What does HDD mean in Unclassified?

Heating degree day

Heating degree day 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, 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. The base temperature is usually an indoor temperature which is adequate for human comfort. 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 or 65 °F —base temperatures that are approximately appropriate for a good proportion of buildings. Also, base temperature of 17 °C is used at least in Finland and Denmark, but also 19 °C 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.

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