Heating Degree Day…what in the world is this?

These days climate issues hit the headline of all media, online or traditional. Lots of words are being spent to illustrate what climate changes are, how much of it is of human origin, and what we can, or could, do to stop the planet from heating up. A good share of human-made emissions come from the heating systems in our home, residential and commercial alike. We have grown familiar, then, with terms like Heating degree days, or HDDs. For most of us, though, the question is still the same: what in the world heating dregree days are?

HDDs are a unit of measurement that relates to the heating of a building. The product of measuring temperature (in degrees) over time (in days), HDDs are measured relative to a baseline outdoor temperature that theoretically represents an “ideal” comfort temperature, below which a building will need to be heated to feel comfortable. In the U.S. the baseline temperature for HDDs is 65 degrees Fahrenheit, 18°C in the Celsius system.

Please be warned a heating degree day is actually not a day at all, rather a unit that can measure temperature over any given period of time. If you use 65 degrees as a baseline, then a day with an average temperature of 60 will accrue 5 HDDs (the difference between a ctual temperature and the comfort temperature, set at 65 degrees). If there were 30 days like that in a row, that month would accrue 150 HDDs.

If we speak in terms of energy efficiency, HDDs allow comparing the energy efficiency of a building over time, or the energy efficiency of several buildings in different climates, a task that can be difficult in force of the influence outdoor temperatures have on a building’s heating energy consumption.

For example, the same building will need to use more heating energy during a cold winter than it did during the previous, milder winter, given the change in outdoor temperature. Likewise, at any given time a building in areas as cold as Vermont or Norway is likely to need more heating energy than a building in Florida or Malta – even if the buildings are equally energy efficient. In both cases, energy consumption fluctuates due to temperature, which makes it difficult to assess the efficiency of one building over another, or of the same building over time.

This is where HDDs come to our help: by isolating the variable of outside temperature, it becomes much clearer which buildings are operating efficiently (or inefficiently), regardless of the outside temperature.

For instance, say a family installs several efficiency measures one summer, but their subsequent winter heating bills stay the same as those of the previous year. By isolating the variable of average winter temperatures, which often fluctuate from year to year, HDDs can help assess the efficiency of the family’s home before and after the efficiency measures were installed. So if the winter after their summer installations is colder than the previous one, their heating needs will be greater – yet the new efficiency measures will help the family maintain a comfortable home while paying much less than they otherwise would have.


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