A Heating Degree Day (HDD): is Heating degree days are used to estimate how cold the climate is and how much energy may be needed to keep buildings warm. HDDs are calculated by subtracting the mean daily temperature from a balance temperature, and summing up only positive values over an entire year. The balance temperature used can vary, but is usually set at 65_F (18_C), 68_F (20_C), or 70_F (21_F).

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