DEGREE DAYS:

In order to calculate how much heat a building will lose during the heating season, or gain during the hot weather, designers use a simple system called Degree Days. Most metropolitan daily papers publish Heating Degree Day and Cooling Degree Day information on the Weather page. If you cannot find cumulative information there, call the paper’s office for complete figures for one full heating season, and one full cooling season.

Heating Degree Days are figured like this: The method assumes that, in the home, various heat sources will keep the house at a comfortable 70°F even when the outside temperature drops to 65°F. So, the count of Heating-Degree-Days starts when the outside temperature falls below 65 degrees, and a day in which the average temperature is 62°F has contributed 3 Degree Days to the seasonal heat load.

Cooling Degree Days are calculated upward from 65°F, and a 24-hour day that averages 68°F will add 3 cooling Degree Days to the season's total.

Getting back to heating Degree Days: The Degree-Day total for the heating season is used to predict the heat loss for a particular building design, considering losses through windows, walls, ceiling, etc. Also, Degree Days are used to estimate the annual savings that might result, for example, from changing from double-glazed to triple-glazed windows.

As examples of annual heating Degree-Day totals, the figure for Boston, Mass is about 5,640, while it is 9,640 in Caribou, Maine. On a side note, the most recent decade has been the warmest decade on record, so the trend may be toward lower heating degree-day totals than those quoted here:

City Area          Heating DD     Cooling DD
                                      
Boston             5,641              678
Milwaukee        7,324              479
Seattle             4,908              190
San Diego        1,256               984
Dallas              2,507             2,603
Miami                 200             4,198

In Figuring Heat Loss you will see how to use your area’s annual Degree-Day total to calculate the heat loss for home or office. Also, you’ll see how to experiment on paper, to see the result of increasing insulation or other energy conservation measures.

If you want, we will calculate heat loss for you. Email us, and we'll send you the worksheets to fill out and return. For a modest fee, we’ll send you the report with recommendations.