Design Temperature Difference
The design temperature difference is the variation in degrees Fahrenheit between the outside and inside design temperatures. It is used in the heat transmission loss formula (see below) and is a crucial factor in heating calculations.
Be careful that you obtain the degree difference between the two temperatures; do not simply subtract the smaller figure from the larger one. For example, if the outside and inside design temperatures are —20 and 70°F, respectively, the design temperature difference will be 90° (20° below zero plus 70° above zero).