What are heating/cooling degree days?

They’re a useful measure of the how much heating is needed on a given day. Twice the Heating degree days means you need twice as much energy to heat the building. Here’s why...

Thermodynamics tells us that the energy transferred between two bodies is proportional to the temperature di erence between them. The higher the temperature gradient, the more energy is transferred (in the form of heat). Heating Degree Days (HDDs) are de ned as the time-integrated temperature di erence between the threshold building temperature (usually 18.3° C) and the outside temperature. So one HDD is equivalent to outdoors being colder than indoors by one degree, for one day. If you double the HDD, you double the temperature gradient, so twice as much energy is transferred.

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