## 3 Results and Discussion
### Comparison with the Oil Boiler System  
The last step in the present analysis regards the comparison of the air-source high-temperature heat pump system with the oil boiler case (baseline scenario). Figure 13  
Figure 12: Heat pump system: monthly energy quantities [kWh] and mean COP for (**a**) Athens (Zone B), and (**b**) Thessaloniki (Zone C). displays the heating and boiler loads for the oil boiler case in Athens and Thessaloniki. The cumulative consumption in fuel energy input was found at 9046 kWh for Athens and 16,965 kWh for Thessaloniki. Table 4 includes the data of the energy, financial, and environmental analysis of the examined scenarios in this study, while Table 5 summarizes the most valuable comparative indexes. Figure 13: Boiler system: loads in [W] and cumulative energy in [kWh] for heating for (**a**) Athens (Zone B) and (**b**) Thessaloniki (Zone C). The primary energy consumption with the heat pump decreased from 9951 kWh to 9247 kWh per year in Athens, leading to a reduction of 7.1%. For Thessaloniki, the primary energy demand was reduced from 18,861 kWh to 18,267 kWh, which presented a reduction of 3.1%. The lower primary energy gain in Thessaloniki can be explained by the reduced mean yearly COP in this city when compared to Athens. The lifetime primary energy savings were found to be 17,588 kWh in Athens and 9857 kWh in Thessaloniki. The financial analysis shows the important benefits of the use of heat pumps, which proves the need for expanding the use of this technology. The simple payback period for the case in Athens was 8.03 years, while in Thessaloniki, it was 6.11 years. Both values encourage the SPP in Thessaloniki to be lower due to the greater yearly economic gain in this city. Practically, the higher demand in Thessaloniki makes the profit greater in this city, while the investment cost for the heat pump is greater in Thessaloniki due to the higher capacity of the system compared to the capacity in Athens. The life cycle cost analysis indicates that the present net savings in Athens were EUR 5956, and in Thessaloniki, the net savings were EUR 7286. Thus, it is clear that the investment leads to satisfying gain, which was higher in Thessaloniki. The environmental analysis indicates that the CO\({}_{2}\) emissions for Athens were reduced by 35% with the heat pump, while the respective reduction in Thessaloniki was 32%. The lifetime CO\({}_{2}\) emissions reduction was calculated at 20,882 kg\({}_{\text{CO}2}\) for Athens and 35,278 kg\({}_{\text{CO}2}\) for Thessaloniki. Although the reduction percentage was higher for Athens, the total CO\({}_{2}\) avoidance was greater in Thessaloniki due to the greater energy demands in this location. Moreover, the present heat pump uses an environmentally friendly working fluid, and this fact adds an extra environmental benefit to the suggested configuration. It is remarkable to state that the examined high-temperature heat pump is an ideal solution for both examined locations. While the performance of the heat pump in Athens was a bit higher than in the case of Thessaloniki, the benefits in Thessaloniki seemed to be greater. This result is explained by the higher heating demand in Thessaloniki, which makes it possible to achieve higher enhancements margins by retrofitting the heating system in Thessaloniki. In every case, it has to be highlighted that, in both locations, there are important advantages that indicate the utilization of high-temperature heat pumps both in the southern and northern Greek regions. Finally, it is obvious that all the energy, economic, and environmental indexes showed that the substitution of the oil boiler with the heat pump is a viable and feasible choice. The relatively low mean COP of around 2 is enough for overcoming the oil boiler performance.