## 3 Results and Discussion
In the HP/CHP/B systems, where the HP has operational priority, the EPI is reduced up to 70% (1000 kW HP unit) compared to the base system with the boiler and CHP unit. Further reductions in _EPI_ could be achieved through the deployment of large-scale HP units. In contrast, when the CHP unit holds operational priority, the  
Figure 10: The share of RES by the systems. Figure 9: The structure of the thermal energy produced in systems. The highest CO\({}_{2}\) emissions are produced in systems that utilize natural gas exclusively: 1443 tons of CO\({}_{2}\) per year in the gas-fired boiler system, and 1360 tons of CO\({}_{2}\) in the CHP/B system. In the systems where HP takes operational priority, annual emissions are, depending on the installed HP size, reduced in range from 71 to 82% compared to the CHP/B system. In the systems where CHP takes operational priority, annual emissions are reduced by a smaller amount compared to the systems with HP operational priority: reduction is in the range from 55 to 61% compared to the CHP/B system. Similar annual CO\({}_{2}\) emissions are achieved when the electricity generated by the CHP is utilised for the HP operation (CHP-HP/B system), which results in 572 tons of CO\({}_{2}\) emissions per year, representing a 58% reduction relative to the CHP-B system. A full lifecycle assessment (LCA) was not conducted due to the unavailability of detailed data on materials, manufacturing, and supply chains. However, according to a recent LCA study on heat pumps [33], the use phase accounts for approximately 94.6% of total emissions, with other phases contributing marginally (-6%). Therefore, the operational CO\({}_{2}\) emissions presented in this study provide a reliable representation of the overall environmental impact. The cost analysis is based on the assumption that the DH system operator invests its funds in the purchase of new equipment, i.e., does not make a loan for the investment. The initial costs for equipment, grid connection, maintenance, and purchased water for HPs are the same for all three price scenarios. However, the energy price differs due to different electricity and natural gas prices. The levelized cost of energy for heating \(C_{\rm LH}\) is presented in Figure 12.  
Figure 11: Indicators of primary energy consumption (_EPI_) and CO\({}_{2}\) emission. In Scenarios 1 and 2, which reflect current and recent energy price conditions, the baseline DH system configuration CHP/B is found to be the most cost-effective. It achieves the lowest levelized cost of heating \(C_{\text{LH}}\) among all evaluated configurations, with values of 0.037 \(\upepsilon\)/kWh and 0.019 \(\upepsilon\)/kWh, respectively. Under these scenarios, high electricity prices favour the operation of the CHP unit, particularly due to the ability to export electricity to the grid. Consequently, systems incorporating HPs exhibit higher \(C_{\text{LH}}\) values compared to the baseline system. The increase in costs is most pronounced in configurations where the HP has operational priority, with \(C_{\text{LH}}\) rising by up to 75% in Scenario 1 and by more than 200% in Scenario 2. Even in configurations where the CHP unit retains operational priority, the \(C_{\text{LH}}\) still increases by up to 31% in Scenario 1 and up to 88% in Scenario 2. The cost increase is more significant in systems with higher installed HP capacity. These results indicate that, under the current and recent electricity price conditions modelled in Scenarios 1 and 2, the installation of HPs, particularly large-scale units, is not economically viable. Additionally, the operation of a system relying solely on a gas-fired boiler under these pricing conditions proves economically unviable due to high fuel costs and the absence of electricity export revenues. In contrast, Scenario 3 assumes a more favourable electricity price structure. Under this scenario, all system variants incorporating HPs achieve lower specific heating costs than the baseline DH system, which records a \(C_{\text{LH}}\) of 0.110 \(\upepsilon\)/kWh.