The normal period of the heating degree days has been changed to the averages of the climatological normal period of 1991-2020 and the coefficients has been calculated to be equivalent to the new normal period. Old reference values can be ordered from the Climate Service Centre.

What is a heating degree day?

With the help of a heating degree day, the consumption of heating energy is standardized in order to compare the energy consumption of a building in different months or years as well as comparing buildings in different municipalities. The heating degree day is used in estimating the need for heating a building because the energy consumption of a building is almost linear to the difference between the outside and inside temperature.

The heating degree day is calculated monthly for 16 cities of comparison. Those cities are Mariehamn, Vantaa, Helsinki-Kaisaniemi, Pori, Turku, Tampere-Pirkkala, Lahti, Lappeenranta, Jyväskylä, Vaasa, Kuopio, Joensuu, Kajaani, Oulu, Sodankylä, and Ivalo. If ordered, heating degree days can also be calculated for other areas.

How is the heating degree day calculated?

The heating degree day is calculated by adding up the differences of the daily indoor and outside temperatures of the whole month. The most commonly used heating degree day is S17, which is calculated by adding up the difference between the presumed indoor temperature +17°C and the daily average outside temperature. The heating degree day of a month is the sum of the daily heating degree days of the month and the heating degree day of a year is the sum of the monthly heating degree days of the year. The average heating degree day of the normal period 1991-2020 is used as the comparative value.

Days when the average temperature is above +10°C in the spring and above +12°C in the autumn are not taken into account in calculating the heating degree day. It is presumed that the heating is turned on or off when the outside temperature goes daily above or below the above-mentioned limits. If the temperature observations are missing, the daily average temperatures are interpolated.