ENERGY-CONSCIOUS PLANNING AND ZONING
3.3.1 Characteristic Values for Describing Thermal Levels

In the legal guidelines VDI 2067, Section 1 and VDI 3807, Section 1, the significant characteristic thermal values for the heating requirements of 29 locations in Baden-Württemberg are outlined. In this context, the following definitions of technical concepts (shortened here) are used. The maximum values occurring in Baden-Württemberg supplement the corresponding definitions.

The potential heating period is established in a pure calendric sense from September 1 to May 31, amounting to nine months in total.

The actual heating period falls within the potential heating period and can only be estimated on the basis of previous weather patterns. The period is delineated by the first outdoor temperature reading below 15°C (as a five-day average) in autumn and the first reading above 15°C in the spring of the following year.

On any given day requiring heating, the daily average of the air temperature lies below 15°C. The number of heating days relates to the months of the heating period as well as the summer months of June, July, and August. In the city of Trochtelfingen, for example, there are on average 269.9 heating days during the heating period, compared with 235.7 in Heidelberg. The average outdoor temperature during the heating period amounts to 3.0°C in Trochtelfingen and 6.3°C in Heidelberg.

The day-to-degree ratio for the heating period is the sum of the differences between the average room temperature of 20°C and the daily averages of the air temperature across all heating days (from September 1 to May 31). Only the heating days falling between the beginning and end of the heating period are taken into consideration for the day-to-degree ratio of the heating period. The day-to-degree ratio for the heating period amounts to 4597 Kd/a for Trochtelfingen, 3226 Kd/a for Heidelberg (Kd/a = Kelvin x Day/Year).

Finally, the heating day-degrees (G15) (see Fig. 3/19) is the sum of the differences between the heating threshold temperature of 15°C and the daily averages of outdoor temperatures across all calendar days with daily average temperatures under 15°C. The average quantity of heating day-degrees (G15,a) amounts to 3425 Kd/a for Trochtelfingen and 2065 Kd/a for Heidelberg.

These data also demarcate the regional span of climatic characteristic values occurring in Baden-Württemberg. In this, exceeding the heating threshold temperature of 15°C (day-to-degree ratio and heating day-degrees) and the maximum temperature difference between inside and outside play significant roles.

The basis for calculating yearly heating needs is primarily the standards booklet DIN 4701, according to which the standard heating requirements of a building are to be calculated. The yearly heating use during the heating period can be estimated beforehand according to the legal guideline VDI 2067. After this, an increase of 1°C in the average temperature difference between outside and inside during the heating period implies a 6% increase in heating energy usage.