Example of Use
HDD provides a simple metric for quantifying the amount of heating that buildings in a particular location need over a certain period (e.g. a particular month or year). In conjunction with the average U-value for a building they provide a means of roughly estimating the amount of energy required to heat the building over that period.
One HDD means that the temperature conditions outside the building were equivalent to being below a defined threshold comfort temperature inside the building by one degree for one day. Thus heat has to be provided inside the building to maintain thermal comfort.
Say we are given the number of heating degree days D in one year and we wish to calculate the energy required by a building. We know that heat needs to be provided at the rate at which it is being lost to the environment. This can be calculated as the sum of the heat losses per degree of each element of the buildings' thermal envelope (such as windows, walls, and roof) or as the average U-value of the building multiplied by the area of the thermal envelope of the building, or quoted directly for the whole building. This gives the buildings' specific heat loss rate Pspecific, generally given in Watts per Kelvin (W/K). Total energy in kWh is then given by:
Q = Pspecific×24×D/1000
Note that as total energy consumption is in kWh and heating degree days are we must convert W/K into kWh per degree per day by dividing by 1000 (to convert W to kW), and multiplying by 24 hours in a day (1 kW = 1kWh per hour). Since one degree temperature difference in Celsius and Kelvin scale are the same, they get cancelled and no conversion is required.
Example: For a typical New York City winter day with high of 40°F and low of 30°F, the average temperature is likely to be around 35°F. For such a day we can approximate the HDD as (65 - 35) = 30. A month of thirty similar days might accumulate 900 HDD . A year (including summer average temperatures above 70°F) might accumulate an annual 5000 HDD .
Read more about this topic: Heating Degree Day
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