating is not necessary in Los Angeles and San Diego if the insulation is sufficient to take into account the colder night temperatures. Thus, even if the heating degree days indicate a demand for heating sufficient insulation of a building can make heating unnecessary.
Read more about Heating Degree Day: Example of Use, Problems, Conversion
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