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Concept
Heating degree day
Summary
Heating degree day (HDD) is a measurement designed to quantify the demand for energy needed to heat a building. HDD is derived from measurements of outside air temperature. The heating requirements for a given building at a specific location are considered to be directly proportional to the number of HDD at that location.
Related measurements include the cooling degree day (CDD), which quantifies demand for air conditioning.
Heating degree days are defined relative to a base temperature—the outside temperature above which a building needs no heating. Base temperatures may be defined for a particular building as a function of the temperature that the building is heated to, or it may be defined for a country or region for example. In the latter case, building standards or conventions may exist for the temperature threshold. These include:
The base temperature does not necessarily correspond to the building mean internal temperature, as standards may consider mean building insulation levels and internal gains to determine an average external temperature at which heating will be required. Base temperatures of 16 °C and 19 °C (61, 66 °F) are also used. The variation in choice of base temperature implies that HDD values cannot always be compared – care must be taken to ensure that only HDDs with equal base temperatures are compared.
There are a number of ways in which HDD can be calculated: the more detailed a record of temperature data, the more accurate the HDD that can be calculated. HDD are often calculated using simple approximation methods that use daily temperature readings instead of more detailed temperature records such as half-hourly readings, the latter of which can be used to estimate an integral. One popular approximation method, that used by the U.S. National Weather Service, is to take the average temperature on any given day (the mean of the high and low temperature) and subtract it from the base temperature. If the value is less than or equal to zero, that day has zero HDD.
Official source
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