## 3 Results and Discussion
### System Performance Comparison for Both Climate Zones  
After the thermodynamic investigation, the next step regards the analysis of the total heating system and its comparison with the oil boiler case. The analysis was done for two representative locations in Greece: Athens and Thessaloniki. Figure 9 shows the results of the heating loads, the electrical demand from the heat pump, and the respective cumulative energy quantities. Figure 9a illustrates the results of the simulation for Athens, and Figure 9b illustrates the results of the simulation for Thessaloniki. The maximum heating load in Athens was found at 6 kW, while in Thessaloniki, it was found at 8 kW. Moreover, it is obvious that the heating period was a bit greater in Thessaloniki compared to Athens. More specifically, the heating period in Athens was 3009 h, while in Thessaloniki it was 3847 h. The yearly heating demand in Athens was found at 6785 kWh and the respective electricity consumption was found at 3189 kWh. The yearly heating demand in Thessaloniki was found at 12,591 kWh and the respective electricity consumption was found at 6236 kWh. It is clear that Thessaloniki had approximately the double thermal loads compared to the Athens case.  
The mean yearly COP or seasonal COP (SCOP) in Athens was calculated at 2.128, while in Thessaloniki it was calculated at 2.019. The lower value in Thessaloniki is reasonable due to the lower ambient temperatures in Thessaloniki compared to Athens, as has been presented in Figure 3. Also, it has to be added that the reduction of the COP with the reduction of the ambient temperature is clear by checking Figure 6. Practically, the lower ambient temperature reduced the evaporator temperature, and consequently, the COP was getting to lower values according to Figure 6. Additionally, Figure 10 depicts the  
Figure 8: Pressure–specific enthalpy depiction (p–h) of the heat pump cycle for the case with ambient temperature at 10 \({}^{\circ}\)C.  
hourly COP variation for both locations during the year. It is clear that the COP values were generally smaller in Thessaloniki compared to the respective values in Athens; the fact verifies the lower mean yearly COP in Thessaloniki. Moreover, Figure 11 shows the variation in the compressor's isentropic efficiency during the year for both locations. Again, it can be said that the isentropic efficiency in Thessaloniki takes lower values compared to the efficiency in Athens. The mean yearly isentropic efficiency in Athens was found at 63.3%, while in Thessaloniki, it was found at 61.9%.  
Figure 9: Heat pump system: loads in [W] and cumulative energy in [kWh] for heating for (**a**) Athens (Zone B) and (**b**) Thessaloniki (Zone C).  
Figure 12 shows the monthly results in energy and efficiency terms for both locations. The heating period for Athens starts in November and stops in April, while for Thessaloniki,  
Figure 11: Heat pump system: variation in the compressor isentropic efficiency during the year of operation for the examined locations.  
Figure 10: Heat pump system: variation in the COP during the year of operation for the examined locations.  
it starts in October and stops in April. The maximum heating energy demand was found for both locations in January, and, more specifically, it was 2039 kWh for Athens and 3435 kWh for Thessaloniki. The minimum COP was found in January for both locations due to the lower ambient temperature during this month compared to the others. More specifically, the mean monthly COP for January was calculated at 2.093 for Athens and 1.959 for Thessaloniki.