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The standard outdoor temperature according to DIN EN 12831-1 is the outside air temperature used to calculate the heating load (standard heat losses) of a building. It represents the cold design case for which the heating system must be sized.
For Germany, this value is specified more precisely in the national supplement DIN/TS 12831-1:2020-04 as the design outdoor temperature. It is based on the coldest two-day mean of the outside air temperature which, statistically, occurs roughly once every two years. The underlying data are climate records from the German Weather Service (DWD) over an 18-year period (1995–2012), evaluated on a postcode basis for Germany. For Berlin-Mitte (postcode 10178), for example, the design outdoor temperature is −11.2 °C. You can look up the design outdoor temperature for your location (only germany) using the climate map provided by the German Heat Pump Association.
Optionally, the design outdoor temperature can be adjusted using the time constant of the building. For thermally heavy, slow-reacting buildings, the temperature can be increased by up to around 4 K, because these buildings cool down more slowly and therefore require less heating reserve.
Historical context: In the past, heating design in Germany was based on “standard outdoor temperatures” according to DIN 4701 or DIN EN 12831:2003, which provided tabulated values for a limited number of locations. Today, design outdoor temperatures according to DIN/TS 12831-1 are used for Germany. In physical terms, however, the idea remains the same: a rare but typical winter cold event forms the basis for determining the heat losses and the heating load.
Further reading and sources:
DIN/TS 12831-1:2020-04 – Method for calculation of the room heating load – Part 1: National supplements to DIN EN 12831-1
