Heating degree days are measured when the outside temperature average over 24 hours becomes low enough to require heating. Measurements are based on a threshold for heating which is fixed at 18 degrees Celsius. If for instance the outside temperature is 10 degrees Celsius, 18–10=8 heating degree days are counted. When it is 4 degrees Celsius below zero, the number of heating degree days is 18–(–4)=22 HDD. When the outside temperature is 20 degrees Celsius, no heating is required, and no heating degree days are recorded. The number of degree days in the text refers to the annual total number of heating degree days.
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