The Degree Day Formula was devised some forty odd years ago by the
American Gas Association and other groups, and has since been revised to reflect
internal heat gains and the levels of
insulation. The Degree Day Formula is based on the
assumption that heat for the interior of a house or building will be obtained from sources
other than the heating system (e.g. sunlight, body heat of the occupants, etc.)
until the outside temperature declines to 65°F. At this point the heating system begins to
operate. The consumption of fuel will be directly
proportionate to the difference between the 65°F base temperature and the mean outdoor temperature. In other words, three times as much fuel
will be used when the mean outdoor temperature is 35°F than when it is 55°F. The mean outdoor temperature can be determined by taking the
sum of the highest and lowest outside temperatures during a twenty-four hour period beginning at midnight and dividing it by two. Each degree in temperature below 65°F is regarded as
one “
degree day.”
The Degree Day Formula is applied by dividing the heat loss figure by 1000 and multiplying the result by the figure for the unit fuel consumption per degree day per 1000 Btu’s design heat loss, which, in turn, is multiplied by the total number of degree days in the heating season (calculated on a 65°F base) and then by the appropriate correction factors.