Heating degree day, HDD, is a measurement directly related to heating costs that can help you accurately calculate the energy consumption of a home’s heating system.

The concept of HDD was devised by heating engineers to correlate exterior temperature to the fuel needed to heat a building. In the Pacific Northwest, HDD is calculated using a base temperature of 65 degrees Fahrenheit—the temperature at which a building requires no heating. If the average outside temperature is more than 65 degrees, the HDD for that day is zero. If the average temperature is less than 65 degrees, simply subtract the number from 65 to determine the HDD. For example, a day with an average temperature of 40 degrees is 25 HDD. Combining the daily HDD values for a year yields the annual HDD. The Portland Metro region averages approximately 4,200 annual HDD, while other parts of Oregon average more than 8,000 HDD.

HDD can help you evaluate a home’s heating needs and seasonal heating requirements. If annual HDD increases by 10 percent from one year to the next, a home can expect to see a 10 percent increase in heating fuel use. This is a more accurate way to determine a home’s heating requirements than comparing year-to-year heating bills.