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Heating degree days are calculated over a period of time (typically a year) by adding up the differences between each day's mean daily temperature and the "balance point" temperature of 18 ° C (or 65 ° F ), above which the building is assumed not to need any heating (the actual indoor temperature will be higher due to Insulation retaining heat from the occupants). Thus three successive winter days with average temperatures of 4 °C, −2 °C and −4 °C totals to 56 HDD. Conversely, three summer days of 26 °C, 28 °C, and 30 °C totals to 30 CDD. For differences between day and night temperatures, the average is taken; so a day maximum of 10 °C with a night minumum of 2 °C averages to 6 °, or 12 HDD. The system has several problems. Heat requirement is not linear with temperature (Valor et al., 2001), and heavily insulated buildings have a lower "balance point": some will need heating below 18 °C, but others won't need any heating until the temperature is much lower (see Superinsulation ). Solar gain (see Passive Solar ) reduces the need for heating on sunny days (but not cloudy days), and Wind increases it (by an amount that depends on how tightly the building is constructed). People also differ in their opinions about what constitutes a comfortable indoor temperature. Note that in the United States , HDD figures appear much higher due to the use of the Fahrenheit system; to convert °F HDD, divide by 1.8 to get °C HDD, multiply by 1.8 for the reverse. SEE ALSO REFERENCES
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