Heating degree days can be used to estimate
seasonal fuel requirements. One heating degree day
(HDD) is defined as a 24-hr. period when inside
temperature is 1°F above outside temperature. Most
published values for HDDs use 65°F as the base
inside temperature. If, in a 24-hr. period, the average
outside temperature was 40°F, there would be
65 – 40 = 25 HDDs for that day. Table 2, Appendix
IV, shows mean annual heating degree days for
selected locations. Consult a local weather station
for specific locations.