Heating degree-days for energy planning under climate change: Validation for Sarajevo and regional extrapolation

• HDD analyzed for Sarajevo (2001–2020) using three temperature-based methods • Updated HDD = 2708°C-days; 12 % lower than reference value (3077°C-days) • Heating season shortened from 211 to 195 days; mean temperature increased by 0.7°C • Regional HDD reductions of 9–15 % observed across five major cities • Validation shows ≈5 % deviation between measured and calculated values Heating degree-days (HDD) are essential for estimating heating energy demand and guiding energy planning under changing climatic conditions. This study analyzes HDD for Sarajevo for the period 2001–2020 using three methods based on monthly, daily, and hourly outdoor temperatures, applying several criteria that vary in the number of consecutive days required to start or stop heating, in order to assess the impact of climate change and rising winter temperatures. The updated HDD value for Sarajevo (2708°C-days) is 12% lower than the current reference value (3077°C-days), which is still officially used despite being calculated in the 1980s from outdated climatic data using the same methodology. The heating season has shortened from 211 to 195 days, while the mean outdoor temperature during the heating season increased from 4.4°C to 5.1°C. Validation against measured natural gas consumption showed an approximately 4% deviation from the calculated values, confirming the reliability of the updated estimates. Extrapolation to five major cities in the former Yugoslavia revealed consistent HDD reductions of 9–15%, particularly in warmer climates. By integrating HDD calculations at multiple temporal scales, the multi-method approach improves the accuracy of heating demand estimates and provides a robust basis for updating building energy standards and improving heating system design under climate change. These findings demonstrate consistent HDD reductions across the region and underscore the importance of updating outdoor design parameters for reliable energy planning in the context of a warming climate.