by Corkscrewed » Mon Mar 24, 2014 7:14 pm
A degree day is the difference between the average temperature of a given day and 65°F, which can be added up throughout a season to get an idea of how far from the "norm" a location is. The 65°F baseline was a technically arbitary temperature agreed to a long time ago by the people who make these decisions for use in the industry.
In heating, the average temperature will be presumably less than 65°F, since it's cold. 65° is the baseline temperature, meaning you want to get the temperature back up to that amount. So you will have to use energy (heat is simply thermal energy) to raise the temperature. Btu/hr is a measure of energy rate needed to get the temperature back up to the baseline, and you multiply that by 24 hours to get the total BTU in a day, which is multiplied by the degree day of that particular day to figure out how much heat is needed that day. Then you add all the days up in a season to get the total amount needed for the season.
The formula basically gets you to the same place but uses the transitive property to aggregate the Degree Days all at once, and then multiply by the other variables to find out how much heat is required.