9101 Degree Days

In theory, if one wanted to predict the heat lost by a building over an extended period of time, Equation 9.12 could be solved for each individual hour, taking into account the relevant changes of the variables. This is possible because the change in the value BLC with respect to temperature is not significant and the indoor temperature (Ti) is normally controlled to a constant value (such as 70°F in winter). That being the case, the total energy transfer could be predicted by knowing the summation of the individual deviations of outdoor temperature (To) from the indoor condition (Ti) over an extended period of time.

The summation described has come to be known as "Degree-Days" and annual tabulations of Degree-Days for various climates are published by NOAA, ASHRAE, and various other public and military organizations. Historically an indoor reference point of 65°F has been used to account for the fact that even the most poorly constructed building is capable of maintaining comfort conditions without heating when the temperature is at least 65°F.

Because of the impracticality of obtaining hour-by-hour temperature data for a wide variety of locations, daily temperature averages are often used to represent 24-hour blocks of time. The daily averages are calculated by taking the average of daily maximum and minimum temperature recordings, which in turn are converted o Degree Days. Quantitatively this is calculated as:

reference T average■

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