To compare the heating costs of different periods (all years are not equally warm or cold), heating costs are reduced to a normal year based on degree days. Outdoor climate has a significant impact on heat consumption, and degree days are used to eliminate discrepancies. In the application, degree days are used to check heating costs. To do so, the consumption of a period of 12 months is multiplied, as well as the heating consumption normalized based on degree days – the horizontal line signifies that in terms of changes in outdoor climate, heating consumption of the building has been even. Rise or decline in the line refers to significant changes in the consumption of heating energy and the consumer should determine its cause.
One degree day represents a 1 °C difference between the calculated indoor temperature and the average outdoor temperature within 24 hours. For example, if the average outdoor temperature over a 24-hour period is 2 °C, then the number of degree days of a 24-hour period (1 day) is 17 – 2 = 15 (°C×d). When calculating degree days and normalizing heating costs based on these, the indoor temperature or bivalent temperature used is lower than the actual indoor temperature, because some of the required heat is covered by the so-called free heat generated by electrical appliances, people and solar radiation. The better a building has been constructed and insulated, the lower its bivalent temperature (the temperature until which additional heating is required), because in case of smaller heat loss, appliances, people and solar radiation provide higher percentage of the heat requirements. As a rule, the bivalent temperature is between the range of 12–17 °C. In case of a non-renovated apartment building, it is usually 17 °C, i.e. 3–5 °C is added as free heat.
Estonia has been divided into six regions based on degree days as follows: 1) Jõhvi, 2) Tartu, 3) Tallinn, 4) Valga, 5) Pärnu and 6) Ristna. In simplified calculations, degree days in Tallinn may be regarded as the Estonian average.
Figure 1. Recommended regions of degree days [www.kredex.ee]
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