Sensitivity of Heating Energy Demand to Indoor Temperature, Effective Air-Change Rate, and Climate: A TRNSYS Case Study Supporting Sustainable Operation of a Multi-Apartment Building

Building heating energy demand is influenced by indoor temperature, air exchange, and climatic conditions. This study examines the individual and combined effects of these parameters on a district-heated multi-apartment residential building in Osijek, Croatia. A TRNSYS model was evaluated using measured space-heating consumption for the analysed period from October 2024 to April 2025 and representative TMYx weather data. In the model, air exchange was represented by an effective air-change rate accounting for both uncontrolled air infiltration through the building envelope and natural ventilation associated with occupant window opening. The measured seasonal consumption was 666.16 MWh, while the model predicted 670.61 MWh, resulting in a deviation of 0.67%. The model was then used for a full-factorial analysis including nine indoor temperatures from 18 to 26 °C, nine effective air-change rates from 0.30 to 0.70 h−1, and six Croatian locations, giving 486 simulation scenarios. Under reference conditions, heating demand in Zadar and Split was 44.05% and 50.93% lower than in Osijek/Čepin. Increasing the indoor temperature from 18 to 26 °C increased seasonal demand by 432.63–525.32 MWh, while increasing the effective air-change rate from 0.30 to 0.70 h−1 increased it by 182.67–296.55 MWh. The analysis also showed a positive interaction between indoor temperature and effective air-change rate. For the analysed building, the results can support more sustainable energy use by informing energy-efficient building operation and district-heating demand planning, while the reported numerical sensitivities remain specific to the case-study building.